Question

JavaDoc defines set as :

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

To verify the same, i created a very simple program:

import java.util.HashSet;

public class CheckHashSet {
    public static void main(String[] args) {
        HashSet<Employee> set = new HashSet<Employee>();
        set.add(new Employee(10));
        set.add(new Employee(10));
        System.out.println(set.size());
        System.out.println(new Employee(10).equals(new Employee(10)));
    }

    private static class Employee implements Comparable<Employee> {
        private final int id;
        public Employee(int id) {
            this.id = id;
        }
        @Override
        public int compareTo(Employee o) {
            return this.id - o.id; 
        }

        @Override
        public boolean equals(Object obj) {
            if(obj instanceof Employee) {
                return compareTo((Employee)obj)==0;
            }
            return false;
        }
    }
}

The output of the program is

2
true

This means new Employee(10).equals(new Employee(10)) returns true whereas set.add(new Employee(10)); set.add(new Employee(10)); adds the object twice.

What is wrong with my code?

Answer 1

Your Employee class doesn’t override hashCode – it needs to do so in order for any hash-based collection to work.

For example:

@Override
public int hashCode() {
    return id;
}