@Override
    public boolean equals(Object that) {
        if (that == null) {
            return false;
        }
        if (that == this) {
            return true;
        }
        if (!(that instanceof ArtistPair)){
            return false;
        }
        ArtistPair thatPair = (ArtistPair) that;
        return thatPair.a1 == this.a1 && thatPair.a2 == this.a2;
    }