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);
}
}
为什么第一个是true,而第二个却是false呢?
Java设计者认为:每次都要开辟新空间会占用大量的资源,因此他们规定在[-128,127] (因为Java设计者认为大家对数的使用大多在这个范围以内)之间的Integer类型的变量,直接指向常量池中的缓存地址,不会new开辟出新的空间,因此当值在[-128,127] 区间内的时候,两个相同值的变量会指向相同地址。
源码如下
/**
* 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;
}
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