Autoboxing wth ++， - java中的運算符

Question

我對java中的自動裝箱拆箱感到困惑。請參閱我的以下兩個progarm。

Integer x = 400; 
Integer y = x; 
x++; x--; 
System.out.println((x==y)); 

The output is false. 
I known why the output is false. Because of autoboxing x. 

---

Integer x = 100; 
Integer y = x; 
x++; x--; 
System.out.println((x==y)); 

The output is true. 
But the program is same as the upper. Why the output is true? 
Please explain me detail. 

非常感謝你。

Answer 1

這是因爲整數-128到127被緩存，所以在第二個例子中x和y指的是相同的Integer實例。

Integer x = 100;   // x refers to cached 100 
x++; 

相當於

int var = x.intValue(); 
var++; 
x = Integer.valueOf(var); // returns cached 100 

見Integer.valueOf（int）的API。