一、常量與物件的比較

這樣的程式碼結果會是什麼？

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

結果是：

        true
        false

知識點：大家都知道這其實是一個常量比較，一個是物件之間比較。根本是比較的指向地址.

然而把上述的數字改為100，結果還會這樣嗎

        int a = 100,b=100;
        System.out.println(a == b);//1
 

        Integer c = 100, d = 100;
        System.out.println(c == d);//2

結果是：

        true
        true

這裡是不是出現了神奇之處了呢（有趣~~哈哈哈）

原因就是，檢視Integer原始碼會發現有一個內部快取類IntegerCache ，它快取了一定範圍的數（Integer127 ~ -128會存在一個快取裡）

下面程式碼來自Integer類裡

/**
     * 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() {}
    }

還有一段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，所有的小整數在內部快取，然後當我們宣告類似——

Integer c = 100; 

的時候，它實際上在內部做的是

Integer i = Integer.valueOf(100); 

類似的Long、Bety 裡面都有類似的快取類。

以上都是自己見解，如有誤，請大家多多指教
參考 ：https://blog.csdn.net/xlgen157387/article/details/51261859