Java坑一

最新推荐文章于 2021-02-12 11:12:52 发布

转载最新推荐文章于 2021-02-12 11:12:52 发布 · 99 阅读

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原文链接：http://www.cnblogs.com/yasepix/p/5773045.html

文章标签：

#java

本文通过一个简单的Java程序示例，介绍了Integer对象在-128到127范围内使用缓存来支持自动装箱的对象身份语义。当Integer值超出此范围时，将不再使用缓存，而是创建新的Integer对象。

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public class Test {

    public static void main(String[] args) {
        Integer a = 12;
        Integer b = 12;
        System.out.println(a == b);
        System.out.println(a.equals(b));
        a = 128;
        b = 128;
        System.out.println(a == b);
        System.out.println(a.equals(b));
    }
}

    /**
     * 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) -1);
            }
            high = h;

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

        private IntegerCache() {}
    }