Java code using HashSet of longs doesn't work?

Question

This simple Java code adds 2 to a set of long, and subsequently prints whether 2 is a member of the set:

import java.util.*;

class A {
    public static void main(String[] args) {
        HashSet<Long> s = new HashSet<Long>();
        long x = 2;
        s.add(x);
        System.out.println(s.contains(2));
    }
}

It should print true since 2 is in the set, but instead it prints false. Why?

$ javac A.java && java A
false

@LuiggiMendoza - A `long` primitive would have worked fine because of autoboxing. The problem is that it was an `int` primitive. — Ted Hopp, Jun 03 '13 at 16:50
Sorry, you're right. The literal 2 was treated as `int` thus autoboxed to `Integer`. — Luiggi Mendoza, Jun 03 '13 at 16:52

Denys Séguret · Accepted Answer · 2013-06-04T05:27:26.357

24

Your set contains instances of Long and you were looking for an Integer (the type into which an int is boxed when an Object is required).

Test

System.out.println(s.contains(Long.valueOf(2)));

or

System.out.println(s.contains(2L));

edited Jun 04 '13 at 05:27

answered Jun 03 '13 at 16:46

Denys Séguret

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I'd rather type "2L" ('L' uppercase) instead of "2l", since 2l might look like twenty-one. – Marlon Bernardes Jun 03 '13 at 16:51
@MarlonBernardes Edited : `2l` changed to `2L`. – Denys Séguret Jun 03 '13 at 16:51

Rahul Bobhate · Answer 2 · 2013-06-03T16:55:20.437

6

When you say s.contains(2), it searches for 2 which by default is an int, which gets boxed to Integer. But the object which you stored was Long. So, it returns false

Try using s.contains(Long.valueOf(2)) instead.

edited Jun 03 '13 at 16:55

answered Jun 03 '13 at 16:48

Rahul Bobhate

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Actually, the object stored was a `Long`, not a `long`. But +1 for getting the essence of the issue correct. – Ted Hopp Jun 03 '13 at 16:53
Oops. Just a typo. Thanks for notifying. :) – Rahul Bobhate Jun 03 '13 at 16:54

Juned Ahsan · Answer 3 · 2013-06-03T16:54:59.520

1

Your Hashset stores object of Long and not int/Integer.. You are trying to get an Integer where int is boxed while an Object is required.

edited Jun 03 '13 at 16:54

answered Jun 03 '13 at 16:46

Juned Ahsan

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Java code using HashSet of longs doesn't work?

3 Answers3