String、Integer和Long类的valueOf()源码分析

String.valueOf(Object obj)

public static String valueOf(Object obj){
        return (obj==null)?"null":obj.toString();
}

Long.valueOf(String s)

//Long.valueOf(String s)
public static Long valueOf(String s) throws NumberFormatException{
       return Long.valueOf(parseLong(s,10));
}

//parseLong(String s,int radix)
public static long parseLong(String s, int radix)
          throws NumberFormatException
{
    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");
    }

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

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

            if (len == 1) // Cannot have lone "+" or "-"
                throw NumberFormatException.forInputString(s);
            i++;
        }
        multmin = limit / radix;
        while (i < len) {
            // Accumulating negatively avoids surprises near MAX_VALUE
            digit = Character.digit(s.charAt(i++),radix);
            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;
        }
    } else {
        throw NumberFormatException.forInputString(s);
    }
    return negative ? result : -result;
}

Long.valueOf(long l) 使用Cache -128~127

//Long.valueOf(long l)
public static Long valueOf(long l) {
    final int offset = 128;
    if (l >= -128 && l <= 127) { // will cache
        return LongCache.cache[(int)l + offset];
    }
    return new Long(l);
}

LongCache类

private static class LongCache {
    private LongCache(){}

    static final Long cache[] = new Long[-(-128) + 127 + 1];

    static {
        for(int i = 0; i < cache.length; i++)
            cache[i] = new Long(i - 128);
    }
}

Integer.valueOf(Stirng s)

//Integer.valueOf(String s)
public static Integer valueOf(String s) throws NumberFormatException {
    return Integer.valueOf(parseInt(s, 10));
}

//Integer.parseInt(String s,int radix)
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;
        while (i < len) {
            // Accumulating negatively avoids surprises near MAX_VALUE
            digit = Character.digit(s.charAt(i++),radix);
            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;
        }
    } else {
        throw NumberFormatException.forInputString(s);
    }
    return negative ? result : -result;
}

Integer.valueOf(int i)

public static Integer valueOf(int i) {
    assert IntegerCache.high >= 127;
    if (i >= IntegerCache.low && i <= IntegerCache.high)
        return IntegerCache.cache[i + (-IntegerCache.low)];
    return new Integer(i);
}

IntegerCache high  low为-128 默认是127

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

    static {
        // high value may be configured by property
        int h = 127;
        String integerCacheHighPropValue =
            sun.misc.VM.getSavedProperty("java.lang.Integer.IntegerCache.high");
        if (integerCacheHighPropValue != null) {
            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);
        }
        high = h;

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

    private IntegerCache() {}
}

 以下有几个例子

1.System.out.println(127==127); //true , int type compare
2.System.out.println(128==128); //true , int type compare
3.System.out.println(new Integer(127) == new Integer(127)); //false, object compare
4.System.out.println(Integer.parseInt("128")==Integer.parseInt("128")); //true, int type compare
5.System.out.println(Integer.valueOf("127")==Integer.valueOf("127")); //true ,object compare, because IntegerCache return a same object
6.System.out.println(Integer.valueOf("128")==Integer.valueOf("128")); //false ,object compare, because number beyond the IntegerCache
7.System.out.println(Integer.parseInt("128")==Integer.valueOf("128")); //true , int type compare

1和2都是int整型数据比较，==比较的是值，所以结果都是true。

3是对象比较，而new Integer()每次构造一个新的Integer对象，所以结果是false。

4是值比较，Integer.parseInt返回int类型数据，所以结果是true。

5,6都是对象比较。Integer.valueOf返回一个Integer对象，默认在-128~127之间时返回缓存中的已有对象（如果存在的话），否则，new Integer()创建一个新对象。所以5的结果是true，6的结果是false。

7是值比较，Integer.valueOf返回一个Integer对象，是int和Integer之间比较。所以结果是true。

结论：

对于整型数据的比较，首先判断是值比较还是对象比较，如果是值比较，则比较值是否相等；对象比较，则看对象是怎么构造出来的，如果采用new Integer()方式，则每次产生新对象，两个new出来的Integer对象比较肯定返回false，如果是Integer.valueOf方式的话，注意值的区间是否在-128~127之间，如果在，则构造的相同值的对象是同一个对象，==比较后返回true，否则返回false。