How to extend the HashSet in Java?

Question

Let's say I have a CategoryKey class:

public class CategoryKey {
    public int layer;
    public int parent;
    public int child;
}

I try to put many instances of it within a Set , but as we know HashSet is not suitable to store instances of custom classes. So how can I extend the java class HashSet to meet my requirement? Or how can I create a new class implementing interface Set to solve the same problem?

Answer 1

but as we know HashSet is not suitable for store instances of customed class

Yes it is, so long as you write your class appropriate. In particular:

• You should override equals() and hashCode()
• You should make your type immutable
• You shouldn't use public fields

For example:

public final class CategoryKey {
  private final int layer;
  private final int parent;
  private final int child;

  public CategoryKey(int layer, int parent, int child) {
    this.layer = layer;
    this.parent = parent;
    this.child = child;
  }

  public int getLayer() {
    return layer;
  }

  public int getParent() {
    return parent;
  }

  public int getChild() {
    return child;
  }

  @Override public boolean equals(Object other) {
    if (!(other instanceof CategoryKey)) {
      return false;
    }
    CategoryKey otherKey = (CategoryKey) other;
    return layer == otherKey.layer
      && parent == otherKey.parent
      && child == otherKey.child;
  }

  @Override public int hashCode() {
    int hash = 23;
    hash = hash * 31 + layer;
    hash = hash * 31 + parent;
    hash = hash * 31 + child;
    return hash;
  }
}

Answer 2

Why is HashSet not suitable to store instances of custom classes?

As long as your class has a proper implementation of equals() and hashCode() it should work fine.

Answer 3

Override equals and hashCode method of CategoryKey to provide unique instance in Set

A collection that contains no duplicate elements. More formally, sets contain no pair of elements e1 and e2 such that e1.equals(e2), and at most one null element.

Refer here