HashSet, remove and contains methods did not work

Issue

I wrote a generic class to rapresent an Object that has two generic types, like this:

public class Arco<N extends Comparable<N>,L extends Comparable<L>> implements Comparable<Arco<N, L>> {
    private N node1, node2;            //start and end node
    private L label;                   //node label

    //Arch constructors
    public Arco() {
        node1 = node2 = null;
        label = null;
    }

    public Arco(N x1, N y1, L l) {
        node1 = x1;
        node2 = y1;
        label = l;
    }

in this class I have overrided the methods equals, compareTo and hashcode like this:

//override methods to compare Archs
public boolean equals(Arco<N, L> a) {
    if (a instanceof Arco) {
        Arco<N, L> arc = a;
        return (node1.equals(arc.node1) && node2.equals(arc.node2) && label.equals(arc.label));
    }

    return false;
}

@Override
public int hashCode() {
    int result = label != null ? label.hashCode() : 0;
    result = 31 * result + (node1 != null ? node1.hashCode() : 0);
    result = 31 * result + (node2 != null ? node2.hashCode() : 0);
    return result;
}

@Override
public int compareTo(Arco<N, L> a) {
    int i = this.label.compareTo(a.label);
    if (i == 0) {
        int j = this.node1.compareTo(a.node1);
        if (j == 0)
            return this.node2.compareTo(a.node2);
        else
            return j;
    } else {
        return i;
    }
}

now in another class I have an Hashmap like this that contains a Set of Arco objects:

public class Grafo <N extends Comparable<N>,L extends Comparable<L>> {
    HashMap<N, Set<Arco<N, L>>> nodi; //lista dei nodi
    int nArchi; //numero Archi
    boolean diretto; //informazione sul tipo di grafo diretto= true non diretto =false

    //Costructor
    public Grafo(boolean diretto) {
        nodi = new HashMap<N, Set<Arco<N,L>>>();
        nArchi = 0;
        this.diretto = diretto;
    }

in this last class i wrote a method:

public boolean containsArch(N x, N y, L label){
    Arco<N,L> a = new Arco<N,L>(x,y,label);
    if (nodi.containsKey(x)&&nodi.containsKey(y)){
        return nodi.get(x).contains(a);
    }
    return false;
}

my problem now is that sems that .contains(a) does not work , I supposed is because of the comparison wrong from the Arco object but I don't know how to do it in a better way. I noticed that also .remove(Arco) doesn't work

I hope you can help me. thank you!

Solution

Unfortunately, you didn't override equals. Instead of

 public boolean equals(Arco<N,L> a) {

you must write

 public boolean equals(Object a) {

and use instanceof and cast appropriately.

Answered By - Louis Wasserman