Java中那些不得不说的坑

看看下面这段代码跟你想的结果一样吗？

        Integer a =127;
        Integer b = 127;
        System.out.println(a==b);//true

        Integer a1 = 128;
        Integer b1 = 128;
        System.out.println(a1==b1);//false

为什么会是这样的结果，我们反编译.class文件看看

    Integer a = Integer.valueOf(127);
    Integer b = Integer.valueOf(127);
    System.out.println(a == b);

    Integer a1 = Integer.valueOf(128);
    Integer b1 = Integer.valueOf(128);
    System.out.println(a1 == b1);

其实是Integer作者在写这个类时，为了避免重复创建对象，对一定范围的Integer做了缓存，如果该值没有在该范围内则new一个新对象返回，从源码中可以看到如下代码：

    public static Integer valueOf(int i) {
        if (i >= IntegerCache.low && i <= IntegerCache.high)
            return IntegerCache.cache[i + (-IntegerCache.low)]; //i如果在该范围内直接返回缓存值
        return new Integer(i);
    }

IntegerCache 是Integer 类的静态内部类，再来看看IntegerCache的实现

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

        static {//当类被载入到java虚拟机是进行缓存创建
            // 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() {}
    }