现象

Integer a = 1000, b = 1000;
System.out.println(a == b);//false
Integer c = 100, d = 100;
System.out.println(c == d);//true

= 运算比较的不是值而是引用**

分析

其实正常情况Integer类型与int是不可能相等的,一个是对象类型(Integer ),一个是基础数据类型(int)。只是Java搞了自动装箱和拆箱机制,会将int转换成Integer,反之亦然。但是转换时正常情况下,装箱与拆箱的对象的内存地址应该是不同的,应该仍然不等才对,但是JVM让对应的-128~127的装箱对象都缓存起来了,这意味着每次装箱在这个范围的,直接从缓存取,不会new,通过这个trick让其相等 但是超出这个范围的默认还是一般原理就不相等了

当我们给一个Integer 对象赋一个int值的时候,会调用Integer类的静态方法valueOf,如果看看valueOf 的源代码就知道发生了什么:

public static Integer valueOf(int i) {
  if (i >= IntegerCache.low && i
      return IntegerCache.cache[i + (-IntegerCache.low)];
  return new Integer(i);
}

IntegerCache 是Integer 的内部类,其代码如下所示:

/**
     * 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() {}
    }

简单的说,如果整型字面量的值在-128 ~ 127之间,那么不会new新的Integer对象,而是直接引用常量池中的Integer对象,所以才会出现上面的现象


一个好奇的人