LinuxQuestions.org - Custom java Map keys

- Programming (https://www.linuxquestions.org/questions/programming-9/)

- - Custom java Map keys (https://www.linuxquestions.org/questions/programming-9/custom-java-map-keys-553892/)

Custom java Map keys

Hi there,

I'm trying to use a map in Java where the keys are a class of my own:

Code:

public class IntPair {

        private final int x;

        private final int y;

                

        public IntPair(final int x, final int y) {

                this.x = x;

                this.y = y;

        }

                

        public IntPair() {

                x = y = 0;

        }

                

        public int getx() { return x; }

                

        public int gety() { return y; }



        public String toString() {

                return String.format("(%d,%d)",x,y);

        }

                                

} // class IntPair

Here is an example of use:

Code:

                

Map<IntPair,Integer> h = new HashMap<IntPair,Integer>();



IntPair r1 = new IntPair(10,2);

h.put(r1, 5);

h.put(new IntPair(10,2), 8);

h.put(new IntPair(10,2), 543);



for (IntPair r : h.keySet()) {

        System.out.println(r + ":" + h.get(r));

}

So, you see, I've used three times the IntPair(10,2), hence after executing this I would like the Map to have only one element, which key the IntPair(10,2) and with value 543, but I instead of that I get three different instances, each one pointing to one different value: 5, 8 and 543.

I've tried overriding the equals method and also the hashCode() method in IntPair(), but I have had no success: I still get three elements in the Map instead of one.

Is there a way to solve this? How?

I think java is using the references/addresses for the objects to calculate the actual keys that it uses internally in the Map. The problem is that I wouldn't want to keep the references to the keys in the Map. I would like, for instance, to add new values using h.put(new IntPair(x,y)) and then being able, in another place in the program, whether new IntPair(x,y) is a key in the Map...

Thank you a lot in advance.

What happens if you use :

Code:

h.put(r1, 5);

h.put(r1, 8);

h.put(r1, 543);

Thereby using the same object as the key.

Hi

I'm not very good in Java, but I guess you need to make an "equals" method in the IntPair class. I think the default "equals" method is the same as "==" so there will only be a match when the references are the same. How else can the HashMap know if two object are "equal"? And I think if you make an equals method, you should make an hashCode method as well. Two equal objects should make the same hashCode, I don't know if they will do that using the default hashCode.

Absolutely. You must override equals and hashcode as explained in Sun's Javadoc. If you tried this already, then sorry, you must have done something wrong.

Yves.

Hi,

Certainly if I use the code proposed by Seagull that works, but I do not want to do that. I want to drop the reference and then be able to create new objects --with the same content-- to retrieve the values in the Map.

theYinYeti: Could you give me some reference to look at (I mean in the javadoc)? I looked at the reference for Map and for that reason I tried overriding the equals and hashCode method. The equals is easy. I simply check that x and y are the same. For the hashCode I tried returning x*1000+y as a hashCode, but it did not work:

Code:

  // ... within class IntPair:

  public boolean equals(IntPair p) {

    return this.x == p.x && this.y == p.y;

  }



  public int hashCode() {

    return x*1000+y;

  }

Thank you again...

Ok, got it. I had to define the equals method as equals(Object o) and not as equals(IntPair p).

Thank you all for the answers.