Two objects with same data are not the same?

Question

I have a test class like so:

public class CompareObjects {

public static class Obj {
    public int i;

    public Obj(int i) {
        this.i = i;
    }
}

public static void main(String[] args) {
    Obj o1 = new Obj(0);
    Obj o2 = new Obj(0);

    if(o1 == o2) {
        System.out.println("Equal");
    }else{
        System.out.println("Not equal");
    }
}

}

I though the test would return "Equal", but it didn't. Why doesn't Java consider two objects with equal components not the same? Am I doing something wrong here? I have a feeling I completely overlooked something when I started learning Java.

Also, I tested the two against each other with the equals() method, and I get the same result. My reason for this question is that I would like to be able to test an ArrayList with the contains() method to see if one object has the same components as another and therefore equal. Any ideas?

one of the most basic things in java is the difference between `equals` and `==` — gefei, Jan 01 '13 at 20:49
Implement your own "equals". == checks to see if they're the same objects, which in this case has nothing to do with their properties. — Dave Newton, Jan 01 '13 at 20:50
@gefei: Yes I know that. They both return "Not equal" though — CoderTheTyler, Jan 01 '13 at 20:50
[Overriding equals and hashCode in Java](http://stackoverflow.com/q/27581/1288408) — Modus Tollens, Jan 01 '13 at 20:51
No benefit at all. You're right, and I apologize. My comment was harsh. I'm just a bit too obsessed about correctness. — JB Nizet, Jan 01 '13 at 20:57
It's all good. You were right, I didn't REALLY know that. I just thought I did. — CoderTheTyler, Jan 01 '13 at 21:00

tckmn · Accepted Answer · 2013-01-01T20:56:54.797

4

== compares the references to the object. For example:

Obj a = new Obj(0);
Obj b = a;
//a == b

Try implementing equals():

public static class Obj {
    public int i;

    public Obj(int i) {
        this.i = i;
    }

    @Override
    public boolean equals(Object other) {
        if (other == this) return true;
        if (!(other instanceof Obj) || other == null) return false;
        return i == ((Obj)other).i;
    }

    @Override
    public int hashCode() {
        return i;
    }
}

Then, you can use if(o1.equals(o2)) {. However, this is not really a good example, read this (link) for more information.

edited Jan 01 '13 at 20:56

answered Jan 01 '13 at 20:51

tckmn

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Please also override `hashCode()`. Not doing it is a bug. – JB Nizet Jan 01 '13 at 20:55
1

@JBNizet that's why I said `However, this is not really a good example` – tckmn Jan 01 '13 at 20:56

score 2 · Answer 2 · answered Jan 01 '13 at 20:50

2

== returns true only if you are comparing the same object [i.e. the same memory location].

If you want to compare objects by their fields, you have to overload the equals() method, in order to induce an equivalence relation over them.

public boolean equals(Object other){

    return this.i == other.i;

}

Be sure that the equals() method respects reflexivity, symmmetry, transitivity.

answered Jan 01 '13 at 20:50

moonwave99

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moonwave99 typed faster than me. [this is a good tutorial for overloading equals.](http://code.hammerpig.com/how-to-compare-two-objects-in-java-by-overriding-equals.html) – Levi Jan 01 '13 at 20:51
3

Please also override `hashCode()`. Not doing it is a bug. – JB Nizet Jan 01 '13 at 20:55

Eng.Fouad · Answer 3 · 2013-01-01T20:56:18.480

1

== compares the reference equality, i.e if they refer to the same object in the memory.

You need to override equals() method, and use it whenever you want to compare their values. Also, override hashCode() which is used by HashMap for example.

edited Jan 01 '13 at 20:56

answered Jan 01 '13 at 20:50

Eng.Fouad

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score 0 · Answer 4 · answered Jan 01 '13 at 20:51

The == operator does not check for equality in class data; rather, it checks to see if they are the same location in memory. If you did o2 = o1 instead of initializing them the same way, they would be the same location in memory, so o2==o1 would return true. However, since they were initialized some separately, it returns false. Instead, you should define an equals method and implement that.

Two objects with same data are not the same?

4 Answers4