Question

The expressions used in the following Java code are nuts and pretty much unacceptable though I tested them just for a general purpose and obtained some unexpected result. The simple code snippet goes following.

package wrapper;

final public class Main 
{
    public static void main(String[] args) 
    {
        Integer j1 = 127;
        Integer j2 = 127;
        System.out.println(j1==j2); //returns true!!!

        Integer k1 = 128;
        Integer k2 = 128;
        System.out.println(k1==k2); //returns false!!!

        Integer w1 = -128;
        Integer w2 = -128;
        System.out.println(w1==w2); //returns true!!!

        Integer m1 = -129;
        Integer m2 = -129;
        System.out.println(m1==m2); //returns false!!!
    }
}

---

Integer j1 = 127;
Integer j2 = 127;
System.out.println(j1==j2);

The above code obviously displays true hence, there is no question about it.

---

Integer k1 = 128;
Integer k2 = 128;
System.out.println(k1==k2);

The above code is expected to display true on the console but surprisingly, it shows false. Why?

---

Integer w1 = -128;
Integer w2 = -128;
System.out.println(w1==w2); 

This code displays true and no question about it.

---

Integer m1 = -129;
Integer m2 = -129;
System.out.println(m1==m2);

The above code again displays false though it is expected to return true. Why?

Answer 1

Integers from the range -128..127 are cached, so j1 and j2 from:

Integer j1 = 127;
Integer j2 = 127;

point to the same object.

Of course, this is a JVM implementation thing, so you should never assume they are cached. For comparing reference types, never use == but use their equals(...) method instead.