hashCode() & equals() method test

[Shailesh Narkhede]

Hi Ranchers,

Just I want confirmation on ....
Whan we create any class and I will overrides equals method for comparing class accoring to any variable in that class.

 package ss;

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

public class HashSetTest {

	private String name;
	
	public HashSetTest(String name) {
		this.name = name;
	}
	
	public boolean equals(Object object) {
		boolean result = false;
		if (object instanceof HashSetTest) {
			HashSetTest newPerson = (HashSetTest) object;
			if(this.name.equals(newPerson.name)) {
				result = true;
			}
		}
		System.out.println("result  " + result);
		return result;
	}
	public static void main(String[] args) {
		   try{
			   Set set = new HashSet();
			   HashSetTest hashSetTest01 = new HashSetTest("arvind");
			   HashSetTest hashSetTest02 = new HashSetTest("arvind");
			   
			   System.out.println("equals ==> "+hashSetTest01.equals(hashSetTest02));// this will return false if we comment equals 
			   
		   } catch (Exception e) {
			e.printStackTrace();
		   }
		}
}
   

Now as shown in above class we can compare objects of HashSetTest class according to property name of that class.
this is what we get from overriding equals method of Object class in our own class.(we did not override hashCode() method).

Now I am adding objects in HashSet. for that I had overridden both hashCode() & equals() methods.as shown below.

 package ss;


import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

public class HashSetTest1 {

	private String name;
	
	public HashSetTest1(String name) {
		this.name = name;
	}
	
	public boolean equals(Object object) {
		boolean result = false;
		if (object instanceof HashSetTest1) {
			HashSetTest1 newPerson = (HashSetTest1) object;
			if(this.name.equalsIgnoreCase(newPerson.name)) {
				result = true;
			}
		}
		System.out.println("result  " + result);
		return result;
	}
	
	public int hashCode() {
		return 1;
	}
	public String toString() {
		StringBuffer buffer = new StringBuffer();
		buffer.append(name);
		return buffer.toString();
	}
	public static void main(String[] args) {
		   try{
			   Set set = new HashSet();
			   HashSetTest1 hashSetTest01 = new HashSetTest1("arvind");
			   HashSetTest1 hashSetTest02 = new HashSetTest1("porlekar");
			   HashSetTest1 hashSetTest03 = new HashSetTest1("Arvind");
			   
			   set.add(hashSetTest01);
			   set.add(hashSetTest02);
			   set.add(hashSetTest03);
			   
			   Iterator itr =  set.iterator();
			   while(itr.hasNext())
				   System.out.println(itr.next().hashCode());
			   
			   
			   System.out.println("\n "+set.size() + " set size  - "+ set.toString());
			   
		   } catch (Exception e) {
			e.printStackTrace();
		   }
		}
}
   

from above example we can say if we override hashCode() method then we should also override equals() method.
As we returning 1 from hashCode() method, for each object of this class it will returns same hashCode i.e. 1,
Now when we add element in HashSet it will check hashcode of object that we going to add to already added elements,
if it is same then it will call equals() on object by passing other object as parameter. In equals we can do custom equal checking as
we checked in frist example. And if it is true then HashSet will not add new Object

[Ankit Garg]

Everything that you said seems fine to me. The only thing that you should remember is that the hashCode method that you've implemented is not very efficient. The hashCode method should be designed in such a way that it returns unequal hashCode for unequal objects. This is also mentioned in the docs

hashCode method API docs wrote:It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hashtables.

[Leonardo Carreira]

Hi..

i'm wondering about, as we know that with the Java IDE like Netbeans, we can generate hashCode() & equals() method..
is this code generated enough?, or do we've to add additional code?..

sorry for the basic question..

Thanks..

[Ankit Garg]

i'm wondering about, as we know that with the Java IDE like Netbeans, we can generate hashCode() & equals() method..
is this code generated enough?, or do we've to add additional code?..

I don't know what type of code Netbeans generates for the hashCode and equals methods, but I'm sure won't be appropriate in many cases. Netbeans doesn't know what properties you actually want to compare in your class...

[Leonardo Carreira]

Thanks Ankit Garg for your reply..

This is equals and hashCode code that generated by Netbeans :

 
public class Employee {
    private Long id;
    private String code;
    private Date birthDate;
    private double salary;

    @Override
    public boolean equals(Object obj) {
        if (obj == null) {
            return false;
        }
        if (getClass() != obj.getClass()) {
            return false;
        }
        final Employee other = (Employee) obj;
        if (this.id != other.id && (this.id == null || !this.id.equals(other.id))) {
            return false;
        }
        if (this.code != other.code && (this.code == null || !this.code.equals(other.code))) {
            return false;
        }
        if (this.birthDate != other.birthDate && (this.birthDate == null || !this.birthDate.equals(other.birthDate))) {
            return false;
        }
        if (this.salary != other.salary) {
            return false;
        }
        return true;
    }

    @Override
    public int hashCode() {
        int hash = 7;
        hash = 97 * hash + (this.id != null ? this.id.hashCode() : 0);
        hash = 97 * hash + (this.code != null ? this.code.hashCode() : 0);
        hash = 97 * hash + (this.birthDate != null ? this.birthDate.hashCode() : 0);
        hash = 97 * hash + (int) (Double.doubleToLongBits(this.salary) ^ (Double.doubleToLongBits(this.salary) >>> 32));
        return hash;
    }
  
}   

[Shailesh Narkhede]

Ankit, Leonardo Thanks for reply ,

I dont want duplicate object in my HashSet (that means should not have same name property as I overrides equals() method).
for achieving that I was returning 1 from overridden hashCode() method.

How can I achieve uniqueness in HashSet by returning different hashCodes from hashCode() method.

[Ankit Garg]

@Leonardo, the equals method implementation created by Netbeans seems wrong to me. If you look at any of the conditions for example

if (this.id != other.id && (this.id == null || !this.id.equals(other.id))) {  
      return false;  
}

Now if I call the equals method on a object and pass the same object as parameter (i.e. emp.equals(emp);), then this.id will be equal to other.id, thus the condition this.id != other.id will result in false and the equals method will return false while it should return true.

@Shailesh, in your hashCode method you can return something like return name.length(). This will return different hash code for objects whose name property have different length. This will give better performance with HashSet. We can't return name.hashCode() as you are using equalsIgnoreCase in your equals method...

[Minhaj Mehmood]

Shailesh Narkhede wrote:
How can I achieve uniqueness in HashSet by returning different hashCodes from hashCode() method.

following is code to achieve unique hashCode for unique objects
(Assume that you have a class in which there is instance variable "id" type of int)

public int hashCode() {
	final int prime = 31;
	int result = 1;
	result = prime * result + id;
	return result;
}

[Minhaj Mehmood]

Leonardo Carreira wrote:Thanks Ankit Garg for your reply..

This is equals and hashCode code that generated by Netbeans :

 
public class Employee {
    private Long id;
    private String code;
    private Date birthDate;
    private double salary;

    @Override
    public boolean equals(Object obj) {
        if (obj == null) {
            return false;
        }
        if (getClass() != obj.getClass()) {
            return false;
        }
        final Employee other = (Employee) obj;
        if (this.id != other.id && (this.id == null || !this.id.equals(other.id))) {
            return false;
        }
        if (this.code != other.code && (this.code == null || !this.code.equals(other.code))) {
            return false;
        }
        if (this.birthDate != other.birthDate && (this.birthDate == null || !this.birthDate.equals(other.birthDate))) {
            return false;
        }
        if (this.salary != other.salary) {
            return false;
        }
        return true;
    }

    @Override
    public int hashCode() {
        int hash = 7;
        hash = 97 * hash + (this.id != null ? this.id.hashCode() : 0);
        hash = 97 * hash + (this.code != null ? this.code.hashCode() : 0);
        hash = 97 * hash + (this.birthDate != null ? this.birthDate.hashCode() : 0);
        hash = 97 * hash + (int) (Double.doubleToLongBits(this.salary) ^ (Double.doubleToLongBits(this.salary) >>> 32));
        return hash;
    }
  
}   

May be this could help?
it`s generated by eclipse.

import java.util.Date;
public class Employee {
	private Long id;  
	private String code;  
	private Date birthDate;  
	private double salary;
	
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result
				+ ((birthDate == null) ? 0 : birthDate.hashCode());
		result = prime * result + ((code == null) ? 0 : code.hashCode());
		result = prime * result + ((id == null) ? 0 : id.hashCode());
		long temp;
		temp = Double.doubleToLongBits(salary);
		result = prime * result + (int) (temp ^ (temp >>> 32));
		return result;
	}
	
	public boolean equals(Object obj) {
		if (this == obj)
			return true;
		if (obj == null)
			return false;
		if (!(obj instanceof Employee))
			return false;
		Employee other = (Employee) obj;
		if (birthDate == null) {
			if (other.birthDate != null)
				return false;
		} else if (!birthDate.equals(other.birthDate))
			return false;
		if (code == null) {
			if (other.code != null)
				return false;
		} else if (!code.equals(other.code))
			return false;
		if (id == null) {
			if (other.id != null)
				return false;
		} else if (!id.equals(other.id))
			return false;
		if (Double.doubleToLongBits(salary) != Double
				.doubleToLongBits(other.salary))
			return false;
		return true;
	}
}