认识达内从这里开始

昆明达内的小编这一期给大家讲在Java中==的一个坑。

观察下面代码，输出结果是什么？

public static void main(String[] args) {

Integer p = 10000;

Integer q = 10000;

System.out.println(p == q);

System.out.println(p.equals(q));

}

运行一次，答案与预想的完全不一样。在比较数字的时候，一定要用euqals，不能用==c

查看Integer的jdk源码，发现如下片段：

/**

* 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 != ull) {

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 fe) {

// 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() {}

}

这儿的IntegerCache有一个静态的Integer数组，在类加载时就将-128到127的Integer对象创建了，并保存在cache数组中，一旦程序调用valueOf方法，如果i的值是在-128到127之间就直接在cache缓存数组中去取Integer对象。

如果是-128到127的值，你不管创建多少个这个范围内的Integer用ValueOf出来的都是同一个对象。