【神奇现象】java整型数的神奇内部缓存类

一、常量与对象的比较

这样的代码结果会是什么？

        int a = 1000,b=1000;
        System.out.println(a == b);//1
        Integer c = 1000, d = 1000;
        System.out.println(c == d);//2

结果是：

        true
        false

知识点：大家都知道这其实是一个常量比较，一个是对象之间比较。根本是比较的指向地址.

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然而把上述的数字改为100，结果还会这样吗

        int a = 100,b=100;
        System.out.println(a == b);//1
        Integer c = 100, d = 100;
        System.out.println(c == d);//2

结果是：

        true
        true

这里是不是出现了神奇之处了呢（有趣~~哈哈哈）

原因就是，查看Integer源码会发现有一个内部缓存类IntegerCache ，它缓存了一定范围的数（Integer127 ~ -128会存在一个缓存里）

下面代码来自Integer类里

/**
     * 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 {@code -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) {
                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() {}
    }

还有一段valueof方法

  /**
     * 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) {
        if (i >= IntegerCache.low && i <= IntegerCache.high)
            return IntegerCache.cache[i + (-IntegerCache.low)];
        return new Integer(i);
    }

所以事情就成了，所有的小整数在内部缓存，然后当我们声明类似——

Integer c = 100; 

的时候，它实际上在内部做的是

Integer i = Integer.valueOf(100); 

类似的Long、Bety 里面都有类似的缓存类。

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以上都是自己见解，如有误，请大家多多指教
参考 ：https://blog.csdn.net/xlgen157387/article/details/51261859