话不多说,根据以下程序运行,打印的结果为什么不同?
Integer a = 100;
Integer b = 100;
System.out.println(a == b);//print : true
Integer a = 200;
Integer b = 200;
System.out.println(a == b);//print : false
以上两段代码在执行后为什么会出现这样神奇的效果,纠察原因在Integer的源码中一段特殊的代码,在代码中会在虚拟机加载时将-128至127的数据提前缓存到内存中,所以会导致双等号比较会出现不一致的问题(内存引用地址已缓存)。
在开发中对象类型的比较一定记得使用eques进行内容比较。
/**
* 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() {}
}