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Overriding hashCode when using HashMap, HashSet, etc.?

The question is in the title. I am using HashMap

like this:

Map<Player, Partner> m = new HashMap<Player, Partner>();
//do some on m

If both Player

and Partner

are only POJO

representing the data model.

public class Player{

    private int id;

    private String name;
    //etc, GET, SET
}

public class Partner{

    private int id;

    private String name;
    //etc, GET, SET
}

I would like to say that two objects of these classes are equivalent to iff , they have the same id

. So, should I write hashCode

something like

public int hashCode(){
    return id;
}

Is this the correct way to do it and why should I ever use it when I am going to use HashMap or something similar?

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2 answers


This hashCode

will work. You will also need to override in a equals

sequential manner:



@Override
public boolean equals(Object other)
{
    if (!(other instanceof Player))
        return false;
    Player op = (Player) other;
    return id == op.id;
}
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Objects are stored in hash codes. Let's say if you have 100 objects. If 50 out of 100 return a hash code value of "123" and the other 50 return "124", they are stored in two separate buckets and therefore reduce the lookup time.

When looking for the first target one has to find the bucket that the object belongs to, after it finds the bucket, then calls the equls method to find the object



Instead of moving all objects in one place (finding is harder), hash-based collections are combined together with objects that have the same hash value, each such group is stored in a place called a hash bucket

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