Autoboxing wth ++,-- operator in java [duplicate]

Question

This question already has answers here:

Weird Integer boxing in Java (12 answers)

Closed 9 years ago.

I am confuse about autoboxing unboxing in java. Please see my following two 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.

Thank you very much.

Answer 1

This is because Integers -128 to 127 are cached, so in second example x and y refer to the same Integer instance.

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

is equivalent to

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

See Integer.valueOf(int) API.