引言
具体事例
public class Test {
public static void main(String[] args) {
Integer a1=127;
Integer a2=127;
Integer b1=128;
Integer b2=128;
System.out.println(a1==a2);
System.out.println(b1==b2);
}
}
请问输出是什么？
no no no！！！你看到的只是表象，其实里面还有小秘密哦！！！
输出的结果如下：
是不是有点不可思议！
剖析原因
请看源码
/**
* 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;
}
分析源码：
回到事例中，由于a1、a2都等于127，而127存储在常量池中的cache数组中，也就是说，a1和a2指向同一个地址，因此为 true 。
而b1和b2都为128，不再数组范围之内，因此需给它们开辟不同的“家”。地址不同，因此 false。
你学会了吗？
