Integer引用类型注意事项

在jdk5后，java提供了自动装箱与自动拆箱

有如下程序代码：

public class IntegerTest {
    public static void main(String[] args) {
        Integer a = 126;
        Integer as = 126;
        System.out.println(a == as);

        Integer b = 128;
        Integer bs = 128;
        System.out.println(b == bs);



    }



}

输出为：

 

 这是因为整形常量池的存在，查看Integer类源码发现：

 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() {}
    }

含有一个cache，它将存储-128~127的常量，也就是说，a和as其实存的地址时相同的。但这种只适用于自动装箱，如果改成以下形式就不行了：

 Integer a = new Integer(126);
        Integer as = new Integer(126);
        System.out.println(a == as);

将会输出：false