Java autoboxing rules

Question

I am a java novice and so confused by the following example. Is it okay to think that \"==\" sign will compare the values between Integers and \"autoboxed\" Integer

Answer 1

Unboxing will be happing when arithmetic operators, comparison operators appear.

eg:

Integer a = 10;
a = a+10; //1.unboxing a to int 2.calculate a+10 3.boxing 20 to Integer.
System.out.print(a > 10); //1.unboxing a to int 2. compare

But when == appear, it depends.

If boxing type appear on both side, it will compare the reference.But if base type appear on one side, and the other side is a boxing type, the boxing type will unboxing to base type.

eg:

Integer a = new Integer(129);
Integer b = new Integer(129);
System.out.println(a == b); // compare reference return false
System.out.println(a == 129); // a will unboxing and compare 129 == 129 return true

PS: In Java.lang.Integer Cache to support the object identity semantics of autoboxing for values between -128 and 127 (inclusive) as required by JLS. See source code

So:

Integer a = 127;
Integer b = 127; //cached, the same as b a==b return ture

Integer c = 129;
Integer d = 129; // not cached, c==d return false