Java中那些不得不说的坑

看看下面这段代码跟你想的结果一样吗？

Integer a =127; Integer b = 127; System.out.println(a==b);//true Integer a1 = 128; Integer b1 = 128; System.out.println(a1==b1);//false

为什么会是这样的结果，我们反编译.class文件看看

Integer a = Integer.valueOf(127); Integer b = Integer.valueOf(127); System.out.println(a == b); Integer a1 = Integer.valueOf(128); Integer b1 = Integer.valueOf(128); System.out.println(a1 == b1);

其实是Integer作者在写这个类时，为了避免重复创建对象，对一定范围的Integer做了缓存，如果该值没有在该范围内则new一个新对象返回，从源码中可以看到如下代码：

public static Integer valueOf(int i) { if (i >= IntegerCache.low && i <= IntegerCache.high) return IntegerCache.cache[i + (-IntegerCache.low)]; //i如果在该范围内直接返回缓存值 return new Integer(i); }

IntegerCache 是Integer 类的静态内部类，再来看看IntegerCache的实现

private static class IntegerCache { static final int low = -128; static final int high; static final Integer cache[]; static {//当类被载入到java虚拟机是进行缓存创建 // 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() {} }