HashSet allows duplicates

Question

I can't seem to get a HashSet instance to work as expected. The code I used is as follows:

import testing.Subclass;
import java.util.HashSet;

public class tester {
  public static void main(String[] args) throws Exception {
    HashSet<Subclass> set = new HashSet<Subclass>();
    set.add(new Subclass("007812"));
    set.add(new Subclass("007813"));
    System.out.println("Set size " + set.size());
    set.add(new Subclass("007812"));
    System.out.println("Set size " + set.size());

    for(Subclass sub : set) {
      System.out.println(" sub acctNbr " + sub.getAcctNbr());
    }
  }
}

Subclass

public class Subclass implements Comparable<Subclass> {

  public Subclass(String acctNbr) {
    this.acctNbr = acctNbr;
  }
  private String acctNbr;
  public String getAcctNbr() {
    return this.acctNbr;
  }
  public int compareTo(Subclass other) {
    return this.getAcctNbr().compareTo(other.getAcctNbr());
  }

  public boolean equals(Subclass other) {
    if(other.getAcctNbr().equals(this.getAcctNbr()))
      return true;
    else
      return false;
  }
  public int hashCode() {
    return acctNbr.hashCode();
  }
}

This code outputs

sross@sross-workstation:~/Documents$ javac testing/Subclass.java
sross@sross-workstation:~/Documents$ javac tester.java
sross@sross-workstation:~/Documents$ java tester
Set size 2
Set size 3
 sub acctNbr 007812
 sub acctNbr 007812
 sub acctNbr 007813
sross@sross-workstation:~/Documents$

Answer 1

You need to override equals(Object). Instead of doing this you've implemented an equals method with signature equals(Subclass). Consequently your HashSet is using the default equals(Object) method defined on Object for equality testing.

The default equals(Object) implementation is based on object identity, and hence the set "allows" you to add two Strings that, whilst semantically equal, are not the same object.