JDK源码分析：Integer.java部分源码解析

1）声明部：

public final class Integer extends Number implements Comparable<Integer>

　　extends Number, 重写方法：

public byte byteValue() {
    return (byte)value;
}
public short shortValue() {
    return (short)value;
}
public int intValue() {
    return value;
}
public long longValue() {
    return (long)value;
}
public float floatValue() {
    return (float)value;
}
public double doubleValue() {
    return (double)value;
}

　　implements Comparable<T>，接口实现如下：

public int compareTo(Integer anotherInteger) {
    return compare(this.value, anotherInteger.value);
}
public static int compare(int x, int y) {
    return (x < y) ? -1 : ((x == y) ? 0 : 1);
}

　　与实现该接口无关观察compareUnsigned方法：

　　compareUnsigned 例子：

Integer a1 = 0b00000000000000000000000000000001;
Integer a2 = 0b00000000000000000000000000000011;
Integer a3 = 0b10000000000000000000000000000001;
Integer a4 = 0b10000000000000000000000000000011;
int r1 = Integer.compareUnsigned(a1.intValue(),a2.intValue());
int r2 = Integer.compareUnsigned(a1.intValue(),a3.intValue());
int r3 = Integer.compareUnsigned(a3.intValue(),a4.intValue());
Out.println("a1,a2,a3,a4分别为：" + a1 +"," + a2 + "," + a3 + "," + a4);
Out.println("r1,r2,r3分别为：" + r1 +"," + r2 + "," + r3);

　　result：

a1,a2,a3,a4分别为：1,3,-2147483647,-2147483645
r1,r2,r3分别为：-1,-1,-1

　　算法分析：比较是无符号比较方法，而默认是有符号整型，所以需要特殊方法处理。先看Integer的范围：
　　0B10000000000000000000000000000000
　　0B10000000000000000000000000000001
　　..
　　0B11111111111111111111111111111111
　　...
　　0B00000000000000000000000000000000
　　0B00000000000000000000000000000001
　　0B00000000000000000000000000000010
　　...
　　0B01111111111111111111111111111111

　　在同一个符号下：除符号位其他位的数字越大，那么该数字越大。+MIN.VALUE就是修改符号位。不影响同一个符号位下的大小比较,
　　假设a=-1,b=1按无符号考虑：a>b;
　　按有符合考虑：a<b，通过修改符号位，a+MIN.VALUE(正数)>b+MIN.VALUE(负数)
　　实现了无符号比较。

2）属性

@Native public static final int   MIN_VALUE = 0x80000000;
@Native public static final int   MAX_VALUE = 0x7fffffff;
/**
 * The {@code Class} instance representing the primitive type {@code int}.
 */
public static final Class<Integer>  TYPE = (Class<Integer>) Class.getPrimitiveClass("int");
/**
 * All possible chars for representing a number as a String
 */
final static char[] digits = {
    '0' , '1' , '2' , '3' , '4' , '5' ,
    '6' , '7' , '8' , '9' , 'a' , 'b' ,
    'c' , 'd' , 'e' , 'f' , 'g' , 'h' ,
    'i' , 'j' , 'k' , 'l' , 'm' , 'n' ,
    'o' , 'p' , 'q' , 'r' , 's' , 't' ,
    'u' , 'v' , 'w' , 'x' , 'y' , 'z'
};
//十位上的数值，可以根据36，取的十位上数为3
final static char [] DigitTens = {
    '0', '0', '0', '0', '0', '0', '0', '0', '0', '0',
    '1', '1', '1', '1', '1', '1', '1', '1', '1', '1',
    '2', '2', '2', '2', '2', '2', '2', '2', '2', '2',
    '3', '3', '3', '3', '3', '3', '3', '3', '3', '3',
    '4', '4', '4', '4', '4', '4', '4', '4', '4', '4',
    '5', '5', '5', '5', '5', '5', '5', '5', '5', '5',
    '6', '6', '6', '6', '6', '6', '6', '6', '6', '6',
    '7', '7', '7', '7', '7', '7', '7', '7', '7', '7',
    '8', '8', '8', '8', '8', '8', '8', '8', '8', '8',
    '9', '9', '9', '9', '9', '9', '9', '9', '9', '9',
} ;
//个位上的数值，可以根据36，取的个位上数为6
final static char [] DigitOnes = {
    '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
    '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
    '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
    '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
    '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
    '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
    '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
    '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
    '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
    '0', '1', '2', '3', '4', '5', '6', '7', '8', '9',
} ;
final static int [] sizeTable = { 9, 99, 999, 9999, 99999, 999999, 9999999,99999999, 999999999, Integer.MAX_VALUE };
//避免除法操作，直接比较大小返回位数
static int stringSize(int x) {
    for (int i=0; ; i++)
        if (x <= sizeTable[i])
            return i+1;
}
public Integer(int value) { this.value = value;}
@Native public static final int SIZE = 32;
public static final int BYTES = SIZE / Byte.SIZE;

　　3）内部私有类

private static class IntegerCache {
    static final int low = -128;
    static final int high;
    static final Integer cache[];  

    static {
        <span style="color:#ff0000;">// high value may be configured by property</span>
        int h = 127;
        String integerCacheHighPropValue =
            sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
        if (integerCacheHighPropValue != null) {
            try {
                int i = parseInt(integerCacheHighPropValue);
                i = Math.max(i, 127);
                // Maximum array size is Integer.MAX_VALUE
                h = Math.min(i, Integer.MAX_VALUE - (-low) -1);
            } catch( NumberFormatException nfe) {
                // If the property cannot be parsed into an int, ignore it.
            }
        }
        high = h;  

        cache = new Integer[(high - low) + 1];
        int j = low;
        for(int k = 0; k < cache.length; k++)
            cache[k] = new Integer(j++);  

        // range [-128, 127] must be interned (JLS7 5.1.7)
        assert IntegerCache.high >= 127;
    }  

    private IntegerCache() {}
}

　　和Short.java和Byte.java一样，都是通过静态代码块初始化缓存对象数组，不过，IntegerCache对象的high可以通过JVM的启动参数设置，缺省为127。

4）初始化方法

　　构造函数：

public Integer(String s) throws NumberFormatException {
    this.value = parseInt(s, 10);
}
public Integer(int value) {
    this.value = value;
}

　　 其他方法：

public static int parseInt(String s, int radix);
public static int parseInt(String s) throws NumberFormatException;
public static Integer valueOf(String s, int radix) throws NumberFormatException;
public static Integer valueOf(String s) throws NumberFormatException;
public static Integer valueOf(int i);
public static int parseUnsignedInt(String s) throws NumberFormatException;
public static int parseUnsignedInt(String s, int radix);

　　观察“其他方法”的代码，难点主要在下面的方法：

public static int parseInt(String s, int radix)
            throws NumberFormatException
{
    /*
     * WARNING: This method may be invoked early during VM initialization
     * before IntegerCache is initialized. Care must be taken to not use
     * the valueOf method.
     */  

    if (s == null) {
        throw new NumberFormatException("null");
    }  

    if (radix < Character.MIN_RADIX) {
        throw new NumberFormatException("radix " + radix +
                                        " less than Character.MIN_RADIX");
    }  

    if (radix > Character.MAX_RADIX) {
        throw new NumberFormatException("radix " + radix +
                                        " greater than Character.MAX_RADIX");
    }  

    int result = 0;
    boolean negative = false;
    int i = 0, len = s.length();
    int limit = -Integer.MAX_VALUE;
    int multmin;
    int digit;  

    if (len > 0) {
        char firstChar = s.charAt(0);
        if (firstChar < '0') { // Possible leading "+" or "-"
            if (firstChar == '-') {
                negative = true;
                limit = Integer.MIN_VALUE;
            } else if (firstChar != '+')
                throw NumberFormatException.forInputString(s);  

            if (len == 1) // Cannot have lone "+" or "-"
                throw NumberFormatException.forInputString(s);
            i++;
        }
        multmin = limit / radix;//设不同进制下的极限值
        while (i < len) {
<span style="color:#ff0000;">           // Accumulating negatively avoids surprises near MAX_VALUE
            //该方法返回该字符根据进制对应的数字，比如2进制1那么返回1，比如16进制F那么返回15
            digit = Character.digit(s.charAt(i++),radix);//i++:0->1
            if (digit < 0) {
                throw NumberFormatException.forInputString(s);
            }
            if (result < multmin) {
                throw NumberFormatException.forInputString(s);
            }
            result *= radix;
            if (result < limit + digit) {
                throw NumberFormatException.forInputString(s);
            }
            result -= digit;</span>
        }
    } else {
        throw NumberFormatException.forInputString(s);
    }
    return negative ? result : -result;
}

　　

该代码段例子分析：

F1F（16进制）

　　　　　  （1）digit 15；result *= radix -》0；result -= digit -》 -15 
                    （2）digit 1；result *= radix -》-15*16；result -= digit -》 -15*16 - 1(相当于累加) 
                    （3）digit 15；result *= radix -》-(15*16+1)*16；result -= digit -》 -(15*16+1)*16 - 15
                    （4）-（ -(15*16+1)*16 - 15）= （15*15+1）*16 + 15

    public static int parseUnsignedInt(String s, int radix)
                throws NumberFormatException {
        if (s == null)  {
            throw new NumberFormatException("null");
        }

        int len = s.length();
        if (len > 0) {
            char firstChar = s.charAt(0);
            if (firstChar == '-') {
                throw new
                    NumberFormatException(String.format("Illegal leading minus sign " +
                                                       "on unsigned string %s.", s));
            } else {
                if (len <= 5 || // Integer.MAX_VALUE in Character.MAX_RADIX is 6 digits
                    (radix == 10 && len <= 9) ) { // Integer.MAX_VALUE in base 10 is 10 digits
                    return parseInt(s, radix);
                } else {
                    long ell = Long.parseLong(s, radix);
                    if ((ell & 0xffff_ffff_0000_0000L) == 0) {
                        return (int) ell;
                    } else {
                        throw new
                            NumberFormatException(String.format("String value %s exceeds " +
                                                                "range of unsigned int.", s));
                    }
                }
            }
        } else {
            throw NumberFormatException.forInputString(s);
        }
    }

　　例子：

Integer a11 = 3;
Integer a6 = Integer.parseInt("11",2);
Integer a7 = Integer.parseUnsignedInt("80000000",16);
Integer a9 = Integer.valueOf(3);
Integer a10 = new Integer(3);
boolean b1 = a6==a9;//true
boolean b2 = a6==a11;//true
boolean b3 = a6==a10;//false

　　初始化对象进行使用parseInt或者valueOf方法，如果值在IntegerCache范围内，可以直接获取对象。

5）一些toXXXString方法：

public static String toString(int i, int radix) {
    if (radix < Character.MIN_RADIX || radix > Character.MAX_RADIX)
        radix = 10;  

    /* Use the faster version */
    if (radix == 10) {
        return toString(i);
    }  

    char buf[] = new char[33];//二进制
    boolean negative = (i < 0);
    int charPos = 32;  

    if (!negative) {
        i = -i;
    }  

    while (i <= -radix) {
        buf[charPos--] = digits[-(i % radix)];
        i = i / radix;
    }
    buf[charPos] = digits[-i];  

    if (negative) {
        buf[--charPos] = '-';
    }
        //从buf数组的charPos位置开始截取33-charPos字符转换为String类型
    return new String(buf, charPos, (33 - charPos));
}

　　分析该函数之前，我们用十进制转换为二进制来解释下算法：比如10，10%2 商5余0;5%2 商2余1；2%2 商1余0；2%1 商0余1，把余数倒序拼接1010,1010就是二进制。i++号在后面的意思是先赋值然后自身加1；++i在前面的是先自身加1后赋值；--同样。

public static String toUnsignedString(int i, int radix);
public static String toHexString(int i);
public static String toOctalString(int i);
public static String toBinaryString(int i){
    return toUnsignedString0(i, 1);
}  

//前面4个方法都是调用该方法，shift：次幂
private static String toUnsignedString0(int val, int shift) {
    // assert shift > 0 && shift <=5 : "Illegal shift value";
    //获取有效的二进制的位数 31位二进制
    int mag = Integer.SIZE - Integer.numberOfLeadingZeros(val);
    //一个算法，可以根据二进制的有效位数，求出8进制或者16进制的位数
    //比如0100 0000 shift=3 -》 （7+（3-1））/3 = 3 八进制需要3位表示
    int chars = Math.max(((mag + (shift - 1)) / shift), 1);
    char[] buf = new char[chars];  

    formatUnsignedInt(val, shift, buf, 0, chars);  

    // Use special constructor which takes over "buf".
    return new String(buf, true);
}  

//转换为2进制，高位-》低位，直至遇到1停止，0的个数
//比如0000 0000 0000 0000 0000 0000 0000 0001-》31
//比如0100 0000 0000 0000 0000 0000 0000 0001-》1
//比如0010 0000 0000 0000 0000 0000 0000 0001-》2
public static int numberOfLeadingZeros(int i) {
    // HD, Figure 5-6
    if (i == 0)
        return 32;
    int n = 1;
    if (i >>> 16 == 0) { n += 16; i <<= 16; }
    if (i >>> 24 == 0) { n +=  8; i <<=  8; }
    if (i >>> 28 == 0) { n +=  4; i <<=  4; }
    if (i >>> 30 == 0) { n +=  2; i <<=  2; }
    n -= i >>> 31;
    return n;
}  

static int formatUnsignedInt(int val, int shift, char[] buf, int offset, int len) {
    int charPos = len;
    int radix = 1 << shift;
    int mask = radix - 1;
    do {
        //val&mask 可以获取末尾使用该进制表示的数值
        //例子 假设16进制
        //  1010 1010 1010 1000
        //& 0000 0000 0000 1111
        //  0000 0000 0000 1000
        buf[offset + --charPos] = Integer.digits[val & mask];
        val >>>= shift;
    } while (val != 0 && charPos > 0);
    return charPos;
}

　　

public static String toString(int i) {
    if (i == Integer.MIN_VALUE)
        return "-2147483648";
    int size = (i < 0) ? stringSize(-i) + 1 : stringSize(i);
    char[] buf = new char[size];
     // 将Integer数读入到char[]数组
    getChars(i, size, buf);
    return new String(buf, true);
}
static void getChars(int i, int index, char[] buf) {
    int q, r;
    int charPos = index;
    char sign = 0;  

    if (i < 0) {
        sign = '-';
        i = -i;
    }  

    // Generate two digits per iteration
    // 处理超过2的16次方的大数
    while (i >= 65536) {
        q = i / 100;
        // really: r = i - (q * 100);
        // 假设 65536 那么等于 36，根据36取的十位数和个位数上的数值
        r = i - ((q << 6) + (q << 5) + (q << 2));
        //655
        i = q;
        //个位 6
        buf [--charPos] = DigitOnes[r];
        //十位 3
        buf [--charPos] = DigitTens[r];
    }  

    // Fall thru to fast mode for smaller numbers
    // assert(i <= 65536, i);
    // 处理<2的16次方的大数
    for (;;) {
        q = (i * 52429) >>> (16+3);//i*52429/524288 ≈0.1000003。相当于i/10
        //获取个位的数字
        r = i - ((q << 3) + (q << 1));  // r = i-(q*10) ...
        buf [--charPos] = digits [r];
        i = q;
        if (i == 0) break;
    }
    if (sign != 0) {
        buf [--charPos] = sign;
    }
}

　一个例子：　

for(int i = 0;i<65536;i++){
    int q = (i * 52429) >>> (16+3);//相当于q = i/10;
    int j = i/10;
    if(q != j){
        Out.println(false);
    }
}
输出：true