Using own implementation of HashSet.add()

Question

I am using a HashSet to add unique Users to the hashset. These users are updated frequently from an online source and when it reads it in from there it gets a new object each time so all the Users get added again.

I have a UserID variable in my user class and would like to use this to add users to the HashSet and ensure uniqueness.

private HashSet<User> userHashSet = new HashSet<User>();

userHashSet.add(user);
//based on the users id it will either add or not

I want to do this without losing efficiency and preferably only one HashSet

score 4 · Answer 1 · answered Jul 16 '15 at 00:52

4

You should implement the equals and hashCode method in your User class.

As an example:

@Override
public int hashCode() {
    return getUserId().hashCode();
}

@Override
public boolean equals(Object obj) {
    return obj instanceof User &&
        getUserId().equals(((User) obj).getUserId());
}

answered Jul 16 '15 at 00:52

kevin

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That was exactly the solution! Thank you – hakobob Jul 30 '15 at 00:26

score 1 · Answer 2 · edited May 23 '17 at 11:58

You need to implement the hashCode and equals methods in your User class. So long as you implement those two methods correctly then you will get unique entries in your HashSet.

There are many articles about how to implement these correctly (e.g. What issues should be considered when overriding equals and hashCode in Java?), but the easiest way to do it is to get your IDE to auto-generate those methods, or use the Apache Commons helpers.

Using own implementation of HashSet.add()

2 Answers2