Using java.util.HashSet for Custom Class

[Rajasekar Elango]

Hi,

I am trying to use java.util.HashSet for storing collection of non-duplicate values of my own custom class.

Here is the sample program which I tried.

 
public class UsingSet {
 
String value;
 
public UsingSet(String value){
this.value = value;
}
 
public String toString() {
return value;
}
 
public boolean equals(UsingSet another) {
 
System.out.println("equals called");
return (this.value.equals(another.value) );
        }
 
public int hashCode() {
 
    int hash = value.hashCode();
System.out.println("hashcode called" + hash);
return hash;
}
 
public static void main(String args[]) {
 
java.util.Set s = new java.util.HashSet();
s.add(new UsingSet("a"));
s.add(new UsingSet("b"));
s.add(new UsingSet("a"));
s.add(new UsingSet("b"));
s.add(new UsingSet("a"));
 
s.add(new Integer(1));
s.add(new Integer(1));
 
System.out.println("s = " + s);
 
}
}
 

Output:

hashcode called97
hashcode called98
hashcode called97
hashcode called98
hashcode called97
s = [a, a, a, 1, b, b]

The UsingSet class has a single field value and two UsingSet instances are equal if their values are equal.

I dont know how add() method works to compare objects. But from the above output the add() method is not using the overloaded equals method (the print statements are getting not printed) and it adds to set even if attribute value has the same value.

Overloading hashCode() method also doesn't seems to work.

I tried the same with java.util.TreeSet() by passing my own comparator to constructor and it works fine.

Is it not possible with HashSet()..?

But again, trying the same for Integer class (look at last two add statements and output) works fine and Integer instances considered as equal if their values are equal..

So I think this should be possible..

Can you please help me if anyone know how to implement this.

Thanks very much,

Raja.

[Petr Blahos]

Hi Rajasekar,

Your signature of equals is wrong. It should be:

public boolean equals(Object o) {
    UsingSet another = (UsingSet)o;
    ....
}

In your code you do not override but overload, therefore
the old good Object.equals is called.

P.
[ October 06, 2004: Message edited by: Petr Blahos ]

[Rajasekar Elango]

Thanks Petr Blahos

Yes It works now.

[Tushar Mishra]

Hi,

I am facing a similar problem. The equals method seems to have been defined correctly
I am trying to use a custom class for HashSet. I have overridden the equals and hashcode method.

Observations
1. if I use Hashset, both the employees get added. However, e1.equals(e2) returns true.
2. If I use TreeSet, Only 1 object gets added.

While debugging, Employee -> equals method doesn't seem to get called from HashSet -> add method

What mistake am I making here ?

   

public class Employee implements Comparable<Object>{

	private int id;
	private String name;

	public Employee(int i, String n){
		this.id=i;
		this.name=n;
	}

	public int getID(){
		return id;
	}

	public String getName(){
		return name;
	}
	
	public String toString()
	{
		return (name + " " + id);
	}
	
	public int compareTo(Object obj){
		int returnValue;
		if(id ==((Employee) obj).getID())
			returnValue=0;
		else
			if(id >((Employee) obj).getID())
				returnValue = 1;
			else
				returnValue =-1;
		
		return returnValue;
	}
	
	public boolean equals(Object obj)
	{
		if(!(obj instanceof Employee))
			return false;
		
		return (id == ((Employee) obj).getID()); 
	}
	
	public int hashCode()
        {  
		return name.hashCode();  
	} 
	
}

 

 

import java.util.*;

public class HashSetTest {
	
	public static void main(String[] args) 
	{
		HashSetTest hs = new HashSetTest();
		hs.doStuff();
	}
	
	
	public void doStuff()
	{
		
		HashSet<Employee> empSet = new HashSet<Employee>();

		Employee e1 = new Employee(123, "aa");
		Employee e2 = new Employee(123, "bb");

		empSet.add(e1);
		empSet.add(e2);

		System.out.println("\nEmployee size :" + empSet.size());
		System.out.println(e1.equals(e2));
	}
	

}

[Rob Spoor]

Please don't kick a 4 year old thread.

[Ernest Friedman-Hill]

Your hashCode() method is based on the name, but equals is based on the id. The behavior of the two methods is therefore inconsistent. In particular, the two objects you're testing with are equals(), but they have different hashCode() values. The HashSet will only call equals() to check for the equality of two objects with the same hashCode().

[Campbell Ritchie]

Also your equals() method does not test whether the two Objects are of the same class; you can suffer a ClassCastException.

Google for Angelika Langer Java equals method and you will find some useful information there.

[Ernest Friedman-Hill]

Campbell Ritchie wrote:Also your equals() method does not test whether the two Objects are of the same class; you can suffer a ClassCastException.

equals() does indeed, although compareTo() does not.

[Tushar Mishra]

Hi Ernest / Campbell,

Thanks for your quick reply.

I changed the hashcode() method and made it consistent with equals() method as below. I am getting the expected results now. Thanks for clarifying the concept regarding equals() and hashcode() methods.

  

	public int hashCode() {  

		return id;  
	} 

  

I am sorry for the late reply.