Question

I have the following code.

public static void doIntCompareProcess() {
    int a = 100;
    int b = 100;

    Integer c = 200;
    Integer d = 200;

    int f = 20000;
    int e = 20000;

    System.out.println(c == d);
    compareInt(e, f);
    compareInt(a, b);
}

public static void compareInt(Integer v1, Integer v2) {
    System.out.println(v1 == v2);
}

That gives me this output:

false
false
true

Where the currect output should be:

false
false
false

Why am I getting the wrong output for the code?

Answer 1

The last line corresponds to:

compareInt(100, 100);

Since compareInt() takes two Integer objects, the two parameters get auto-boxed. During that process, instances of Integer(n) for small values of n get interned. In other words, compareInt() receives two references to the same Integer(100) object. This is what’s causing the last comparison to evaluate to true.

See Using == operator in Java to compare wrapper objects

The bottom line is to not use the == operator for directly comparing Integer objects. For further discussion, see https://stackoverflow.com/a/1515811/367273