Java – what happens when we add two integer objects?

When we add two integer objects to Java, can anyone explain the internal behavior? (just like un@R_778_2419 @Object to a primitive, then add two integers, and finally wrap them in an integer object)

Integer sum = new Integer(2) + new Integer(4);

Solution

It compiles into:

Integer sum = Integer.valueOf(new Integer(2).intValue()+new Integer(4).intValue());

You can verify this by looking at the bytecode disassembly obtained using javap - C

This corresponds to the new integer (2) Part of intvalue(), leaving int 2 in the stack:

0:  new #2; //class java/lang/Integer
3:  dup
4:  iconst_2
5:  invokespecial   #3; //Method java/lang/Integer."<init>":(I)V
8:  invokevirtual   #4; //Method java/lang/Integer.intValue:()I

This is with the new integer (4) For the part corresponding to intvalue(), put int 4 on the stack:

11: new #2; //class java/lang/Integer
14: dup
15: iconst_4
16: invokespecial   #3; //Method java/lang/Integer."<init>":(I)V
19: invokevirtual   #4; //Method java/lang/Integer.intValue:()I

And here and 2 are calculated with Iadd by calling integer Valueof wraps the sum as an integer and stores the result in the first local variable (astore_1):

22: iadd
23: invokestatic    #5; //Method java/lang/Integer.valueOf:(I)Ljava/lang/Integer;
26: astore_1