Question

I have a question about the following piece of code:

public class Equivalence {
    public static void main(String[] args) {
        Integer n1 = new Integer(47);
        Integer n2 = new Integer(47);
        System.out.println(n1 == n2);
        System.out.println(n1 != n2);
    }
}

The resulting output surprised me:

false
true

I have checked the constructor in Javadoc online, nothing help from there.

Thanks in advance

Thank you

Answer 1

Only primitive types can be reliably compared with ==. For objects (and Integer is an object type), the equals() method should be used.

== can be used for objects but only to check whether two objects are in fact the same. So for example:

Integer n1 = new Integer(47);
Integer n2 = n1;
// n1 == n2 will be true

It’s best if you think of ints as the number itself and Integers as a post-it note with the number written on it. == will only return true if you’re talking about the same post-it note, equals() however will return true for any two notes with the same number on it.

What complicates the issue is that since version 1.5 Java also has autoboxing, that is, ints are automatically converted to Integer and vice versa when required. This can lead to very surprising results if you’re not careful.