Integer 的小知识

先看代码

public class IntegerTest{    public static void main(String[] args) {        Integer a1 = 12;        Integer a2 = 12;        //#1        System.out.println(a1 == a2 ); //true        Integer b1 = 128;        Integer b2 = 128;        //#2        System.out.println(b1 == b2 ); //false        //#3        Integer c1 = 12 ;        int c2 = 12 ;        System.out.println(c1 == c2 ); //true        //#4        Integer d1 = new Integer(12);        Integer d2 = new Integer(12);        System.out.println(d1 == d2 ); //false    }}//#1和#2 的结果不相同的原因是啥了？a. Integer a =12 ; java 反编译的时候，成为了 Integer a = Integer.valueOf(12);public static Integer valueOf(int i) {    if (i >= IntegerCache.low && i <= IntegerCache.high)         return IntegerCache.cache[i + (-IntegerCache.low)];    return new Integer(i); }当 i 值为[-128,127]的时候 从IntegerCache中取值。     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) {                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() {}    }    看完IntegerCache类 就知道是啥事了。    //那为啥#4是false了?    因为通过构造函数形成的对象，不会从IntegerCache数组中取实例。    //#3 是从JDSK1.5开始 Integer和int比较的时候 Integer自动拆箱，再进行比较。