Java Set removes “complex object”

Question

I'm always confused about the Java Collections (set, map) remove "complex object", which I mean some self-defined class rather than just primitive type.

I'm experimenting like:

public class Main {
    public static void main(String[] args) {        
        // set
        Set<Node> set = new HashSet<>();
        set.add(new Node(1,2));
        set.add(new Node(3,4));
        System.out.println(set);
        
        set.remove(new Node(1,2));
        System.out.println(set + "\n");
            
            
        // tree set
        TreeSet<Node> tset = new TreeSet<>((a, b) -> a.name - b.name);
        tset.add(new Node(1,2));
        tset.add(new Node(3,4));
        System.out.println(tset);
        
        tset.remove(new Node(1,2));
        System.out.println(tset);
    }
}

class Node {
    int name;
    int price;
    Node(int name, int price) {
        this.name = name;
        this.price = price;
    }
}

In the example above, the printout would be:

 Set:
[Node@5ba23b66, Node@2ff4f00f]
[Node@5ba23b66, Node@2ff4f00f]

 TreeSet:
[Node@48140564, Node@58ceff1]
[Node@58ceff1]

Obviously, the general Set can't remove new Node(1, 2) , which is treated as a different object. But interestingly the TreeSet can remove, which I think because the hashing code is based on the lambda comparator I defined here?

And if I change to remove new Node(1, 6) , interestingly it's the same printout, where obviously the remove in TreeSet is based on only the name value.

I think I still lack of deep understanding of how Set build up hashing and how comparator would affect this.

Answer 1

For HashMap and HashSet , you need to overwrite hashCode() and equals(Object) , where if two objects are equal, they should have equal hash codes. Eg, in your case, you could implement it like this:

@Override
public boolean equals(Object o) {
    if (o == null || getClass() != o.getClass()) {
        return false;
    }
    Node node = (Node) o;
    return name == node.name && price == node.price;
}

@Override
public int hashCode() {
    return Objects.hash(name, price);
}

For TreeMap and TreeSet , the notion of equality is based on a comparison (whether the class implements Comparable , or you supply a custom Comparator ). In the code you provided, you have a custom Comparator that only takes the name in to consideration, so it would consider any two Node s with the same name as being equal, regardless of their price .

Answer 2

javadoc comes to rescue

https://docs.oracle.com/javase/7/docs/api/java/util/Set.html#remove(java.lang.Object)

Removes the specified element from this set if it is present (optional operation). More formally, removes an element e such that (o==null ? e==null : o.equals(e)), if this set contains such an element. Returns true if this set contained the element (or equivalently, if this set changed as a result of the call). (This set will not contain the element once the call returns.)

So just change your Node class and override equals(Object o) method with your own.