hashCode() and equals() methods in java

Usage of hashCode() and equals()
hashCode() method is used to get a unique integer for given object. This integer is used for determining the bucket location, when this object needs to be stored in some HashTable like data structure. By default, Object’s hashCode() method returns and integer representation of memory address where object is stored.

equals() method, as name suggest, is used to simply verify the equality of two objects. Default implementation simply check the object references of two objects to verify their equality.

Overriding the default behavior
Everything works fine until you do not override any of these methods in your classes. But, sometimes application needs to change the default behavior of some objects.

Lets take an example where your application has Employee object. Lets create a minimal possible structure of Employee class

public class Employee
{
    private Integer id;
    private String firstname;
    private String lastName;
    private String department;

    public Integer getId() {
        return id;
    }
    public void setId(Integer id) {
        this.id = id;
    }
    public String getFirstname() {
        return firstname;
    }
    public void setFirstname(String firstname) {
        this.firstname = firstname;
    }
    public String getLastName() {
        return lastName;
    }
    public void setLastName(String lastName) {
        this.lastName = lastName;
    }
    public String getDepartment() {
        return department;
    }
    public void setDepartment(String department) {
        this.department = department;
    }
}

Above Employee class has some very basic attributes and there accessor methods. Now consider a simple situation where you need to compare two employee objects.

public class EqualsTest {
    public static void main(String[] args) {
        Employee e1 = new Employee();
        Employee e2 = new Employee();

        e1.setId(100);
        e2.setId(100);
        //Prints false in console
        System.out.println(e1.equals(e2));
    }
}

Above method will print “false“. But, is it really correct after knowing that both objects represent same employee. In a real time application, this must return true.

To achieve correct behavior, we need to override equals method as below:

public boolean equals(Object o) {
        if(o == null)
        {
            return false;
        }
        if (o == this)
        {
           return true;
        }
        if (getClass() != o.getClass())
        {
            return false;
        }
        Employee e = (Employee) o;
        return (this.getId() == e.getId());
}

Add this method to your Employee class, and EqualsTest will start returning “true“.

So are we done? Not yet. Lets test again above modified Employee class in different way.

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

public class EqualsTest
{
    public static void main(String[] args)
    {
        Employee e1 = new Employee();
        Employee e2 = new Employee();

        e1.setId(100);
        e2.setId(100);

        //Prints 'true'
        System.out.println(e1.equals(e2));

        Set employees = new HashSet();
        employees.add(e1);
        employees.add(e2);
        //Prints two objects
        System.out.println(employees);
    }
}

Above class prints two objects in second print statement. If both employee objects have been equal, in a Set which stores only unique objects, there must be only one instance inside HashSet, after all both objects refer to same employee. What is it we are missing??

We are missing the second important method hashCode(). As java docs say, if you override equals() method then you must override hashCode() method. So lets add another method in our Employee class.

@Override
 public int hashCode()
 {
    final int PRIME = 31;
    int result = 1;
    result = PRIME * result + getId();
    return result;
 }

Once above method is added in Employee class, the second statement start printing only single object in second statement, and thus validating the true equality of e1 and e2.

Key Points

• Always use same attributes of an object to generate hashCode() and equals() both. As in our case, we have used employee id.
• equals() must be consistent (if the objects are not modified, then it must keep returning the same value).
• Whenever a.equals(b), then a.hashCode() must be same as b.hashCode().
• If you override one, then you should override the other.

If you feel, I am missing something or wrong somewhere, please leave a comment. I will update this post again to help others.