Integer的“==”返回值

 

/**
 * Created by spring on 10/26/13.
 */
public class Test {
    public static void main(String args[]){

        Integer i_128 = -128;
        Integer j_128 = -128;

        System.out.println(i_128==j_128);

        Integer i_129 = -129;
        Integer j_129 = -129;

        System.out.println(i_129==j_129);


        Integer i128 = 128;
        Integer j128 = 128;

        System.out.println(i128==j128);

        Integer i127 = -127;
        Integer j127 = -127;

        System.out.println(i127==j127);


    }
}

 输出：

true
false
false
true

 

这个原因想必大家都知道，Integer是一个不可变类，所以对一些常用的值（-128--127）进行了缓存，避免对象的频繁创建和销毁。但是从jdk1.5开始这个缓存的范围可以由开发者根据需要自己指定，不过下限还是-128,上限可以修改。jdk的源码如下：

    /**
     * Cache to support the object identity semantics of autoboxing for values between
     * -128 and 127 (inclusive) as required by JLS.
     *
     * The cache is initialized on first usage.  The size of the cache
     * may be controlled by the -XX:AutoBoxCacheMax=<size> option.
     * During VM initialization, java.lang.Integer.IntegerCache.high property
     * may be set and saved in the private system properties in the
     * sun.misc.VM class.
     */

    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));
            }
            high = h;

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

 

    /**
     * Returns an {@code Integer} instance representing the specified
     * {@code int} value.  If a new {@code Integer} instance is not
     * required, this method should generally be used in preference to
     * the constructor {@link #Integer(int)}, as this method is likely
     * to yield significantly better space and time performance by
     * caching frequently requested values.
     *
     * This method will always cache values in the range -128 to 127,
     * inclusive, and may cache other values outside of this range.
     *
     * @param  i an {@code int} value.
     * @return an {@code Integer} instance representing {@code i}.
     * @since  1.5
     */
    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);
    }