java基本类型对应包装类的静态缓存

阅读jdk源码，可以发现，对于基本类型的包装类，是有静态缓存。

1、对Boolean类型,true和false都有对应的缓存，源码为：

/**
 * The {@code Boolean} object corresponding to the primitive
 * value {@code true}.
 */
public static final Boolean TRUE = new Boolean(true);

/**
 * The {@code Boolean} object corresponding to the primitive
 * value {@code false}.
 */
public static final Boolean FALSE = new Boolean(false);

2、对于Integer,范围在-128至127内的，都有对应的缓存，源码为：

/**
 * 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= 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() {}
}

3、对于Byte/Short/Long，是和Integer类似的，范围在-128至127内的，都有对应的缓存

总结，对于上述基本类型对应的包装类，只要在缓存范围内，即-128至127范围（一个字节，补码表示的范围）内，

我们是可以直接用=判断相相等的。

4、注意，对于Float和Double类型，并没有对应的缓存。