Interget中的缓存

判断一下下面的代码运行结果：

public class IntegerCache {
	public static void main(String[] args) {
    	Integer a = 100, b = 100, c = 129, d = 129;
System.out.println(a == b);
System.out.println(c == d);
    }
}

点击查看结果

true
false

那为什么会出现这样的情况呢？

下面解释一下：

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() {}
}

通过分析源码我们可以得知，JAVA在 -128 ～ 127之间做了缓存。考虑到高频数值的复用场景，这样做还是很合理的，提高代码运行效率