Integeter127与128

package com.hangao.basic;

/**
 * @author hangao hangao1204@hotmail.com
 *
 */
public class TestInt {
    public static void main(String[] args) {
        Integer ie1 = 127;
        Integer ie2 = 127;
        System.out.println("Integer 127==127:" + (ie1 == ie2));//Integer 127==127:true
        
        Integer ie3 = new Integer(127);
        Integer ie4 = new Integer(127);
        System.out.println("new Integer 127==127:" + (ie3 == ie4));//new Integer 127==127:false
        
        Integer ie5 = 128;
        Integer ie6 = 128;
        System.out.println("Integer 128==128:" + (ie5 == ie6));//Integer 128==128:false
        
        Integer ie7 = new Integer(128);
        Integer ie8 = new Integer(128);
        System.out.println("new Integer 128==128:" + (ie7 == ie8));//new Integer 128==128:false
        
        analysis();
    }

    private static void analysis() {
        Integer ie0 = 127;
        System.out.println(ie0);
        /* L0
        LINENUMBER 5 L0
        BIPUSH 127
        INVOKESTATIC java/lang/Integer.valueOf (I)Ljava/lang/Integer;
        ASTORE 1*/
        /*从字节码文件可以看出此处有一个自动装箱的动作*/
        
        /*public static Integer valueOf(int i) {
            if (i >= IntegerCache.low && i <= IntegerCache.high)
                return IntegerCache.cache[i + (-IntegerCache.low)];
            return new Integer(i);
        }*/
        /*而Integeter这个类的装箱过程从Integeter源代码可以看出在(-128 - 127)之间的int值
        其实返回的是IntegerCache里面的cache[]数组里面的对象*/
        
        /*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 = Integer.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() {}
        }*/
        /*IntegerCache是Integer的内部类，在被jdk加载的时候，会去初始化创建（-128 - 127）之间所有int值对应的Integer类数组
        使用的时候可以从里面取*/
    }
}