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本博文整理者：刘军

本博文出自于： 《Java8 编程官方参考教程》一书

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第7章 方法和类的深入分析

7.1 重载方法

  1.   1 package Chap7;
  2.   2
  3.   3 /**
  4.   4  * Demonstrate method overloading.
  5.   5  *
  6.   6  * @ClassName: OverloadDemo
  7.   7  * @Description:
  8.   8  * @author 刘军/shall_liu (1136808529@qq.com)
  9.   9  * @date 2017年9月14日 上午12:15:21
  10.  10  *
  11.  11  */
  12.  12 class OverloadDemo {
  13.  13 	void test() {
  14.  14 		System.out.println("No parameters");
  15.  15 	}
  16.  16
  17.  17 	// Overload test for one integer parameter.
  18.  18 	void test(int a) {
  19.  19 		System.out.println("a: " + a);
  20.  20 	}
  21.  21
  22.  22 	// Overload test for two integer parameters.
  23.  23 	void test(int a, int b) {
  24.  24 		System.out.println("a and b: " + a + " " + b);
  25.  25 	}
  26.  26
  27.  27 	// overload test for a double parameter
  28.  28 	double test(double a) {
  29.  29 		System.out.println("double a: " + a);
  30.  30 		return a * a;
  31.  31 	}
  32.  32 }
  33.  33
  34.  34
  35.  35 package Chap7;
  36.  36
  37.  37 /**
  38.  38  *
  39.  39  * @ClassName: Overload
  40.  40  * @Description:
  41.  41  * @author 刘军/shall_liu (1136808529@qq.com)
  42.  42  * @date 2017年9月14日 下午11:35:25
  43.  43  *
  44.  44  */
  45.  45 class Overload {
  46.  46 	public static void main(String args[]) {
  47.  47 		OverloadDemo ob = new OverloadDemo();
  48.  48 		double result;
  49.  49
  50.  50 		// call all versions of test()
  51.  51 		ob.test();
  52.  52 		ob.test(10);
  53.  53 		ob.test(10, 20);
  54.  54 		result = ob.test(123.25);
  55.  55 		System.out.println("Result of ob.test(123.25): " + result);
  56.  56 	}
  57.  57 }
  58.  58 //其运行结果为：
  59.  59 No parameters
  60.  60 a: 10
  61.  61 a and b: 10 20
  62.  62 double a: 123.25
  63.  63 Result of ob.test(123.25): 15190.5625
  64.  64
  65.  65
  66.  66
  67.  67 

  1.   1 package Chap7;
  2.   2
  3.   3 /**
  4.   4  * Demonstrate method overloading.
  5.   5  *
  6.   6  * @ClassName: OverloadDemo
  7.   7  * @Description:
  8.   8  * @author 刘军/shall_liu (1136808529@qq.com)
  9.   9  * @date 2017年9月14日 上午12:15:21
  10.  10  *
  11.  11  */
  12.  12 class OverloadDemo {
  13.  13 	void test() {
  14.  14 		System.out.println("No parameters");
  15.  15 	}
  16.  16
  17.  17 	// Overload test for two integer parameters.
  18.  18 	void test(int a, int b) {
  19.  19 		System.out.println("a and b: " + a + " " + b);
  20.  20 	}
  21.  21
  22.  22 	// overload test for a double parameter and return type
  23.  23 	void test(double a) {
  24.  24 		System.out.println("Inside test(double) a: " + a);
  25.  25 	}
  26.  26 }
  27.  27
  28.  28 package Chap7;
  29.  29
  30.  30 /**
  31.  31  *
  32.  32  * @ClassName: Overload
  33.  33  * @Description:
  34.  34  * @author 刘军/shall_liu (1136808529@qq.com)
  35.  35  * @date 2017年9月14日 下午11:35:25
  36.  36  *
  37.  37  */
  38.  38 class Overload {
  39.  39 	public static void main(String args[]) {
  40.  40 		OverloadDemo ob = new OverloadDemo();
  41.  41 		int i = 88;
  42.  42
  43.  43 		ob.test();
  44.  44 		ob.test(10, 20);
  45.  45
  46.  46 		ob.test(i); // this will invoke test(double)
  47.  47 		ob.test(123.2); // this will invoke test(double)
  48.  48 	}
  49.  49 }
  50.  50 //其运行结果为：
  51.  51 No parameters
  52.  52 a and b: 10 20
  53.  53 Inside test(double) a: 88.0
  54.  54 Inside test(double) a: 123.2
  55.  55

7.1.1 重载构造函数

  1.   1 package Chap7;
  2.   2
  3.   3 /**
  4.   4  *
  5.   5 * @ClassName: Box
  6.   6 * @Description:  重载构造器
  7.   7 * @author  刘军/shall_liu (1136808529@qq.com)
  8.   8 * @date 2017年9月14日 下午11:47:01
  9.   9 *
  10.  10  */
  11.  11 class Box {
  12.  12 	double width;
  13.  13 	double height;
  14.  14 	double depth;
  15.  15
  16.  16 	// This is the constructor for Box.
  17.  17 	Box(double w, double h, double d) {
  18.  18 		width = w;
  19.  19 		height = h;
  20.  20 		depth = d;
  21.  21 	}
  22.  22
  23.  23 	// compute and return volume
  24.  24 	double volume() {
  25.  25 		return width * height * depth;
  26.  26 	}
  27.  27 }

  1.   1 package Chap7;
  2.   2
  3.   3 /**
  4.   4  *
  5.   5  * @ClassName: Box
  6.   6  * @Description: 重载构造器
  7.   7  * Here, Box defines three constructors to initialize the dimensions of a box
  8.   8  * various ways.
  9.   9  * @author 刘军/shall_liu (1136808529@qq.com)
  10.  10  * @date 2017年9月14日 下午11:47:01
  11.  11  *
  12.  12  */
  13.  13 class Box {
  14.  14 	double width;
  15.  15 	double height;
  16.  16 	double depth;
  17.  17
  18.  18 	// constructor used when all dimensions specified
  19.  19 	Box(double w, double h, double d) {
  20.  20 		width = w;
  21.  21 		height = h;
  22.  22 		depth = d;
  23.  23 	}
  24.  24
  25.  25 	// constructor used when no dimensions specified
  26.  26 	Box() {
  27.  27 		width = -1; // use -1 to indicate
  28.  28 		height = -1; // an uninitialized
  29.  29 		depth = -1; // box
  30.  30 	}
  31.  31
  32.  32 	// constructor used when cube is created
  33.  33 	Box(double len) {
  34.  34 		width = height = depth = len;
  35.  35 	}
  36.  36
  37.  37 	// compute and return volume
  38.  38 	double volume() {
  39.  39 		return width * height * depth;
  40.  40 	}
  41.  41 }
  42.  42
  43.  43
  44.  44
  45.  45 package Chap7;
  46.  46
  47.  47 /**
  48.  48  *
  49.  49 * @ClassName: OverloadCons
  50.  50 * @Description:  重载构造器
  51.  51 * @author  刘军/shall_liu (1136808529@qq.com)
  52.  52 * @date 2017年9月14日 下午11:49:30
  53.  53 *
  54.  54  */
  55.  55
  56.  56 class OverloadCons {
  57.  57 	public static void main(String args[]) {
  58.  58 		// create boxes using the various constructors
  59.  59 		Box mybox1 = new Box(10, 20, 15);
  60.  60 		Box mybox2 = new Box();
  61.  61 		Box mycube = new Box(7);
  62.  62
  63.  63 		double vol;
  64.  64
  65.  65 		// get volume of first box
  66.  66 		vol = mybox1.volume();
  67.  67 		System.out.println("Volume of mybox1 is " + vol);
  68.  68
  69.  69 		// get volume of second box
  70.  70 		vol = mybox2.volume();
  71.  71 		System.out.println("Volume of mybox2 is " + vol);
  72.  72
  73.  73 		// get volume of cube
  74.  74 		vol = mycube.volume();
  75.  75 		System.out.println("Volume of mycube is " + vol);
  76.  76 	}
  77.  77 }
  78.  78
  79.  79 //其运行结果为：
  80.  80 Volume of mybox1 is 3000.0
  81.  81 Volume of mybox2 is -1.0
  82.  82 Volume of mycube is 343.0
  83.  83
  84.  84
  85.  85

7.2 将对象用作参数

  1.   1 class PassOb {
  2.   2   public static void main(String args[]) {
  3.   3     Test ob1 = new Test(100, 22);
  4.   4     Test ob2 = new Test(100, 22);
  5.   5     Test ob3 = new Test(-1, -1);
  6.   6
  7.   7     System.out.println("ob1 == ob2: " + ob1.equalTo(ob2));
  8.   8
  9.   9     System.out.println("ob1 == ob3: " + ob1.equalTo(ob3));
  10.  10   }
  11.  11 }
  12.  12
  13.  13 package Chap7;
  14.  14
  15.  15 /**
  16.  16  * Objects may be passed to methods.
  17.  17  *
  18.  18  * @ClassName: Test
  19.  19  * @Description:
  20.  20  * @author 刘军/shall_liu (1136808529@qq.com)
  21.  21  * @date 2017年9月14日 下午11:52:55
  22.  22  *
  23.  23  */
  24.  24 class Test {
  25.  25 	int a, b;
  26.  26
  27.  27 	Test(int i, int j) {
  28.  28 		a = i;
  29.  29 		b = j;
  30.  30 	}
  31.  31
  32.  32 	// return true if o is equal to the invoking object
  33.  33 	boolean equalTo(Test o) {
  34.  34 		if (o.a == a && o.b == b)
  35.  35 			return true;
  36.  36 		else
  37.  37 			return false;
  38.  38 	}
  39.  39 }
  40.  40 //其运行结果为：
  41.  41 ob1 == ob2: true
  42.  42 ob1 == ob3: false
  43.  43 

  1.   1 package Chap7;
  2.   2
  3.   3 /**
  4.   4  * Here, Box allows one object to initialize another.
  5.   5  *
  6.   6  * @ClassName: Box
  7.   7  * @Description:
  8.   8  * @author 刘军/shall_liu (1136808529@qq.com)
  9.   9  * @date 2017年9月15日 上午12:00:54
  10.  10  *
  11.  11  */
  12.  12
  13.  13 class Box {
  14.  14 	double width;
  15.  15 	double height;
  16.  16 	double depth;
  17.  17
  18.  18 	// construct clone of an object
  19.  19 	Box(Box ob) { // pass object to constructor
  20.  20 		width = ob.width;
  21.  21 		height = ob.height;
  22.  22 		depth = ob.depth;
  23.  23 	}
  24.  24
  25.  25 	// constructor used when all dimensions specified
  26.  26 	Box(double w, double h, double d) {
  27.  27 		width = w;
  28.  28 		height = h;
  29.  29 		depth = d;
  30.  30 	}
  31.  31
  32.  32 	// constructor used when no dimensions specified
  33.  33 	Box() {
  34.  34 		width = -1; // use -1 to indicate
  35.  35 		height = -1; // an uninitialized
  36.  36 		depth = -1; // box
  37.  37 	}
  38.  38
  39.  39 	// constructor used when cube is created
  40.  40 	Box(double len) {
  41.  41 		width = height = depth = len;
  42.  42 	}
  43.  43
  44.  44 	// compute and return volume
  45.  45 	double volume() {
  46.  46 		return width * height * depth;
  47.  47 	}
  48.  48 }
  49.  49
  50.  50 package Chap7;
  51.  51
  52.  52 /**
  53.  53  *
  54.  54  * @ClassName: OverloadCons2
  55.  55  * @Description:
  56.  56  * @author 刘军/shall_liu (1136808529@qq.com)
  57.  57  * @date 2017年9月15日 上午12:01:21
  58.  58  *
  59.  59  */
  60.  60 class OverloadCons2 {
  61.  61 	public static void main(String args[]) {
  62.  62 		// create boxes using the various constructors
  63.  63 		Box mybox1 = new Box(10, 20, 15);
  64.  64 		Box mybox2 = new Box();
  65.  65 		Box mycube = new Box(7);
  66.  66
  67.  67 		Box myclone = new Box(mybox1);
  68.  68
  69.  69 		double vol;
  70.  70
  71.  71 		// get volume of first box
  72.  72 		vol = mybox1.volume();
  73.  73 		System.out.println("Volume of mybox1 is " + vol);
  74.  74
  75.  75 		// get volume of second box
  76.  76 		vol = mybox2.volume();
  77.  77 		System.out.println("Volume of mybox2 is " + vol);
  78.  78
  79.  79 		// get volume of cube
  80.  80 		vol = mycube.volume();
  81.  81 		System.out.println("Volume of cube is " + vol);
  82.  82
  83.  83 		// get volume of clone
  84.  84 		vol = myclone.volume();
  85.  85 		System.out.println("Volume of clone is " + vol);
  86.  86 	}
  87.  87 }
  88.  88
  89.  89
  90.  90 //其运行结果为：
  91.  91 Volume of mybox1 is 3000.0
  92.  92 Volume of mybox2 is -1.0
  93.  93 Volume of cube is 343.0
  94.  94 Volume of clone is 3000.0
  95.  95
  96.  96
  97.  97

7.3 参数传递的深入分析

  1.   1 package Chap7;
  2.   2
  3.   3 /**
  4.   4  * Objects may be passed to methods.
  5.   5  *
  6.   6  * @ClassName: Test
  7.   7  * @Description:
  8.   8  * @author 刘军/shall_liu (1136808529@qq.com)
  9.   9  * @date 2017年9月14日 下午11:52:55
  10.  10  *
  11.  11  */
  12.  12 // Simple Types are passed by value.
  13.  13 class Test {
  14.  14 	void meth(int i, int j) {
  15.  15 		i *= 2;
  16.  16 		j /= 2;
  17.  17 	}
  18.  18 }
  19.  19
  20.  20 package Chap7;
  21.  21
  22.  22 class CallByValue {
  23.  23 	public static void main(String args[]) {
  24.  24 		Test ob = new Test();
  25.  25 		int a = 15, b = 20;
  26.  26
  27.  27 		System.out.println("a and b before call: " + a + " " + b);
  28.  28
  29.  29 		ob.meth(a, b);
  30.  30
  31.  31 		System.out.println("a and b after call: " + a + " " + b);
  32.  32 	}
  33.  33 }
  34.  34
  35.  35 //其运行结果为：、
  36.  36 a and b before call: 15 20
  37.  37 a and b after call: 15 20
  38.  38
  39.  39 

  1.   1 package Chap7;
  2.   2
  3.   3 /**
  4.   4  * Objects are passed through their references.
  5.   5  *
  6.   6  * @ClassName: Test
  7.   7  * @Description:
  8.   8  * @author 刘军/shall_liu (1136808529@qq.com)
  9.   9  * @date 2017年9月14日 下午11:52:55
  10.  10  *
  11.  11  */
  12.  12
  13.  13 class Test {
  14.  14 	int a, b;
  15.  15
  16.  16 	Test(int i, int j) {
  17.  17 		a = i;
  18.  18 		b = j;
  19.  19 	}
  20.  20
  21.  21 	// pass an object
  22.  22 	void meth(Test o) {
  23.  23 		o.a *= 2;
  24.  24 		o.b /= 2;
  25.  25 	}
  26.  26 }
  27.  27
  28.  28
  29.  29 package Chap7;
  30.  30
  31.  31 /**
  32.  32  *
  33.  33  * @ClassName: PassObjRef
  34.  34  * @Description:
  35.  35  * @author 刘军/shall_liu (1136808529@qq.com)
  36.  36  * @date 2017年9月15日 上午12:10:14
  37.  37  *
  38.  38  */
  39.  39 class PassObjRef {
  40.  40 	public static void main(String args[]) {
  41.  41 		Test ob = new Test(15, 20);
  42.  42
  43.  43 		System.out.println("ob.a and ob.b before call: " + ob.a + " " + ob.b);
  44.  44
  45.  45 		ob.meth(ob);
  46.  46
  47.  47 		System.out.println("ob.a and ob.b after call: " + ob.a + " " + ob.b);
  48.  48 	}
  49.  49 }
  50.  50
  51.  51 //其运行结果为；
  52.  52 ob.a and ob.b before call: 15 20
  53.  53 ob.a and ob.b after call: 30 10
  54.  54

7.4 返回对象

  1.   1 package Chap7;
  2.   2
  3.   3 /**
  4.   4  * Objects are passed through their references.
  5.   5  *
  6.   6  * @ClassName: Test
  7.   7  * @Description:
  8.   8  * @author 刘军/shall_liu (1136808529@qq.com)
  9.   9  * @date 2017年9月14日 下午11:52:55
  10.  10  *
  11.  11  */
  12.  12
  13.  13 class Test {
  14.  14 	int a, b;
  15.  15
  16.  16
  17.  17 	public Test() {
  18.  18 	}
  19.  19
  20.  20 	Test(int i) {
  21.  21 		a = i;
  22.  22 	}
  23.  23
  24.  24 	Test(int i, int j) {
  25.  25 		a = i;
  26.  26 		b = j;
  27.  27 	}
  28.  28
  29.  29 	// pass an object
  30.  30 	void meth(Test o) {
  31.  31 		o.a *= 2;
  32.  32 		o.b /= 2;
  33.  33 	}
  34.  34
  35.  35 	Test incrByTen() {
  36.  36 		Test temp = new Test(a + 10);
  37.  37 		return temp;
  38.  38 	}
  39.  39 }
  40.  40
  41.  41
  42.  42
  43.  43 package Chap7;
  44.  44
  45.  45 class RetOb {
  46.  46 	public static void main(String args[]) {
  47.  47 		Test ob1 = new Test(2);
  48.  48 		Test ob2;
  49.  49
  50.  50 		ob2 = ob1.incrByTen();
  51.  51 		System.out.println("ob1.a: " + ob1.a);
  52.  52 		System.out.println("ob2.a: " + ob2.a);
  53.  53
  54.  54 		ob2 = ob2.incrByTen();
  55.  55 		System.out.println("ob2.a after second increase: " + ob2.a);
  56.  56 	}
  57.  57 }
  58.  58
  59.  59 //
  60.  60 ob1.a: 2
  61.  61 ob2.a: 12
  62.  62 ob2.a after second increase: 22
  63.  63
  64.  64
  65.  65

7.5 递归

  1.   1 package Chap7;
  2.   2
  3.   3 //A simple example of recursion.
  4.   4 class Factorial {
  5.   5 	// this is a recusive function
  6.   6 	int fact(int n) {
  7.   7 		int result;
  8.   8
  9.   9 		if (n == 1)
  10.  10 			return 1;
  11.  11 		result = fact(n - 1) * n;
  12.  12 		return result;
  13.  13 	}
  14.  14 }
  15.  15
  16.  16
  17.  17 package Chap7;
  18.  18
  19.  19 class Recursion {
  20.  20 	public static void main(String args[]) {
  21.  21 		Factorial f = new Factorial();
  22.  22
  23.  23 		System.out.println("Factorial of 3 is " + f.fact(3));
  24.  24 		System.out.println("Factorial of 4 is " + f.fact(4));
  25.  25 		System.out.println("Factorial of 5 is " + f.fact(5));
  26.  26 	}
  27.  27 }
  28.  28 //
  29.  29
  30.  30
  31.  31 Factorial of 3 is 6
  32.  32 Factorial of 4 is 24
  33.  33 Factorial of 5 is 120
  34.  34 


  1.   1 package Chap7;
  2.   2
  3.   3 //Another example that uses recursion.
  4.   4
  5.   5 class RecTest {
  6.   6 	int values[];
  7.   7
  8.   8 	RecTest(int i) {
  9.   9 		values = new int[i];
  10.  10 	}
  11.  11
  12.  12 	// display arrary -- recursively
  13.  13 	void printArray(int i) {
  14.  14 		if (i == 0)
  15.  15 			return;
  16.  16 		else
  17.  17 			printArray(i - 1);
  18.  18 		System.out.println("[" + (i - 1) + "] " + values[i - 1]);
  19.  19 	}
  20.  20 }
  21.  21
  22.  22 package Chap7;
  23.  23
  24.  24 class Recursion2 {
  25.  25 	public static void main(String args[]) {
  26.  26 		RecTest ob = new RecTest(10);
  27.  27 		int i;
  28.  28
  29.  29 		for (i = 0; i < 10; i++)
  30.  30 			ob.values[i] = i;
  31.  31
  32.  32 		ob.printArray(10);
  33.  33 	}
  34.  34 }
  35.  35
  36.  36 //
  37.  37 [0] 0
  38.  38 [1] 1
  39.  39 [2] 2
  40.  40 [3] 3
  41.  41 [4] 4
  42.  42 [5] 5
  43.  43 [6] 6
  44.  44 [7] 7
  45.  45 [8] 8
  46.  46 [9] 9
  47.  47
  48.  48

7.6 访问控制

  1.   1 package Chap7;
  2.   2
  3.   3 /* This program demonstrates the difference between
  4.   4 public and private.
  5.   5 */
  6.   6 class Test {
  7.   7 	int a; // default access
  8.   8 	public int b; // public access
  9.   9 	private int c; // private access
  10.  10
  11.  11 	// methods to access c
  12.  12 	void setc(int i) { // set c's value
  13.  13 		c = i;
  14.  14 	}
  15.  15
  16.  16 	int getc() { // get c's value
  17.  17 		return c;
  18.  18 	}
  19.  19 }
  20.  20
  21.  21
  22.  22 package Chap7;
  23.  23
  24.  24 class AccessTest {
  25.  25 	public static void main(String args[]) {
  26.  26 		Test ob = new Test();
  27.  27
  28.  28 		// These are OK, a and b may be accessed directly
  29.  29 		ob.a = 10;
  30.  30 		ob.b = 20;
  31.  31
  32.  32 		// This is not OK and will cause an error
  33.  33 		// ob.c = 100; // Error!
  34.  34
  35.  35 		// You must access c through its methods
  36.  36 		ob.setc(100); // OK
  37.  37
  38.  38 		System.out.println("a, b, and c: " + ob.a + " " + ob.b + " " + ob.getc());
  39.  39 	}
  40.  40 }
  41.  41 //
  42.  42
  43.  43 a, b, and c: 10 20 100
  44.  44
  45.  45
  46.  46 

  1.   1 package Chap7;
  2.   2
  3.   3 //This class defines an integer stack that can hold 10 values.
  4.   4 class Stack {
  5.   5 	/*
  6.   6 	 * Now, both stck and tos are private. This means that they cannot be
  7.   7 	 * accidentally or maliciously altered in a way that would be harmful to the
  8.   8 	 * stack.
  9.   9 	 */
  10.  10 	private int stck[] = new int[10];
  11.  11 	private int tos;
  12.  12
  13.  13 	// Initialize top-of-stack
  14.  14 	Stack() {
  15.  15 		tos = -1;
  16.  16 	}
  17.  17
  18.  18 	// Push an item onto the stack
  19.  19 	void push(int item) {
  20.  20 		if (tos == 9)
  21.  21 			System.out.println("Stack is full.");
  22.  22 		else
  23.  23 			stck[++tos] = item;
  24.  24 	}
  25.  25
  26.  26 	// Pop an item from the stack
  27.  27 	int pop() {
  28.  28 		if (tos < 0) {
  29.  29 			System.out.println("Stack underflow.");
  30.  30 			return 0;
  31.  31 		} else
  32.  32 			return stck[tos--];
  33.  33 	}
  34.  34 }
  35.  35 

  1.   1 class TestStack {
  2.   2   public static void main(String args[]) {
  3.   3     Stack mystack1 = new Stack();
  4.   4     Stack mystack2 = new Stack();
  5.   5
  6.   6     // push some numbers onto the stack
  7.   7     for(int i=0; i<10; i++) mystack1.push(i);
  8.   8     for(int i=10; i<20; i++) mystack2.push(i);
  9.   9
  10.  10     // pop those numbers off the stack
  11.  11     System.out.println("Stack in mystack1:");
  12.  12     for(int i=0; i<10; i++)
  13.  13        System.out.println(mystack1.pop());
  14.  14
  15.  15     System.out.println("Stack in mystack2:");
  16.  16     for(int i=0; i<10; i++)
  17.  17        System.out.println(mystack2.pop());
  18.  18
  19.  19     // these statements are not legal
  20.  20     // mystack1.tos = -2;
  21.  21     // mystack2.stck[3] = 100;
  22.  22   }
  23.  23 }

7.7 理解st

  1.   1 // Demonstrate static variables, methods, and blocks.
  2.   2 class UseStatic {
  3.   3   static int a = 3;
  4.   4   static int b;
  5.   5
  6.   6   static void meth(int x) {
  7.   7     System.out.println("x = " + x);
  8.   8     System.out.println("a = " + a);
  9.   9     System.out.println("b = " + b);
  10.  10   }
  11.  11
  12.  12   static {
  13.  13     System.out.println("Static block initialized.");
  14.  14     b = a * 4;
  15.  15   }
  16.  16
  17.  17   public static void main(String args[]) {
  18.  18     meth(42);
  19.  19   }
  20.  20 }

  1.   1 class StaticDemo {
  2.   2   static int a = 42;
  3.   3   static int b = 99;
  4.   4   static void callme() {
  5.   5     System.out.println("a = " + a);
  6.   6   }
  7.   7 }
  8.   8
  9.   9 class StaticByName {
  10.  10   public static void main(String args[]) {
  11.  11     StaticDemo.callme();
  12.  12     System.out.println("b = " + StaticDemo.b);
  13.  13   }
  14.  14 }
  15.  15

7.8 final介绍

7.9 重新审视数组

  1.   1 // This program demonstrates the length array member.
  2.   2 class Length {
  3.   3   public static void main(String args[]) {
  4.   4     int a1[] = new int[10];
  5.   5     int a2[] = {3, 5, 7, 1, 8, 99, 44, -10};
  6.   6     int a3[] = {4, 3, 2, 1};
  7.   7
  8.   8     System.out.println("length of a1 is " + a1.length);
  9.   9     System.out.println("length of a2 is " + a2.length);
  10.  10     System.out.println("length of a3 is " + a3.length);
  11.  11   }
  12.  12 }
  13.  13 

  1.   1 // Improved Stack class that uses the length array member.
  2.   2 class Stack {
  3.   3   private int stck[];
  4.   4   private int tos;
  5.   5
  6.   6   // allocate and initialize stack
  7.   7   Stack(int size) {
  8.   8     stck = new int[size];
  9.   9     tos = -1;
  10.  10   }
  11.  11
  12.  12   // Push an item onto the stack
  13.  13   void push(int item) {
  14.  14     if(tos==stck.length-1) // use length member
  15.  15       System.out.println("Stack is full.");
  16.  16     else
  17.  17       stck[++tos] = item;
  18.  18   }
  19.  19
  20.  20   // Pop an item from the stack
  21.  21   int pop() {
  22.  22     if(tos < 0) {
  23.  23       System.out.println("Stack underflow.");
  24.  24       return 0;
  25.  25     }
  26.  26     else
  27.  27       return stck[tos--];
  28.  28   }
  29.  29 }

  1.   1 class TestStack2 {
  2.   2   public static void main(String args[]) {
  3.   3     Stack mystack1 = new Stack(5);
  4.   4     Stack mystack2 = new Stack(8);
  5.   5
  6.   6     // push some numbers onto the stack
  7.   7     for(int i=0; i<5; i++) mystack1.push(i);
  8.   8     for(int i=0; i<8; i++) mystack2.push(i);
  9.   9
  10.  10     // pop those numbers off the stack
  11.  11     System.out.println("Stack in mystack1:");
  12.  12     for(int i=0; i<5; i++)
  13.  13        System.out.println(mystack1.pop());
  14.  14
  15.  15     System.out.println("Stack in mystack2:");
  16.  16     for(int i=0; i<8; i++)
  17.  17        System.out.println(mystack2.pop());
  18.  18   }
  19.  19 }

7.10 嵌套类和内部类

  1.   1 // Demonstrate an inner class.
  2.   2 class Outer {
  3.   3   int outer_x = 100;
  4.   4
  5.   5   void test() {
  6.   6     Inner inner = new Inner();
  7.   7     inner.display();
  8.   8   }
  9.   9
  10.  10   // this is an innner class
  11.  11   class Inner {
  12.  12     void display() {
  13.  13       System.out.println("display: outer_x = " + outer_x);
  14.  14     }
  15.  15   }
  16.  16 }
  17.  17
  18.  18 class InnerClassDemo {
  19.  19   public static void main(String args[]) {
  20.  20     Outer outer = new Outer();
  21.  21     outer.test();
  22.  22   }
  23.  23 }
  24.  24 

  1.   1 // This program will not compile.
  2.   2 class Outer {
  3.   3   int outer_x = 100;
  4.   4
  5.   5   void test() {
  6.   6     Inner inner = new Inner();
  7.   7     inner.display();
  8.   8   }
  9.   9
  10.  10   // this is an innner class
  11.  11   class Inner {
  12.  12     int y = 10; // y is local to Inner
  13.  13     void display() {
  14.  14       System.out.println("display: outer_x = " + outer_x);
  15.  15     }
  16.  16   }
  17.  17
  18.  18   void showy() {
  19.  19     System.out.println(y); // error, y not known here!
  20.  20   }
  21.  21 }
  22.  22
  23.  23 class InnerClassDemo {
  24.  24   public static void main(String args[]) {
  25.  25     Outer outer = new Outer();
  26.  26     outer.test();
  27.  27   }
  28.  28 }
  29.  29 

  1.   1
  2.   2 // Define an inner class within a for loop.
  3.   3 class Outer {
  4.   4   int outer_x = 100;
  5.   5
  6.   6   void test() {
  7.   7     for(int i=0; i<10; i++) {
  8.   8       class Inner {
  9.   9         void display() {
  10.  10           System.out.println("display: outer_x = " + outer_x);
  11.  11         }
  12.  12       }
  13.  13       Inner inner = new Inner();
  14.  14       inner.display();
  15.  15     }
  16.  16   }
  17.  17 }
  18.  18
  19.  19 class InnerClassDemo {
  20.  20   public static void main(String args[]) {
  21.  21     Outer outer = new Outer();
  22.  22     outer.test();
  23.  23   }
  24.  24 }
  25.  25

7.11 String类介绍

  1.   1 // Demonstrating Strings.
  2.   2 class StringDemo {
  3.   3   public static void main(String args[]) {
  4.   4     String strOb1 = "First String";
  5.   5     String strOb2 = "Second String";
  6.   6     String strOb3 = strOb1 + " and " + strOb2;
  7.   7
  8.   8     System.out.println(strOb1);
  9.   9     System.out.println(strOb2);
  10.  10     System.out.println(strOb3);
  11.  11   }
  12.  12 }

  1.   1 // Demonstrating some String methods.
  2.   2 class StringDemo2 {
  3.   3   public static void main(String args[]) {
  4.   4     String strOb1 = "First String";
  5.   5     String strOb2 = "Second String";
  6.   6     String strOb3 = strOb1;
  7.   7
  8.   8     System.out.println("Length of strOb1: " +    strOb1.length());
  9.   9
  10.  10     System.out.println("Char at index 3 in strOb1: " + strOb1.charAt(3));
  11.  11
  12.  12     if(strOb1.equals(strOb2)) {
  13.  13       System.out.println("strOb1 == strOb2");
  14.  14     }else{
  15.  15       System.out.println("strOb1 != strOb2");
  16.  16     }
  17.  17
  18.  18     if(strOb1.equals(strOb3)) {
  19.  19       System.out.println("strOb1 == strOb3");}
  20.  20     else{
  21.  21       System.out.println("strOb1 != strOb3");}
  22.  22   }
  23.  23 }
  24.  24 

  1.   1 // Demonstrate String arrays.
  2.   2 class StringDemo3 {
  3.   3   public static void main(String args[]) {
  4.   4     String str[] = { "one", "two", "three" };
  5.   5
  6.   6     for(int i=0; i<str.length; i++)
  7.   7       System.out.println("str[" + i + "]: " + str[i]);
  8.   8   }
  9.   9 }
  10.  10

7.12 使用命令行参数

  1.   1 class CommandLine {
  2.   2   public static void main(String args[]) {
  3.   3     for(int i=0; i<args.length; i++)
  4.   4       System.out.println("args[" + i + "]: " +   args[i]);
  5.   5   }
  6.   6 }
  7.   7

7.13 varargs：可变长度参数

  1.   1 // Use an array to pass a variable number of
  2.   2 // arguments to a method.
  3.   3 class PassArray {
  4.   4   static void vaTest(int v[]) {
  5.   5     System.out.print("Number of args: " + v.length +  " Contents: ");
  6.   6
  7.   7     for(int x : v){
  8.   8       System.out.print(x + " ");
  9.   9     }
  10.  10     System.out.println();
  11.  11   }
  12.  12
  13.  13   public static void main(String args[])
  14.  14   {
  15.  15     // Notice how an array must be created to
  16.  16     // hold the arguments.
  17.  17     int n1[] = { 10 };
  18.  18     int n2[] = { 1, 2, 3 };
  19.  19     int n3[] = { };
  20.  20
  21.  21     vaTest(n1); // 1 arg
  22.  22     vaTest(n2); // 3 args
  23.  23     vaTest(n3); // no args
  24.  24   }
  25.  25 }

  1.   1 // Demonstrate variable-length arguments.
  2.   2 class VarArgs {
  3.   3
  4.   4   // vaTest() now uses a vararg.
  5.   5   static void vaTest(int ... v) {
  6.   6     System.out.print("Number of args: " + v.length +  " Contents: ");
  7.   7
  8.   8     for(int x : v) {
  9.   9       System.out.print(x + " ");
  10.  10     }
  11.  11     System.out.println();
  12.  12   }
  13.  13
  14.  14   public static void main(String args[])
  15.  15   {
  16.  16
  17.  17     // Notice how vaTest() can be called with a
  18.  18     // variable number of arguments.
  19.  19     vaTest(10);      // 1 arg
  20.  20     vaTest(1, 2, 3); // 3 args
  21.  21     vaTest();        // no args
  22.  22   }
  23.  23 }
  24.  24 

  1.   1 // Use varargs with standard arguments.
  2.   2 class VarArgs2 {
  3.   3
  4.   4   // Here, msg is a normal parameter and v is a
  5.   5   // varargs parameter.
  6.   6   static void vaTest(String msg, int ... v) {
  7.   7     System.out.print(msg + v.length +
  8.   8                         " Contents: ");
  9.   9
  10.  10     for(int x : v) {
  11.  11       System.out.print(x + " ");
  12.  12      }
  13.  13
  14.  14     System.out.println();
  15.  15   }
  16.  16
  17.  17   public static void main(String args[])
  18.  18   {
  19.  19     vaTest("One vararg: ", 10);
  20.  20     vaTest("Three varargs: ", 1, 2, 3);
  21.  21     vaTest("No varargs: ");
  22.  22   }
  23.  23 }
  24.  24

7.13.1 重载varargs方法

  1.   1 // Varargs and overloading.
  2.   2 class VarArgs3 {
  3.   3
  4.   4   static void vaTest(int ... v) {
  5.   5     System.out.print("vaTest(int ...): " +  "Number of args: " + v.length +  " Contents: ");
  6.   6
  7.   7     for(int x : v) {
  8.   8       System.out.print(x + " ");
  9.   9     }
  10.  10     System.out.println();
  11.  11   }
  12.  12
  13.  13   static void vaTest(boolean ... v) {
  14.  14     System.out.print("vaTest(boolean ...) " +  "Number of args: " + v.length +  " Contents: ");
  15.  15
  16.  16     for(boolean x : v) {
  17.  17       System.out.print(x + " ");
  18.  18     }
  19.  19     System.out.println();
  20.  20   }
  21.  21
  22.  22   static void vaTest(String msg, int ... v) {
  23.  23     System.out.print("vaTest(String, int ...): " +  msg + v.length +  " Contents: ");
  24.  24
  25.  25     for(int x : v) {
  26.  26       System.out.print(x + " ");
  27.  27      }
  28.  28     System.out.println();
  29.  29   }
  30.  30
  31.  31   public static void main(String args[])
  32.  32   {
  33.  33     vaTest(1, 2, 3);
  34.  34     vaTest("Testing: ", 10, 20);
  35.  35     vaTest(true, false, false);
  36.  36

7.13.2 varargs方法与模糊性

  1.   1 // Varargs, overloading, and ambiguity.
  2.   2 //
  3.   3 // This program contains an error and will
  4.   4 // not compile!
  5.   5 class VarArgs4 {
  6.   6
  7.   7   static void vaTest(int ... v) {
  8.   8     System.out.print("vaTest(Integer ...): " +  "Number of args: " + v.length +  " Contents: ");
  9.   9
  10.  10     for(int x : v) {
  11.  11       System.out.print(x + " ");
  12.  12     }
  13.  13     System.out.println();
  14.  14   }
  15.  15
  16.  16   static void vaTest(boolean ... v) {
  17.  17     System.out.print("vaTest(boolean ...) " +  "Number of args: " + v.length +  " Contents: ");
  18.  18
  19.  19     for(boolean x : v) {
  20.  20       System.out.print(x + " ");
  21.  21    }
  22.  22     System.out.println();
  23.  23   }
  24.  24
  25.  25
  26.  26   public static void main(String args[])
  27.  27   {
  28.  28     vaTest(1, 2, 3);  // OK
  29.  29     vaTest(true, false, false); // OK
  30.  30
  31.  31     vaTest(); // Error: Ambiguous!
  32.  32   }
  33.  33 }

第8章 继承

8.1 继承的基础知识

  1.   1 // A simple example of inheritance.
  2.   2
  3.   3 // Create a superclass.
  4.   4 class A {
  5.   5   int i, j;
  6.   6
  7.   7   void showij() {
  8.   8     System.out.println("i and j: " + i + " " + j);
  9.   9   }
  10.  10 }
  11.  11
  12.  12 // Create a subclass by extending class A.
  13.  13 class B extends A {
  14.  14   int k;
  15.  15
  16.  16   void showk() {
  17.  17     System.out.println("k: " + k);
  18.  18   }
  19.  19   void sum() {
  20.  20     System.out.println("i+j+k: " + (i+j+k));
  21.  21   }
  22.  22 }
  23.  23
  24.  24 class SimpleInheritance {
  25.  25   public static void main(String args[]) {
  26.  26     A superOb = new A();
  27.  27     B subOb = new B();
  28.  28
  29.  29     // The superclass may be used by itself.
  30.  30     superOb.i = 10;
  31.  31     superOb.j = 20;
  32.  32     System.out.println("Contents of superOb: ");
  33.  33     superOb.showij();
  34.  34     System.out.println();
  35.  35
  36.  36     /* The subclass has access to all public members of
  37.  37        its superclass. */
  38.  38     subOb.i = 7;
  39.  39     subOb.j = 8;
  40.  40     subOb.k = 9;
  41.  41     System.out.println("Contents of subOb: ");
  42.  42     subOb.showij();
  43.  43     subOb.showk();
  44.  44     System.out.println();
  45.  45
  46.  46     System.out.println("Sum of i, j and k in subOb:");
  47.  47     subOb.sum();
  48.  48   }
  49.  49 }
  50.  50

8.1.1 成员访问与继承

  1.   1 /* In a class hierarchy, private members remain
  2.   2    private to their class.
  3.   3
  4.   4    This program contains an error and will not
  5.   5    compile.
  6.   6 */
  7.   7
  8.   8 // Create a superclass.
  9.   9 class A {
  10.  10   int i; // public be default
  11.  11   private int j; // private to A
  12.  12
  13.  13   void setij(int x, int y) {
  14.  14     i = x;
  15.  15     j = y;
  16.  16   }
  17.  17 }
  18.  18
  19.  19 // A's j is not accessible here.
  20.  20 class B extends A {
  21.  21   int total;
  22.  22
  23.  23   void sum() {
  24.  24     total = i + j; // ERROR, j is not accessible here
  25.  25   }
  26.  26 }
  27.  27
  28.  28 class Access {
  29.  29   public static void main(String args[]) {
  30.  30     B subOb = new B();
  31.  31
  32.  32     subOb.setij(10, 12);
  33.  33
  34.  34     subOb.sum();
  35.  35     System.out.println("Total is " + subOb.total);
  36.  36   }
  37.  37 }
  38.  38

8.1.2 一个更实际的例子

  1.   1 // This program uses inheritance to extend Box.
  2.   2 class Box {
  3.   3   double width;
  4.   4   double height;
  5.   5   double depth;
  6.   6
  7.   7   // construct clone of an object
  8.   8   Box(Box ob) { // pass object to constructor
  9.   9     width = ob.width;
  10.  10     height = ob.height;
  11.  11     depth = ob.depth;
  12.  12   }
  13.  13
  14.  14   // constructor used when all dimensions specified
  15.  15   Box(double w, double h, double d) {
  16.  16     width = w;
  17.  17     height = h;
  18.  18     depth = d;
  19.  19   }
  20.  20
  21.  21   // constructor used when no dimensions specified
  22.  22   Box() {
  23.  23     width = -1;  // use -1 to indicate
  24.  24     height = -1; // an uninitialized
  25.  25     depth = -1;  // box
  26.  26   }
  27.  27
  28.  28   // constructor used when cube is created
  29.  29   Box(double len) {
  30.  30     width = height = depth = len;
  31.  31   }
  32.  32
  33.  33   // compute and return volume
  34.  34   double volume() {
  35.  35     return width * height * depth;
  36.  36   }
  37.  37 }
  38.  38
  39.  39 // Here, Box is extened to include weight.
  40.  40 class BoxWeight extends Box {
  41.  41   double weight; // weight of box
  42.  42
  43.  43   // constructor for BoxWeight
  44.  44   BoxWeight(double w, double h, double d, double m) {
  45.  45     width = w;
  46.  46     height = h;
  47.  47     depth = d;
  48.  48     weight = m;
  49.  49   }
  50.  50 }
  51.  51
  52.  52 class DemoBoxWeight {
  53.  53   public static void main(String args[]) {
  54.  54     BoxWeight mybox1 = new BoxWeight(10, 20, 15, 34.3);
  55.  55     BoxWeight mybox2 = new BoxWeight(2, 3, 4, 0.076);
  56.  56     double vol;
  57.  57
  58.  58     vol = mybox1.volume();
  59.  59     System.out.println("Volume of mybox1 is " + vol);
  60.  60     System.out.println("Weight of mybox1 is " + mybox1.weight);
  61.  61     System.out.println();
  62.  62
  63.  63     vol = mybox2.volume();
  64.  64     System.out.println("Volume of mybox2 is " + vol);
  65.  65     System.out.println("Weight of mybox2 is " + mybox2.weight);
  66.  66   }
  67.  67 }
  68.  68 

  1.   1 // Here, Box is extended to include color.
  2.   2 class ColorBox extends Box {
  3.   3   int color; // color of box
  4.   4
  5.   5   ColorBox(double w, double h, double d, int c) {
  6.   6     width = w;
  7.   7     height = h;
  8.   8     depth = d;
  9.   9     color = c;
  10.  10   }
  11.  11 }
  12.  12

8.1.3 超类变量可以引用子类对象

  1.   1 class RefDemo {
  2.   2   public static void main(String args[]) {
  3.   3     BoxWeight weightbox = new BoxWeight(3, 5, 7, 8.37);
  4.   4     Box plainbox = new Box();
  5.   5     double vol;
  6.   6
  7.   7     vol = weightbox.volume();
  8.   8     System.out.println("Volume of weightbox is " + vol);
  9.   9     System.out.println("Weight of weightbox is " + weightbox.weight);
  10.  10     System.out.println();
  11.  11
  12.  12     // assign BoxWeight reference to Box reference
  13.  13     plainbox = weightbox;
  14.  14
  15.  15     vol = plainbox.volume(); // OK, volume() defined in Box
  16.  16     System.out.println("Volume of plainbox is " + vol);
  17.  17
  18.  18     /* The following statement is invalid because plainbox
  19.  19        does not define a weight member. */
  20.  20 //  System.out.println("Weight of plainbox is " + plainbox.weight);
  21.  21   }
  22.  22 }

8.2 使用super关键字

8.2.1 使用super调用超类的构造函数

  1.   1 // BoxWeight now uses super to initialize its Box attributes.
  2.   2 class BoxWeight extends Box {
  3.   3   double weight; // weight of box
  4.   4
  5.   5   // initialize width, height, and depth using super()
  6.   6   BoxWeight(double w, double h, double d, double m) {
  7.   7     super(w, h, d); // call superclass constructor
  8.   8     weight = m;
  9.   9   }
  10.  10 }

  1.   1  // A complete implementation of BoxWeight.
  2.   2 class Box {
  3.   3   private double width;
  4.   4   private double height;
  5.   5   private double depth;
  6.   6
  7.   7   // construct clone of an object
  8.   8   Box(Box ob) { // pass object to constructor
  9.   9     width double h, double d) {
  10.  10     width = w;
  11.  11     height = h;
  12.  12     depth = d;
  13.  13   }
  14.  14
  15.  15   // constructor used when no dimensions specified
  16.  16   Box() {
  17.  17     width = -1;  // use -1 to indicate
  18.  18     height = -1; // an uninitialized
  19.  19     depth = -1;  // box
  20.  20   }
  21.  21  = ob.width;
  22.  22     height = ob.height;
  23.  23     depth = ob.depth;
  24.  24   }
  25.  25
  26.  26   // constructor used when all dimensions specified
  27.  27   Box(double w,
  28.  28   // constructor used when cube is created
  29.  29   Box(double len) {
  30.  30     width = height = depth = len;
  31.  31   }
  32.  32
  33.  33   // compute and return volume
  34.  34   double volume() {
  35.  35     return width * height * depth;
  36.  36   }
  37.  37 }
  38.  38
  39.  39 // BoxWeight now fully implements all constructors.
  40.  40 class BoxWeight extends Box {
  41.  41   double weight; // weight of box
  42.  42
  43.  43   // construct clone of an object
  44.  44   BoxWeight(BoxWeight ob) { // pass object to constructor
  45.  45     super(ob);
  46.  46     weight = ob.weight;
  47.  47   }
  48.  48
  49.  49   // constructor when all parameters are specified
  50.  50   BoxWeight(double w, double h, double d, double m) {
  51.  51     super(w, h, d); // call superclass constructor
  52.  52     weight = m;
  53.  53   }
  54.  54
  55.  55   // default constructor
  56.  56   BoxWeight() {
  57.  57     super();
  58.  58     weight = -1;
  59.  59   }
  60.  60
  61.  61   // constructor used when cube is created
  62.  62   BoxWeight(double len, double m) {
  63.  63     super(len);
  64.  64     weight = m;
  65.  65   }
  66.  66 }
  67.  67
  68.  68 class DemoSuper {
  69.  69   public static void main(String args[]) {
  70.  70     BoxWeight mybox1 = new BoxWeight(10, 20, 15, 34.3);
  71.  71     BoxWeight mybox2 = new BoxWeight(2, 3, 4, 0.076);
  72.  72     BoxWeight mybox3 = new BoxWeight(); // default
  73.  73     BoxWeight mycube = new BoxWeight(3, 2);
  74.  74     BoxWeight myclone = new BoxWeight(mybox1);
  75.  75     double vol;
  76.  76
  77.  77     vol = mybox1.volume();
  78.  78     System.out.println("Volume of mybox1 is " + vol);
  79.  79     System.out.println("Weight of mybox1 is " + mybox1.weight);
  80.  80     System.out.println();
  81.  81
  82.  82     vol = mybox2.volume();
  83.  83     System.out.println("Volume of mybox2 is " + vol);
  84.  84     System.out.println("Weight of mybox2 is " + mybox2.weight);
  85.  85     System.out.println();
  86.  86
  87.  87     vol = mybox3.volume();
  88.  88     System.out.println("Volume of mybox3 is " + vol);
  89.  89     System.out.println("Weight of mybox3 is " + mybox3.weight);
  90.  90     System.out.println();
  91.  91
  92.  92     vol = myclone.volume();
  93.  93     System.out.println("Volume of myclone is " + vol);
  94.  94     System.out.println("Weight of myclone is " + myclone.weight);
  95.  95     System.out.println();
  96.  96
  97.  97     vol = mycube.volume();
  98.  98     System.out.println("Volume of mycube is " + vol);
  99.  99     System.out.println("Weight of mycube is " + mycube.weight);
  100. 100     System.out.println();
  101. 101   }
  102. 102 }
  103. 103 

  1.   1 // construct clone of an object
  2.   2 BoxWeight(BoxWeight ob) { // pass object to constructor
  3.   3   super(ob);
  4.   4   weight = ob.weight;
  5.   5 }
  6.   6

8.2.2 super的另一种用法

  1.   1 // Using super to overcome name hiding.
  2.   2 class A {
  3.   3   int i;
  4.   4 }
  5.   5
  6.   6 // Create a subclass by extending class A.
  7.   7 class B extends A {
  8.   8   int i; // this i hides the i in A
  9.   9
  10.  10   B(int a, int b) {
  11.  11     super.i = a; // i in A
  12.  12     i = b; // i in B
  13.  13   }
  14.  14
  15.  15   void show() {
  16.  16     System.out.println("i in superclass: " + super.i);
  17.  17     System.out.println("i in subclass: " + i);
  18.  18   }
  19.  19 }
  20.  20
  21.  21 class UseSuper {
  22.  22   public static void main(String args[]) {
  23.  23     B subOb = new B(1, 2);
  24.  24
  25.  25     subOb.show();
  26.  26   }
  27.  27 }
  28.  28

8.3 创建多级继承层次

  1.   1 // Extend BoxWeight to include shipping costs.
  2.   2
  3.   3 // Start with Box.
  4.   4 class Box {
  5.   5   private double width;
  6.   6   private double height;
  7.   7   private double depth;
  8.   8
  9.   9   // construct clone of an object
  10.  10   Box(Box ob) { // pass object to constructor
  11.  11     width = ob.width;
  12.  12     height = ob.height;
  13.  13     depth = ob.depth;
  14.  14   }
  15.  15
  16.  16   // constructor used when all dimensions specified
  17.  17   Box(double w, double h, double d) {
  18.  18     width = w;
  19.  19     height = h;
  20.  20     depth = d;
  21.  21   }
  22.  22
  23.  23   // constructor used when no dimensions specified
  24.  24   Box() {
  25.  25     width = -1;  // use -1 to indicate
  26.  26     height = -1; // an uninitialized
  27.  27     depth = -1;  // box
  28.  28   }
  29.  29
  30.  30   // constructor used when cube is created
  31.  31   Box(double len) {
  32.  32     width = height = depth = len;
  33.  33   }
  34.  34
  35.  35   // compute and return volume
  36.  36   double volume() {
  37.  37     return width * height * depth;
  38.  38   }
  39.  39 }
  40.  40
  41.  41 // Add weight.
  42.  42 class BoxWeight extends Box {
  43.  43   double weight; // weight of box
  44.  44
  45.  45   // construct clone of an object
  46.  46   BoxWeight(BoxWeight ob) { // pass object to constructor
  47.  47     super(ob);
  48.  48     weight = ob.weight;
  49.  49   }
  50.  50
  51.  51   // constructor when all parameters are specified
  52.  52   BoxWeight(double w, double h, double d, double m) {
  53.  53     super(w, h, d); // call superclass constructor
  54.  54     weight = m;
  55.  55   }
  56.  56
  57.  57   // default constructor
  58.  58   BoxWeight() {
  59.  59     super();
  60.  60     weight = -1;
  61.  61   }
  62.  62
  63.  63   // constructor used when cube is created
  64.  64   BoxWeight(double len, double m) {
  65.  65     super(len);
  66.  66     weight = m;
  67.  67   }
  68.  68 }
  69.  69
  70.  70 // Add shipping costs
  71.  71 class Shipment extends BoxWeight {
  72.  72   double cost;
  73.  73
  74.  74   // construct clone of an object
  75.  75   Shipment(Shipment ob) { // pass object to constructor
  76.  76     super(ob);
  77.  77     cost = ob.cost;
  78.  78   }
  79.  79
  80.  80   // constructor when all parameters are specified
  81.  81   Shipment(double w, double h, double d,
  82.  82             double m, double c) {
  83.  83     super(w, h, d, m); // call superclass constructor
  84.  84     cost = c;
  85.  85   }
  86.  86
  87.  87   // default constructor
  88.  88   Shipment() {
  89.  89     super();
  90.  90     cost = -1;
  91.  91   }
  92.  92
  93.  93   // constructor used when cube is created
  94.  94   Shipment(double len, double m, double c) {
  95.  95     super(len, m);
  96.  96     cost = c;
  97.  97   }
  98.  98 }
  99.  99
  100. 100 class DemoShipment {
  101. 101   public static void main(String args[]) {
  102. 102     Shipment shipment1 =
  103. 103                new Shipment(10, 20, 15, 10, 3.41);
  104. 104     Shipment shipment2 =
  105. 105                new Shipment(2, 3, 4, 0.76, 1.28);
  106. 106
  107. 107     double vol;
  108. 108
  109. 109     vol = shipment1.volume();
  110. 110     System.out.println("Volume of shipment1 is " + vol);
  111. 111     System.out.println("Weight of shipment1 is "
  112. 112

8.4 构造函数的调用时机

  1.   1 // Demonstrate when constructors are called.
  2.   2
  3.   3 // Create a super class.
  4.   4 class A {
  5.   5   A() {
  6.   6     System.out.println("Inside A's constructor.");
  7.   7   }
  8.   8 }
  9.   9
  10.  10 // Create a subclass by extending class A.
  11.  11 class B extends A {
  12.  12   B() {
  13.  13     System.out.println("Inside B's constructor.");
  14.  14   }
  15.  15 }
  16.  16
  17.  17 // Create another subclass by extending B.
  18.  18 class C extends B {
  19.  19   C() {
  20.  20     System.out.println("Inside C's constructor.");
  21.  21   }
  22.  22 }
  23.  23
  24.  24 class CallingCons {
  25.  25   public static void main(String args[]) {
  26.  26     C c = new C();
  27.  27   }
  28.  28 }
  29.  29

8.5 方法重写

  1.   1 // Method overriding.
  2.   2 class A {
  3.   3   int i, j;
  4.   4
  5.   5   A(int a, int b) {
  6.   6     i = a;
  7.   7     j = b;
  8.   8   }
  9.   9
  10.  10   // display i and j
  11.  11   void show() {
  12.  12     System.out.println("i and j: " + i + " " + j);
  13.  13   }
  14.  14 }
  15.  15
  16.  16 class B extends A {
  17.  17   int k;
  18.  18
  19.  19   B(int a, int b, int c) {
  20.  20     super(a, b);
  21.  21     k = c;
  22.  22   }
  23.  23
  24.  24   // display k -- this overrides show() in A
  25.  25   void show() {
  26.  26     System.out.println("k: " + k);
  27.  27   }
  28.  28 }
  29.  29
  30.  30 class Override {
  31.  31   public static void main(String args[]) {
  32.  32     B subOb = new B(1, 2, 3);
  33.  33
  34.  34     subOb.show(); // this calls show() in B
  35.  35   }
  36.  36 }
  37.  37 

  1.   1 class B extends A {
  2.   2   int k;
  3.   3
  4.   4   B(int a, int b, int c) {
  5.   5     super(a, b);
  6.   6     k = c;
  7.   7   }
  8.   8
  9.   9   void show() {
  10.  10     super.show(); // this calls A's show()
  11.  11     System.out.println("k: " + k);
  12.  12   }
  13.  13 }
  14.  14 

  1.   1 // Methods with differing type signatures are overloaded -- not overridden.
  2.   2 class A {
  3.   3   int i, j;
  4.   4
  5.   5   A(int a, int b) {
  6.   6     i = a;
  7.   7     j = b;
  8.   8   }
  9.   9
  10.  10   // display i and j
  11.  11   void show() {
  12.  12     System.out.println("i and j: " + i + " " + j);
  13.  13   }
  14.  14 }
  15.  15
  16.  16 // Create a subclass by extending class A.
  17.  17 class B extends A {
  18.  18   int k;
  19.  19
  20.  20   B(int a, int b, int c) {
  21.  21     super(a, b);
  22.  22     k = c;
  23.  23   }
  24.  24
  25.  25   // overload show()
  26.  26   void show(String msg) {
  27.  27     System.out.println(msg + k);
  28.  28   }
  29.  29 }
  30.  30
  31.  31 class Override {
  32.  32   public static void main(String args[]) {
  33.  33     B subOb = new B(1, 2, 3);
  34.  34
  35.  35     subOb.show("This is k: "); // this calls show() in B
  36.  36     subOb.show(); // this calls show() in A
  37.  37   }
  38.  38 }
  39.  39

8.6 动态方法调度

  1.   1 // Dynamic Method Dispatch
  2.   2 class A {
  3.   3    void callme() {
  4.   4      System.out.println("Inside A's callme method");
  5.   5   }
  6.   6 }
  7.   7
  8.   8 class B extends A {
  9.   9   // override callme()
  10.  10   void callme() {
  11.  11     System.out.println("Inside B's callme method");
  12.  12   }
  13.  13 }
  14.  14
  15.  15 class C extends A {
  16.  16   // override callme()
  17.  17   void callme() {
  18.  18     System.out.println("Inside C's callme method");
  19.  19   }
  20.  20 }
  21.  21
  22.  22 class Dispatch {
  23.  23   public static void main(String args[]) {
  24.  24     A a = new A(); // object of type A
  25.  25     B b = new B(); // object of type B
  26.  26     C c = new C(); // object of type C
  27.  27     A r; // obtain a reference of type A
  28.  28
  29.  29     r = a; // r refers to an A object
  30.  30     r.callme(); // calls A's version of callme
  31.  31
  32.  32     r = b; // r refers to a B object
  33.  33     r.callme(); // calls B's version of callme
  34.  34
  35.  35     r = c; // r refers to a C object
  36.  36     r.callme(); // calls C's version of callme
  37.  37   }
  38.  38 }
  39.  39

8.6.1 重写方法的目的

8.6.2 应用方法重写

  1.   1 // Using run-time polymorphism.
  2.   2 class Figure {
  3.   3   double dim1;
  4.   4   double dim2;
  5.   5
  6.   6   Figure(double a, double b) {
  7.   7     dim1 = a;
  8.   8     dim2 = b;
  9.   9   }
  10.  10
  11.  11   double area() {
  12.  12     System.out.println("Area for Figure is undefined.");
  13.  13     return 0;
  14.  14   }
  15.  15 }
  16.  16
  17.  17 class Rectangle extends Figure {
  18.  18   Rectangle(double a, double b) {
  19.  19     super(a, b);
  20.  20   }
  21.  21
  22.  22   // override area for rectangle
  23.  23   double area() {
  24.  24     System.out.println("Inside Area for Rectangle.");
  25.  25     return dim1 * dim2;
  26.  26   }
  27.  27 }
  28.  28
  29.  29 class Triangle extends Figure {
  30.  30   Triangle(double a, double b) {
  31.  31     super(a, b);
  32.  32   }
  33.  33
  34.  34   // override area for right triangle
  35.  35   double area() {
  36.  36     System.out.println("Inside Area for Triangle.");
  37.  37     return dim1 * dim2 / 2;
  38.  38   }
  39.  39 }
  40.  40
  41.  41 class FindAreas {
  42.  42   public static void main(String args[]) {
  43.  43     Figure f = new Figure(10, 10);
  44.  44     Rectangle r = new Rectangle(9, 5);
  45.  45     Triangle t = new Triangle(10, 8);
  46.  46
  47.  47     Figure figref;
  48.  48
  49.  49     figref = r;
  50.  50     System.out.println("Area is " + figref.area());
  51.  51
  52.  52     figref = t;
  53.  53     System.out.println("Area is " + figref.area());
  54.  54
  55.  55     figref = f;
  56.  56     System.out.println("Area is " + figref.area());
  57.  57   }
  58.  58 }
  59.  59

8.7 使用抽象类

  1.   1 // A Simple demonstration of abstract.
  2.   2 abstract class A {
  3.   3   abstract void callme();
  4.   4
  5.   5   // concrete methods are still allowed in abstract classes
  6.   6   void callmetoo() {
  7.   7     System.out.println("This is a concrete method.");
  8.   8   }
  9.   9 }
  10.  10
  11.  11 class B extends A {
  12.  12   void callme() {
  13.  13     System.out.println("B's implementation of callme.");
  14.  14   }
  15.  15 }
  16.  16
  17.  17 class AbstractDemo {
  18.  18   public static void main(String args[]) {
  19.  19     B b = new B();
  20.  20
  21.  21     b.callme();
  22.  22     b.callmetoo();
  23.  23   }
  24.  24 }

  1.   1 // Using abstract methods and classes.
  2.   2 abstract class Figure {
  3.   3   double dim1;
  4.   4   double dim2;
  5.   5
  6.   6   Figure(double a, double b) {
  7.   7     dim1 = a;
  8.   8     dim2 = b;
  9.   9   }
  10.  10
  11.  11   // area is now an an abstract method
  12.  12   abstract double area();
  13.  13 }
  14.  14
  15.  15 class Rectangle extends Figure {
  16.  16   Rectangle(double a, double b) {
  17.  17     super(a, b);
  18.  18   }
  19.  19
  20.  20   // override area for rectangle
  21.  21   double area() {
  22.  22     System.out.println("Inside Area for Rectangle.");
  23.  23     return dim1 * dim2;
  24.  24   }
  25.  25 }
  26.  26
  27.  27 class Triangle extends Figure {
  28.  28   Triangle(double a, double b) {
  29.  29     super(a, b);
  30.  30   }
  31.  31
  32.  32   // override area for right triangle
  33.  33   double area() {
  34.  34     System.out.println("Inside Area for Triangle.");
  35.  35     return dim1 * dim2 / 2;
  36.  36   }
  37.  37 }
  38.  38
  39.  39 class AbstractAreas {
  40.  40   public static void main(String args[]) {
  41.  41   // Figure f = new Figure(10, 10); // illegal now
  42.  42     Rectangle r = new Rectangle(9, 5);
  43.  43     Triangle t = new Triangle(10, 8);
  44.  44
  45.  45     Figure figref; // this is OK, no object is created
  46.  46
  47.  47     figref = r;
  48.  48     System.out.println("Area is " + figref.area());
  49.  49
  50.  50     figref = t;
  51.  51     System.out.println("Area is " + figref.area());
  52.  52   }
  53.  53 }

8.8 在继承中使用final关键字

8.8.1 使用final关键字阻止重写

  1.   1 class A {
  2.   2   final void meth() {
  3.   3     System.out.println("This is a final method.");
  4.   4   }
  5.   5 }
  6.   6
  7.   7 class B extends A {
  8.   8   void meth() { // ERROR! Can't override.
  9.   9     System.out.println("Illegal!");
  10.  10   }
  11.  11 }
  12.  12

8.8.2 使用final关键字阻止继承

  1.   1 final class A {
  2.   2   // ...
  3.   3 }
  4.   4
  5.   5 // The following class is illegal.
  6.   6 class B extends A { // ERROR! Can't subclass A
  7.   7   // ...
  8.   8 }

8.9 Object类

第9章 包和接口

9.1 包

9.1.1 定义包

9.1.2 包查找与CLASSPATH

9.1.3 一个简短的包示例

  1.   1 // A simple package
  2.   2 package MyPack;
  3.   3
  4.   4 class Balance {
  5.   5   String name;
  6.   6   double bal;
  7.   7
  8.   8   Balance(String n, double b) {
  9.   9     name = n;
  10.  10     bal = b;
  11.  11   }
  12.  12
  13.  13   void show() {
  14.  14     if(bal<0)
  15.  15       System.out.print("-->> ");
  16.  16     System.out.println(name + ": $" + bal);
  17.  17   }
  18.  18 }
  19.  19
  20.  20 class AccountBalance {
  21.  21   public static void main(String args[]) {
  22.  22     Balance current[] = new Balance[3];
  23.  23
  24.  24     current[0] = new Balance("K. J. Fielding", 123.23);
  25.  25     current[1] = new Balance("Will Tell", 157.02);
  26.  26     current[2] = new Balance("Tom Jackson", -12.33);
  27.  27
  28.  28     for(int i=0; i<3; i++) current[i].show();
  29.  29   }
  30.  30 }

9.2 访问保护

  1.   1 package p1;
  2.   2
  3.   3 public class Protection {
  4.   4   int n = 1;
  5.   5   private int n_pri = 2;
  6.   6   protected int n_pro = 3;
  7.   7   public int n_pub = 4;
  8.   8
  9.   9   public Protection() {
  10.  10     System.out.println("base constructor");
  11.  11     System.out.println("n = " + n);
  12.  12     System.out.println("n_pri = " + n_pri);
  13.  13     System.out.println("n_pro = " + n_pro);
  14.  14     System.out.println("n_pub = " + n_pub);
  15.  15   }
  16.  16 }
  17.  17
  18.  18 class Derived extends Protection {
  19.  19   Derived() {
  20.  20     System.out.println("derived constructor");
  21.  21     System.out.println("n = " + n);
  22.  22
  23.  23 //  class only
  24.  24 //  System.out.println("n_pri = " + n_pri);
  25.  25
  26.  26     System.out.println("n_pro = " + n_pro);
  27.  27     System.out.println("n_pub = " + n_pub);
  28.  28   }
  29.  29 }
  30.  30
  31.  31 class SamePackage {
  32.  32   SamePackage() {
  33.  33     Protection p = new Protection();
  34.  34     System.out.println("same package constructor");
  35.  35     System.out.println("n = " + p.n);
  36.  36
  37.  37 //  class only
  38.  38 //  System.out.println("n_pri = " + p.n_pri);
  39.  39
  40.  40     System.out.println("n_pro = " + p.n_pro);
  41.  41     System.out.println("n_pub = " + p.n_pub);
  42.  42   }
  43.  43 }
  44.  44 

  1.   1 package p2;
  2.   2
  3.   3 class Protection2 extends p1.Protection {
  4.   4   Protection2() {
  5.   5     System.out.println("derived other package constructor");
  6.   6
  7.   7 //  class or package only
  8.   8 //  System.out.println("n = " + n);
  9.   9
  10.  10 //  class only
  11.  11 //  System.out.println("n_pri = " + n_pri);
  12.  12
  13.  13     System.out.println("n_pro = " + n_pro);
  14.  14     System.out.println("n_pub = " + n_pub);
  15.  15   }
  16.  16 }
  17.  17
  18.  18 class OtherPackage {
  19.  19   OtherPackage() {
  20.  20     p1.Protection p = new p1.Protection();
  21.  21     System.out.println("other package constructor");
  22.  22
  23.  23 //  class or package only
  24.  24 //  System.out.println("n = " + p.n);
  25.  25
  26.  26 //  class only
  27.  27 //  System.out.println("n_pri = " + p.n_pri);
  28.  28
  29.  29 //  class, subclass or package only
  30.  30 //  System.out.println("n_pro = " + p.n_pro);
  31.  31
  32.  32     System.out.println("n_pub = " + p.n_pub);
  33.  33   }
  34.  34 }

  1.   1 // Demo package p1.
  2.   2 package p1;
  3.   3
  4.   4 // Instantiate the various classes in p1.
  5.   5 public class Demo {
  6.   6   public static void main(String args[]) {
  7.   7     Protection ob1 = new Protection();
  8.   8     Derived ob2 = new Derived();
  9.   9     SamePackage ob3 = new SamePackage();
  10.  10   }
  11.  11 }
  12.  12 

  1.   1 // Demo package p2.
  2.   2 package p2;
  3.   3
  4.   4 // Instantiate the various classes in p2.
  5.   5 public class Demo {
  6.   6   public static void main(String args[]) {
  7.   7     Protection2 ob1 = new Protection2();
  8.   8     OtherPackage ob2 = new OtherPackage();
  9.   9   }
  10.  10 }
  11.  11

9.3 导入包

  1.   1 package JavaBase_ReferenceNanual.Chap9.listing8.MyPack;
  2.   2
  3.   3 /**
  4.   4  * 项目名称：JavaEE_Base
  5.   5  * 类名称：Balance
  6.   6  * 类描述： Now, the Balance class, its constructor,
  7.   7  * and its show() method are public. This means that they can be used by
  8.   8  * non-subclass code outside their package.
  9.   9  * 创建人： shall_liu(1136808529@qq.com)
  10.  10  * 创建时间：2017年9月21日 上午12:01:36
  11.  11  * 修改人：amin
  12.  12  * 修改时间：2017年9月21日 上午12:01:36
  13.  13  * 修改备注：
  14.  14  * @version
  15.  15  */
  16.  16 public class Balance {
  17.  17 	String name;
  18.  18 	double bal;
  19.  19
  20.  20 	public Balance(String n, double b) {
  21.  21 		name = n;
  22.  22 		bal = b;
  23.  23 	}
  24.  24
  25.  25 	public void show() {
  26.  26 		if (bal < 0)
  27.  27 			System.out.print("-->> ");
  28.  28 		System.out.println(name + ": $" + bal);
  29.  29 	}
  30.  30 }

  1.   1 package JavaBase_ReferenceNanual.Chap9.listing9.MyPack;
  2.   2
  3.   3 import JavaBase_ReferenceNanual.Chap9.listing8.MyPack.*;
  4.   4 /**
  5.   5  *
  6.   6 *
  7.   7 * 项目名称：JavaEE_Base
  8.   8 * 类名称：TestBalance
  9.   9 * 类描述：
  10.  10 * 创建人：  shall_liu(1136808529@qq.com)
  11.  11 * 创建时间：2017年9月21日 上午12:06:30
  12.  12 * 修改人：amin
  13.  13 * 修改时间：2017年9月21日 上午12:06:30
  14.  14 * 修改备注：
  15.  15 * @version
  16.  16 *
  17.  17  */
  18.  18 class TestBalance {
  19.  19   public static void main(String args[]) {
  20.  20
  21.  21     /* Because Balance is public, you may use Balance
  22.  22        class and call its constructor. */
  23.  23     Balance test = new Balance("J. J. Jaspers", 99.88);
  24.  24
  25.  25     test.show(); // you may also call show()
  26.  26   }
  27.  27 }
  28.  28

9.4 接口

9.4.1 定义接口

9.4.2 实现接口

  1.   1 package JavaBase_ReferenceNanual.Chap9.listing9.MyPack;
  2.   2
  3.   3 import javax.security.auth.callback.Callback;
  4.   4
  5.   5 class Client implements Callback {
  6.   6 	// Implement Callback's interface
  7.   7 	public void callback(int p) {
  8.   8 		System.out.println("callback called with " + p);
  9.   9 	}
  10.  10
  11.  11 	void nonIfaceMeth() {
  12.  12 		System.out.println("Classes that implement interfaces " + "may also define other members, too.");
  13.  13 	}
  14.  14 }
  15.  15
  16.  16 package JavaBase_ReferenceNanual.Chap9.listing9.MyPack;
  17.  17
  18.  18 import javax.security.auth.callback.Callback;
  19.  19 /**
  20.  20  *
  21.  21 *
  22.  22 * 项目名称：JavaEE_Base
  23.  23 * 类名称：TestIface
  24.  24 * 类描述：
  25.  25 * 创建人：  shall_liu(1136808529@qq.com)
  26.  26 * 创建时间：2017年9月21日 上午1:00:06
  27.  27 * 修改人：amin
  28.  28 * 修改时间：2017年9月21日 上午1:00:06
  29.  29 * 修改备注：
  30.  30 * @version
  31.  31 *
  32.  32  */
  33.  33 class TestIface {
  34.  34 	  public static void main(String args[]) {
  35.  35 	    Callback c = new Client();
  36.  36 	    ((Client) c).callback(42);
  37.  37 	  }
  38.  38 	}
  39.  39 //运行结果为
  40.  40 callback called with 42
  41.  41 

  1.   1 package JavaBase_ReferenceNanual.Chap9.listing9.MyPack;
  2.   2
  3.   3 import javax.security.auth.callback.Callback;
  4.   4
  5.   5 /**
  6.   6  *
  7.   7 *
  8.   8 * 项目名称：JavaEE_Base
  9.   9 * 类名称：AnotherClient
  10.  10 * 类描述：  Another implementation of Callback.
  11.  11 * 创建人：  shall_liu(1136808529@qq.com)
  12.  12 * 创建时间：2017年9月21日 上午1:03:26
  13.  13 * 修改人：amin
  14.  14 * 修改时间：2017年9月21日 上午1:03:26
  15.  15 * 修改备注：
  16.  16 * @version
  17.  17 *
  18.  18  */
  19.  19 class AnotherClient implements Callback {
  20.  20 	// Implement Callback's interface
  21.  21 	public void callback(int p) {
  22.  22 		System.out.println("Another version of callback");
  23.  23 		System.out.println("p squared is " + (p * p));
  24.  24 	}
  25.  25 }
  26.  26
  27.  27
  28.  28 package JavaBase_ReferenceNanual.Chap9.listing9.MyPack;
  29.  29
  30.  30 import javax.security.auth.callback.Callback;
  31.  31
  32.  32 class Client implements Callback {
  33.  33 	// Implement Callback's interface
  34.  34 	public void callback(int p) {
  35.  35 		System.out.println("callback called with " + p);
  36.  36 	}
  37.  37
  38.  38 	void nonIfaceMeth() {
  39.  39 		System.out.println("Classes that implement interfaces " + "may also define other members, too.");
  40.  40 	}
  41.  41 }
  42.  42
  43.  43 package JavaBase_ReferenceNanual.Chap9.listing9.MyPack;
  44.  44
  45.  45 import javax.security.auth.callback.Callback;
  46.  46
  47.  47 /**
  48.  48  *
  49.  49 *
  50.  50 * 项目名称：JavaEE_Base
  51.  51 * 类名称：TestIface2
  52.  52 * 类描述：
  53.  53 * 创建人：  shall_liu(1136808529@qq.com)
  54.  54 * 创建时间：2017年9月21日 上午1:04:49
  55.  55 * 修改人：amin
  56.  56 * 修改时间：2017年9月21日 上午1:04:49
  57.  57 * 修改备注：
  58.  58 * @version
  59.  59 *
  60.  60  */
  61.  61
  62.  62 class TestIface2 {
  63.  63 	public static void main(String args[]) {
  64.  64 		Callback c = new Client();
  65.  65 		AnotherClient ob = new AnotherClient();
  66.  66
  67.  67 		((Client) c).callback(42);
  68.  68
  69.  69 		c = ob; // c now refers to AnotherClient object
  70.  70 		((Client) c).callback(42);
  71.  71 	}
  72.  72 }
  73.  73

9.4.3 嵌套接口

  1.   1 // A nested interface example.
  2.   2
  3.   3 // This class contains a member interface.
  4.   4 class A {
  5.   5   // this is a nested interface
  6.   6   public interface NestedIF {
  7.   7     boolean isNotNegative(int x);
  8.   8   }
  9.   9 }
  10.  10
  11.  11 // B implements the nested interface.
  12.  12 class B implements A.NestedIF {
  13.  13   public boolean isNotNegative(int x) {
  14.  14     return x < 0 ? false : true;
  15.  15   }
  16.  16 }
  17.  17
  18.  18 class NestedIFDemo {
  19.  19   public static void main(String args[]) {
  20.  20
  21.  21     // use a nested interface reference
  22.  22     A.NestedIF nif = new B();
  23.  23
  24.  24     if(nif.isNotNegative(10))
  25.  25       System.out.println("10 is not negative");
  26.  26     if(nif.isNotNegative(-12))
  27.  27       System.out.println("this won't be displayed");
  28.  28   }
  29.  29 }
  30.  30

9.4.4 应用接口

  1.   1 // Define an integer stack interface.
  2.   2 interface IntStack {
  3.   3   void push(int item); // store an item
  4.   4   int pop(); // retrieve an item
  5.   5 }
  6.   6
  7.   7 // An implementation of IntStack that uses fixed storage.
  8.   8 class FixedStack implements IntStack {
  9.   9   private int stck[];
  10.  10   private int tos;
  11.  11
  12.  12   // allocate and initialize stack
  13.  13   FixedStack(int size) {
  14.  14     stck = new int[size];
  15.  15     tos = -1;
  16.  16   }
  17.  17
  18.  18   // Push an item onto the stack
  19.  19   public void push(int item) {
  20.  20     if(tos==stck.length-1) // use length member
  21.  21       System.out.println("Stack is full.");
  22.  22     else
  23.  23       stck[++tos] = item;
  24.  24   }
  25.  25
  26.  26   // Pop an item from the stack
  27.  27   public int pop() {
  28.  28     if(tos < 0) {
  29.  29       System.out.println("Stack underflow.");
  30.  30       return 0;
  31.  31     }
  32.  32     else
  33.  33       return stck[tos--];
  34.  34   }
  35.  35 }
  36.  36
  37.  37 class IFTest {
  38.  38   public static void main(String args[]) {
  39.  39     FixedStack mystack1 = new FixedStack(5);
  40.  40     FixedStack mystack2 = new FixedStack(8);
  41.  41
  42.  42     // push some numbers onto the stack
  43.  43     for(int i=0; i<5; i++) mystack1.push(i);
  44.  44     for(int i=0; i<8; i++) mystack2.push(i);
  45.  45
  46.  46     // pop those numbers off the stack
  47.  47     System.out.println("Stack in mystack1:");
  48.  48     for(int i=0; i<5; i++)
  49.  49        System.out.println(mystack1.pop());
  50.  50
  51.  51     System.out.println("Stack in mystack2:");
  52.  52     for(int i=0; i<8; i++)
  53.  53        System.out.println(mystack2.pop());
  54.  54   }
  55.  55 }

  1.   1 // Define an integer stack interface.
  2.   2 interface IntStack {
  3.   3   void push(int item); // store an item
  4.   4   int pop(); // retrieve an item
  5.   5 }
  6.   6
  7.   7 // Implement a "growable" stack.
  8.   8 class DynStack implements IntStack {
  9.   9   private int stck[];
  10.  10   private int tos;
  11.  11
  12.  12   // allocate and initialize stack
  13.  13   DynStack(int size) {
  14.  14     stck = new int[size];
  15.  15     tos = -1;
  16.  16   }
  17.  17
  18.  18   // Push an item onto the stack
  19.  19   public void push(int item) {
  20.  20     // if stack is full, allocate a larger stack
  21.  21     if(tos==stck.length-1) {
  22.  22       int temp[] = new int[stck.length * 2]; // double size
  23.  23       for(int i=0; i<stck.length; i++) temp[i] = stck[i];
  24.  24       stck = temp;
  25.  25       stck[++tos] = item;
  26.  26     }
  27.  27     else
  28.  28       stck[++tos] = item;
  29.  29   }
  30.  30
  31.  31   // Pop an item from the stack
  32.  32   public int pop() {
  33.  33     if(tos < 0) {
  34.  34       System.out.println("Stack underflow.");
  35.  35       return 0;
  36.  36     }
  37.  37     else
  38.  38       return stck[tos--];
  39.  39   }
  40.  40 }
  41.  41
  42.  42 class IFTest2 {
  43.  43   public static void main(String args[]) {
  44.  44     DynStack mystack1 = new DynStack(5);
  45.  45     DynStack mystack2 = new DynStack(8);
  46.  46
  47.  47     // these loops cause each stack to grow
  48.  48     for(int i=0; i<12; i++) mystack1.push(i);
  49.  49     for(int i=0; i<20; i++) mystack2.push(i);
  50.  50
  51.  51     System.out.println("Stack in mystack1:");
  52.  52     for(int i=0; i<12; i++)
  53.  53        System.out.println(mystack1.pop());
  54.  54
  55.  55     System.out.println("Stack in mystack2:");
  56.  56     for(int i=0; i<20; i++)
  57.  57        System.out.println(mystack2.pop());
  58.  58   }
  59.  59 }
  60.  60 

  1.   1 /* Create an interface variable and
  2.   2    access stacks through it.
  3.   3 */
  4.   4 class IFTest3 {
  5.   5   public static void main(String args[]) {
  6.   6     IntStack mystack; // create an interface reference variable
  7.   7     DynStack ds = new DynStack(5);
  8.   8     FixedStack fs = new FixedStack(8);
  9.   9
  10.  10     mystack = ds; // load dynamic stack
  11.  11     // push some numbers onto the stack
  12.  12     for(int i=0; i<12; i++) mystack.push(i);
  13.  13
  14.  14     mystack = fs; // load fixed stack
  15.  15     for(int i=0; i<8; i++) mystack.push(i);
  16.  16
  17.  17
  18.  18     mystack = ds;
  19.  19     System.out.println("Values in dynamic stack:");
  20.  20     for(int i=0; i<12; i++)
  21.  21        System.out.println(mystack.pop());
  22.  22
  23.  23     mystack = fs;
  24.  24     System.out.println("Values in fixed stack:");
  25.  25     for(int i=0; i<8; i++)
  26.  26        System.out.println(mystack.pop());
  27.  27   }
  28.  28 }
  29.  29

9.4.5 接口中的变量

  1.   1 import java.util.Random;
  2.   2
  3.   3 interface SharedConstants {
  4.   4   int NO = 0;
  5.   5   int YES = 1;
  6.   6   int MAYBE = 2;
  7.   7   int LATER = 3;
  8.   8   int SOON = 4;
  9.   9   int NEVER = 5;
  10.  10 }
  11.  11
  12.  12 class Question implements SharedConstants {
  13.  13   Random rand = new Random();
  14.  14   int ask() {
  15.  15     int prob = (int) (100 * rand.nextDouble());
  16.  16     if (prob < 30)
  17.  17       return NO;           // 30%
  18.  18     else if (prob < 60)
  19.  19       return YES;          // 30%
  20.  20     else if (prob < 75)
  21.  21       return LATER;        // 15%
  22.  22     else if (prob < 98)
  23.  23       return SOON;         // 13%
  24.  24     else
  25.  25       return NEVER;        // 2%
  26.  26   }
  27.  27 }
  28.  28
  29.  29 class AskMe implements SharedConstants {
  30.  30   static void answer(int result) {
  31.  31     switch(result) {
  32.  32       case NO:
  33.  33         System.out.println("No");
  34.  34         break;
  35.  35       case YES:
  36.  36         System.out.println("Yes");
  37.  37         break;
  38.  38       case MAYBE:
  39.  39         System.out.println("Maybe");
  40.  40         break;
  41.  41       case LATER:
  42.  42         System.out.println("Later");
  43.  43         break;
  44.  44       case SOON:
  45.  45         System.out.println("Soon");
  46.  46         break;
  47.  47       case NEVER:
  48.  48         System.out.println("Never");
  49.  49         break;
  50.  50     }
  51.  51   }
  52.  52
  53.  53   public static void main(String args[]) {
  54.  54     Question q = new Question();
  55.  55     answer(q.ask());
  56.  56     answer(q.ask());
  57.  57     answer(q.ask());
  58.  58     answer(q.ask());
  59.  59   }
  60.  60 }

9.4.6 接口可以扩展

  1.   1 // One interface an extend another.
  2.   2 interface A {
  3.   3   void meth1();
  4.   4   void meth2();
  5.   5 }
  6.   6
  7.   7 // B now includes meth1() and meth2() -- it adds meth3().
  8.   8 interface B extends A {
  9.   9   void meth3();
  10.  10 }
  11.  11
  12.  12 // This class must implement all of A and B
  13.  13 class MyClass implements B {
  14.  14   public void meth1() {
  15.  15     System.out.println("Implement meth1().");
  16.  16   }
  17.  17
  18.  18   public void meth2() {
  19.  19     System.out.println("Implement meth2().");
  20.  20   }
  21.  21
  22.  22   public void meth3() {
  23.  23     System.out.println("Implement meth3().");
  24.  24   }
  25.  25 }
  26.  26
  27.  27 class IFExtend {
  28.  28   public static void main(String arg[]) {
  29.  29     MyClass ob = new MyClass();
  30.  30
  31.  31     ob.meth1();
  32.  32     ob.meth2();
  33.  33     ob.meth3();
  34.  34   }
  35.  35 }

9.5 默认接口方法

9.5.1 默认方法的基础知识

  1.   1 public interface MyIF {
  2.   2   // This is a "normal" interface method declaration.
  3.   3   // It does NOT define a default implementation.
  4.   4   int getNumber();
  5.   5
  6.   6   // This is a default method. Notice that it provides
  7.   7   // a default implementation.
  8.   8   default String getString() {
  9.   9     return "Default String";
  10.  10   }
  11.  11 }
  12.  12 

  1.   1 // Implement MyIF.
  2.   2 class MyIFImp implements MyIF {
  3.   3   // Only getNumber() defined by MyIF needs to be implemented.
  4.   4   // getString() can be allowed to default.
  5.   5   public int getNumber() {
  6.   6     return 100;
  7.   7   }
  8.   8 }
  9.   9 

  1.   1 // Use the default method.
  2.   2 class DefaultMethodDemo {
  3.   3   public static void main(String args[]) {
  4.   4
  5.   5     MyIFImp obj = new MyIFImp();
  6.   6
  7.   7     // Can call getNumber(), because it is explicitly
  8.   8     // implemented by MyIFImp:
  9.   9     System.out.println(obj.getNumber());
  10.  10
  11.  11     // Can also call getString(), because of default
  12.  12     // implementation:
  13.  13     System.out.println(obj.getString());
  14.  14   }
  15.  15 }
  16.  16 

  1.   1 class MyIFImp2 implements MyIF {
  2.   2   // Here, implementations for both getNumber( ) and getString( ) are provided.
  3.   3   public int getNumber() {
  4.   4     return 100;
  5.   5   }
  6.   6
  7.   7   public String getString() {
  8.   8     return "This is a different string.";
  9.   9   }
  10.  10 }

9.5.2 一个更加实用的例子

  1.   1 interface IntStack {
  2.   2   void push(int item); // store an item
  3.   3   int pop(); // retrieve an item
  4.   4
  5.   5   // Because clear( ) has a default, it need not be
  6.   6   // implemented by a preexisting class that uses IntStack.
  7.   7   default void clear() {
  8.   8     System.out.println("clear() not implemented.");
  9.   9   }
  10.  10 }

9.5.3 多级继承的问题

9.6 在接口中使用静态方法

  1.   1 public interface MyIF {
  2.   2   // This is a "normal" interface method declaration.
  3.   3   // It does NOT define a default implementation.
  4.   4   int getNumber();
  5.   5
  6.   6   // This is a default method. Notice that it provides
  7.   7   // a default implementation.
  8.   8   default String getString() {
  9.   9     return "Default String";
  10.  10   }
  11.  11
  12.  12   // This is a static interface method.
  13.  13   static int getDefaultNumber() {
  14.  14     return 0;
  15.  15   }
  16.  16 }

9.7 关于包和接口的最后说明

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