Overriding equals() when using Lists in Java

Question

I am in a bit of confusion. Suppose I have class A and some List<A> in some other class B.

So, because I am using a list, duplicates are allowed. Because duplicates are allowed (that means no comparison needed), do I have to override method equals() and hashCode() in class A?

So the emphasis here is on List (duplicates allowed - no need for comparison).

And I should override these 2 methods when I am using Set (duplicates NOT allowed - we need to define how objects will be compared)? (99% sure this is correct answer, just want to check it and make sure I got 100% correct answer for both: List and Set)

Note: I did not find answer on stack because no one asked excact same thing with using lists and duplicates allowed.

The question about `Set` should be a separate post, not bundled into this question. — Louis Wasserman, Sep 30 '20 at 20:40
Have you checked out those two? [Why do I need to override the equals and hashCode methods in Java?](https://stackoverflow.com/q/2265503/12323248), [What issues should be considered when overriding equals and hashCode in Java?](https://stackoverflow.com/q/27581/12323248) — akuzminykh, Sep 30 '20 at 20:43

Louis Wasserman · Answer 1 · 2020-09-30T21:00:18.090

3

If you do not plan to ever compare any A objects to each other -- meaning you're neither using a Set, nor List methods like indexOf or contains -- then you do not need to implement equals or hashCode.

However, if you do ever intend to use indexOf or contains -- or if you plan to test your code, e.g. by asserting that the A in the List is equal to some expected A -- then you do need to implement equals, and it is poor practice to implement equals without also implementing hashCode. You don't need a good implementation of hashCode, but you should have one.

edited Sep 30 '20 at 21:00

answered Sep 30 '20 at 20:40

Louis Wasserman

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Why I do not need to implement equals (and hashCode) when using Set? How Set will know what are duplicates? Based on what? – Cluv Sep 30 '20 at 20:48
@Cluv: I believe you're misreading the answer, which says the opposite. I've tweaked the wording to be clearer. – Louis Wasserman Sep 30 '20 at 20:59
I meant this. Let's suppose I will not compare any A objects to each other. But I want to create Set set = new Set<>(). After creating this Set "set", I want to just add 3 objects A to set. Also, let's suppose class A has 3 fields: a,b,c. Also suppose that 2 of these 3 added objects have the same a,b, and c fields. My question is: How Set "set" will know that those 2 objects A are the same? Or "set" will not know - and he will store all of those 3 objects A – Cluv Sep 30 '20 at 21:09
@Cluv , it's not that the comparison amongst the objects is required only while adding them into Sets. Let's say the Set has all the objects of Type A now that you wanted to add, later you invoke contains() for checking if some object is present in this set or not. To accomplish this a comparison will be done on the objects of Type A. If you do no specify the equals() then the default implementation (Object class) will be used, which is not generally we we would want – Mayank Madhav Sep 30 '20 at 21:15
@Cluv: The set _won't_ know, and will store all three objects. That's why `equals` and `hashCode` are necessary for _anything_ going into a `Set`. – Louis Wasserman Sep 30 '20 at 21:16
@Cluv I think if you don't define `equals` and `hashCode` (together), then just the reference address is used with the default implementations, which will obviously always return "not equal" for objects that seem equal. Anyone, please feel free to correct me. – akuzminykh Sep 30 '20 at 21:20
Yes, that is the answer I was supposing it was correct. Thanks! – Cluv Sep 30 '20 at 21:22
Learning Java after C++ and that is why I have some doubts :) – Cluv Sep 30 '20 at 21:23
And to make it clear. For JUST adding objects A in LIST, no equal and hashCode is needed? :) – Cluv Sep 30 '20 at 21:24
As I stated: that's true...if you're not going to use `contains`, `indexOf`, or any other such method on that `List`, and not going to write tests that verify the contents of that list. – Louis Wasserman Sep 30 '20 at 21:25
@Cluv Is your question answered at this point? Or is there anything unclear? – akuzminykh Sep 30 '20 at 21:33

Overriding equals() when using Lists in Java

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