CODEWITHSUNDEEP
javaobjectcollectionshashequals
Simon Kiely
Simon Kiely

Reputation: 6050

Why can equals not be inferred in HashSets where the type is specified?

I don't understand something silly in Java and was hoping you could clear it up for me.

I have defined Hashset<Point> myHashSet = new HashSet<Point>();

Then, I create two equal Point points, Point p1 and Point p2, and put them in different variables/memory locations. Then I override the .equals() method for public boolean equals(Point other) and added my first point, p1, to the HashSet.

Then I call System.out.println(myHashSet.contains(p2)); // prints false

Why can the compiler not infer "Oh, this hashset is of type Point" from the when it is being created and say "I should check to see if Point has overridden the default equals method...yup, let's call that one!".

Instead, I believe it calls the generic equals method for objects, thus comparing memory location, I believe ?

Is the reasoning for this simply that the HashSet could contain a subclass of Point which uses a different Equals method ? This is the only reason I can see for the current behaviour, though I am sure I am overlooking something :). Thanks a lot.

Upvotes: 1

Views: 139

Answers (4)

AmitG
AmitG

Reputation: 10553

Try adding @Override annotation. You must have misspelled equals method and its signature.

    @Override
    public boolean equals(Object obj) {
        // TODO Auto-generated method stub
        return super.equals(obj);
    }

Upvotes: 1

Bystam
Bystam

Reputation: 53

Your problem lies in public boolean equals(Point other), because that is NOT the default equals method defined in the Object class.

You must override public boolean equals(Object other) (notice Object instead of Point) if you wish the HashMap to use your implementation. See the the documentation for Object and note also that if you override equals you SHOULD also override the hashCode() method.

Upvotes: 2

Dmitry
Dmitry

Reputation: 902

You should override both equals() and hashCode() according to this doc http://docs.oracle.com/javase/6/docs/api/java/lang/Object.html#hashCode%28%29, since HashSet use both of these methods to check if objects are equal

Upvotes: 2

Peter Lawrey
Peter Lawrey

Reputation: 533870

Collections use Object.equals(Object) which you have to override. If you create another method like equals(Point) it won't call it.

Instead, I believe it calls the generic equals method for objects, thus comparing memory location, I believe ?

Yes, as the only method which HashSet can generically call is equals(Object)

Is the reasoning for this simply that the HashSet could contain a subclass of Point which uses a different Equals method ?

The HashSet has no way of know that you want to use this method instead at runtime.

Upvotes: 6

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