Question

I have a class

final class BuildingPair {

    int mBA;
    int mBB;

    public BuildingPair(int pBuildingA,int pBuildingB) {
        mBA = pBuildingA;
        mBB = pBuildingB;
    }

    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + mBA;
        result = prime * result + mBB;
        return result;
    }

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

I want to compare two objects , and when both have the same buildings ids they are equal

so they need to be equal in both directions when :

BuildingPair(1,2) vs BuildingPair(2,1) 
BuildingPair(1,2) vs BuildingPair(1,2)
BuildingPair(2,1) vs BuildingPair(1,2)

i think equals method is ok, but hashcode is wrong.

Answer 1

You need something that computes the same result whether passed A,B or B,A. There may be far more subtle solutions, but I’d probably just go for:

@Override
public int hashCode() {
    return mBA * mBB;
}

Or anything else which uses an operator that is commutative.

---

Alternatively, you could change your constructor so that it always stores min(a,b) in mBA and max(a,b) in mBB – you can then simplify your comparison code and keep your hash code as it currently is.