Java : How to check if a key is exists in a Hashmap

Question

I have defined a hashmap of type :

HashMap<Position, Double> list = new HashMap<>();

Where Position is class with two attributes x and y .

I would like to verify if a position is already in the list or not, I have tried this :

public void addToList(Position p, double somme) {
    if (this.list.containsKey(p)) {
        this.list.replace(p, this.list.get(p) + somme);//add the old value somme 
    } else {
        this.list.put(p, somme);
    }
}

I think I should be more specific and verify values of x any y instead of check if key is exist because each time the check failed to detect an exist position.

How can check if a position is in the list or not?

Answer 1

You should override the equals and hashCode methods of your Position object so that two elements with the same value for x and y are equals.

@Override
public boolean equals(Object obj) { 
    if (obj == this) {
        return true;
    }
    if (obj == null || obj.getClass() != this.getClass()) {
        return false;
    }

    Position otherPos = (Position) obj;
    return x == otherPos.x
            && y == otherPos.y;
}

@Override
public int hashCode() {
    return Objects.hash(x, y);
}