考虑如下代码的输出：
Integer a = 0;
Integer b = 0;
System.out.println(a == b);
System.out.println(a.equals(b));
//output:
//true
//true
再考虑这个代码的结果：
Integer a = 1000;
Integer b = 1000;
System.out.println(a == b);
System.out.println(a.equals(b));
//output:
//false
//true
为什么会有两个不同的结果，这个结果有什么因素导致？
原来 Java 设有 Integer 的常量池，这个常量池缓存了 \([-128,127]\) 的所有整数，所以在比较 \([-128,127]\) 之间的 Integer 的时候是直接比较的同一常量值，所以a==b，当比较范围之外的值的时候比较的是新对象的引用地址，所以 a!=b
注：上面的代码
Integer a = 0等，使用到了自动装箱，而 Integer 的自动装箱使用的是Integer.valueof(int i)
其它包装器
- Boolean：(全部缓存)
- Byte：(全部缓存)
- Character(<= 127缓存)
- Short(-128 — 127缓存)
- Long(-128 — 127缓存)
- Float(没有缓存)
- Doulbe(没有缓存)
附 JDK 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. During VM initialization the
* getAndRemoveCacheProperties method may be used to get and remove any system
* properites that configure the cache size. At this time, the size of the
* cache may be controlled by the vm option -XX:AutoBoxCacheMax=<size>.
*/
// value of java.lang.Integer.IntegerCache.high property (obtained during VM init)
private static String integerCacheHighPropValue;
static void getAndRemoveCacheProperties() {
if (!sun.misc.VM.isBooted()) {
Properties props = System.getProperties();
integerCacheHighPropValue =
(String)props.remove("java.lang.Integer.IntegerCache.high");
if (integerCacheHighPropValue != null)
System.setProperties(props); // remove from system props
}
}
private static class IntegerCache {
static final int high;
static final Integer cache[];
static {
final int low = -128;
// high value may be configured by property
int h = 127;
//可以配置这一常量池的范围
if (integerCacheHighPropValue != null) {
// Use Long.decode here to avoid invoking methods that
// require Integer's autoboxing cache to be initialized
int i = Long.decode(integerCacheHighPropValue).intValue();
i = Math.max(i, 127);
// Maximum array size is Integer.MAX_VALUE
h = Math.min(i, Integer.MAX_VALUE- -low);
}
high = h;
cache = new Integer[(high - low) + 1];
int j = low;
for(int k = 0; k < cache.length; k++) //缓存区间数据
cache[k] = new Integer(j++);
}
private IntegerCache() {}
}
/**
* Returns a <tt>Integer</tt> instance representing the specified
* <tt>int</tt> value.
* If a new <tt>Integer</tt> instance is not required, this method
* should generally be used in preference to the constructor
* {@link #Integer(int)}, as this method is likely to yield
* significantly better space and time performance by caching
* frequently requested values.
*
* @param i an <code>int</code> value.
* @return a <tt>Integer</tt> instance representing <tt>i</tt>.
* @since 1.5
*/
public static Integer valueOf(int i) {
if(i >= -128 && i <= IntegerCache.high)
return IntegerCache.cache[i + 128];
else
return new Integer(i);
}
参考：
https://www.cnblogs.com/csniper/p/5882760.html
http://blog.csdn.net/fanxiaobin577328725/article/details/52431508