【Java基础】枚举和注解

在Java1.5版本中，引入了两个类型：枚举类型enum type和注解类型annotation type。

Num1：用enum代替int常量

枚举类型enum type是指由一组固定的常量组成合法值的类型。比如：

public enum Planet {
	MERCURY(3.302e+23, 2.439e6), VENUS(4.869e+24, 6.052e6), EARTH(5.975e+24,
			6.378e6), MARS(6.419e+23, 3.393e6), JUPITER(1.899e+27, 7.149e7), SATURN(
			5.685e+26, 6.027e7), URANUS(8.683e+25, 2.556e7), NEPTUNE(1.024e+26,
			2.477e7);
	private final double mass; // In kilograms
	private final double radius; // In meters
	private final double surfaceGravity; // In m / s^2

	// Universal gravitational constant in m^3 / kg s^2
	private static final double G = 6.67300E-11;

	// Constructor
	Planet(double mass, double radius) {
		this.mass = mass;
		this.radius = radius;
		surfaceGravity = G * mass / (radius * radius);
	}

	public double mass() {
		return mass;
	}

	public double radius() {
		return radius;
	}

	public double surfaceGravity() {
		return surfaceGravity;
	}

	public double surfaceWeight(double mass) {
		return mass * surfaceGravity; // F = ma
	}
}

public class WeightTable {
	public static void main(String[] args) {
		double earthWeight = Double.parseDouble(args[0]);
		double mass = earthWeight / Planet.EARTH.surfaceGravity();
		for (Planet p : Planet.values())
			System.out.printf("Weight on %s is %f%n", p, p.surfaceWeight(mass));
	}
}

Num2：用EnumSet代替位域

示例代码：

public class Text {
	public enum Style {
		BOLD, ITALIC, UNDERLINE, STRIKETHROUGH
	}

	// Any Set could be passed in, but EnumSet is clearly best
	public void applyStyles(Set<Style> styles) {
		// Body goes here
	}

	// Sample use
	public static void main(String[] args) {
		Text text = new Text();
		text.applyStyles(EnumSet.of(Style.BOLD, Style.ITALIC));
	}
}

总而言之，正是因为枚举类型要用在集合Set中，所以没有理由用位域来表示它。

Num3：注解优先于命名模式

命名模式有两个缺点：

• 文字拼写错误会导致失败，且没有任何提示。
• 无法确保它们只用于相应的程序元素上。
• 它们没有提供将参数值与程序元素关联起来的好方法。

针对以上几个问题，注解很好地解决了所有这些问题。

/**
 * Indicates that the annotated method is a test method. Use only on
 * parameterless static methods.
 */
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface Test {
}

Test注解类型的声明就是它自身通过Retention和Target注解进行了注解。注解类型声明中的这张注解被称作”元注解“meta-annotaition。

那么声明的Test注解，则称作为”标记注解”marker annotation。因为它没有参数，只是“标注”被注解的元素。

示例代码：

public class Sample {
	@Test
	public static void m1() {
	} // Test should pass

	public static void m2() {
	}

	@Test
	public static void m3() { // Test Should fail
		throw new RuntimeException("Boom");
	}

	public static void m4() {
	}

	@Test
	public void m5() {
	} // INVALID USE: nonstatic method

	public static void m6() {
	}

	@Test
	public static void m7() { // Test should fail
		throw new RuntimeException("Crash");
	}

	public static void m8() {
	}
}

另外一种注解方式：

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface ExceptionTest {
	Class<? extends Exception>[] value();
}

使用方式

public class Sample2 {
	@ExceptionTest(ArithmeticException.class)
	public static void m1() { // Test should pass
		int i = 0;
		i = i / i;
	}

	@ExceptionTest(ArithmeticException.class)
	public static void m2() { // Should fail (wrong exception)
		int[] a = new int[0];
		int i = a[1];
	}

	@ExceptionTest(ArithmeticException.class)
	public static void m3() {
	} // Should fail (no exception)

	// Code containing an annotation with an array parameter - Page 174
	@ExceptionTest({ IndexOutOfBoundsException.class,
			NullPointerException.class })
	public static void doublyBad() {
		List<String> list = new ArrayList<String>();

		// The spec permits this method to throw either
		// IndexOutOfBoundsException or NullPointerException
		list.addAll(5, null);
	}
}