HashSet and Equals and HashCode

[Neeraj Vij]

Hi All,

I have not overridden the equals and hashcode method in the below mentioned code. I am unable to get, why contains method returns true.

package test;

import java.util.HashSet;
import java.util.Set;
import java.util.TreeSet;

public class CheckEquals {
	int i;
	String name;
	public static void main(String args[]){
		
		CheckEquals checkEquals = new CheckEquals();
		
		checkEquals.i=5;
		checkEquals.name="neeraj";
		
		CheckEquals checkEquals1 = new CheckEquals();
		
		checkEquals1.i=15;
		checkEquals1.name="neeraj1";
		
		CheckEquals checkEquals2 = new CheckEquals();
		CheckEquals checkEquals3 = new CheckEquals();
		checkEquals2=checkEquals1;
		
		Set testSet = new HashSet();
		
		testSet.add(checkEquals);
		testSet.add(checkEquals1);
		
		System.out.println(" testSet.find -> " + testSet.contains(checkEquals));
		System.out.println(" testSet.find -> " + testSet.contains(checkEquals1));
		System.out.println(" testSet.find -> " + testSet.contains(checkEquals3));
		
		System.out.println(" checkEquals.equals(checkEquals1) -> " + (checkEquals.equals(checkEquals1)));
		
		System.out.println(" checkEquals ==  checkEquals1 -> " + (checkEquals==checkEquals1));
		System.out.println(" checkEquals2 ==  checkEquals1 -> " + (checkEquals2==checkEquals1));
		
		System.out.println(" checkEquals2.hashCode -> " + checkEquals2.hashCode());
		
		System.out.println(" checkEquals1.hashCode -> " + checkEquals1.hashCode());
		
		System.out.println(" checkEquals.hashCode -> " + checkEquals.hashCode());
		
		
	}

}

Output is as follows:

 testSet.find -> true
 testSet.find -> true
 testSet.find -> false
 checkEquals.equals(checkEquals1) -> false
 checkEquals ==  checkEquals1 -> false
 checkEquals2 ==  checkEquals1 -> true
 checkEquals2.hashCode -> 8567361
 checkEquals1.hashCode -> 8567361
 checkEquals.hashCode -> 17523401

[Mohamed Sanaulla]

Neeraj Vij wrote:Hi All,

I have not overridden the equals and hashcode method in the below mentioned code. I am unable to get, why contains method returns true.

Which "true" are you not able to understand? If its this: "checkEquals2 == checkEquals1 -> true" - Then in your code you explicitly assigned them to be equal: checkEquals2=checkEquals1;

[Christophe Verré]

contains() uses equals(). If you don't override the equals() method, an instance of a class will only be equal to itself. If you add A into the set, and check if the set contains A, it will be true, because A.equals(A) is true.

What do you mean you can't get ?

[Neeraj Vij]

Hi,
thanks both for your inputs.

I was expecting the following to return false

System.out.println(" testSet.find -> " + testSet.contains(checkEquals));   
System.out.println(" testSet.find -> " + testSet.contains(checkEquals1));   

as it did for

System.out.println(" checkEquals.equals(checkEquals1) -> " + (checkEquals.equals(checkEquals1)));   

then why its required to overide equals when using custom objects with hashset?

[Christophe Verré]

checkEquals.equals(checkEquals1) returns false because checkEquals and checkEquals1 are different instances. testSet.contains returns true for the reason I have explained above.

then why its required to overide equals when using custom objects with hashset?

When you don't want two objects meaningfully equal to be in the same set. Let's say that you don't want two CheckEquals instance with the same name to be there. With your actual code, you can insert two different instances with the same name :

		Set<CheckEquals> testSet = new HashSet<CheckEquals>();
		CheckEquals checkEquals4 = new CheckEquals();		
		CheckEquals checkEquals5 = new CheckEquals();
		checkEquals4.name = "Neeraj";
		checkEquals5.name = "Neeraj";
		System.out.println(testSet.add(checkEquals4));
		System.out.println(testSet.add(checkEquals5));
		

Both will print true, proving that both instances were inserted in the set.
If you want to avoid that, you'll have to override equals() and hashCode():

	@Override
	public boolean equals(Object obj) {
		if ( !(obj instanceof CheckEquals)) {
			return false;
		}
		CheckEquals to = (CheckEquals)obj;
		return name.equals(to.name);
	}
	
	@Override
	public int hashCode() {
		return name.hashCode();
	}

(not testing for null here)
Now, true and false will be output. The second instance will not be inserted because the equally same object was already in there.

[Neeraj Vij]

Thanks a ton for your clarification...I got it...it will work when you have the exact reference

[Christophe Verré]

Same. containsKey will use the equals() method, which explains why youre getting true. If you don't override hashCode, all your objects will end up in the same bucket, which is not very efficient if you have many objects in your map. That won't prevent you from fetching them though.