Override equals() and hashCode() method in java


Hi! In this post we will learn how to override equals() and hashCode() method.

By overriding equals() and hashCode() method we could >

What we should do to override equals() and hashCode() >

1)  Check whether obj is null or not.

  if(obj==null) //If obj is null, return without comparing obj & Employee class.


2)  check whether  obj is instance of Employee class or not.

if(this.getClass()!=obj.getClass()) //identifies whether obj is instance of Employee class or not.

3) Then, type cast obj into employee instance.
 Employee emp=(Employee)obj;  //type cast obj into employee instance.


   @Override
   public boolean equals(Object obj){
         
          if(obj==null)
                 return false;
         
          if(this.getClass()!=obj.getClass())
                 return false;
  
          Employee emp=(Employee)obj;
          return (emp.id==this.id || emp.id.equals(this.id))
                              && (emp.name==this.name || emp.name.equals(this.name));      
   }
         
   @Override
   public int hashCode(){
          int hash=(this.id==null ? 0: this.id.hashCode() ) +
                       (this.name==null ? 0: this.name.hashCode() );
          return hash;     
   }



Let’s say in an organisation there exists a employee with id=1 and name=’sam’     and some data is stored corresponding to him, but if modifications have to be made in data, previous data must be overridden.


Full Program/SouceCode >
import java.util.HashMap;
class Employee {
  
   private Integer id;
   private String name;
  
   public Employee(Integer id, String name) { // constructor
          this.id = id;
          this.name = name;
   }
   @Override
   public String toString() {
          return "Employee[id=" + id + ", name=" + name + "] ";
   }
   @Override
   public boolean equals(Object obj){
         
          if(obj==null) //If obj is null, return without comparing obj & Employee class.
                 return false;
         
          if(this.getClass()!=obj.getClass())   //identifies whether obj is instance of Employee class or not.
                 return false;
  
          Employee emp=(Employee)obj; //type cast obj into employee instance.
          return (emp.id==this.id || emp.id.equals(this.id))
                              && (emp.name==this.name || emp.name.equals(this.name));      
   }
         
   @Override
   public int hashCode(){
          int hash=(this.id==null ? 0: this.id.hashCode() ) +
                       (this.name==null ? 0: this.name.hashCode() );
          return hash;     
   }
}

/** Copyright (c), AnkitMittal JavaMadeSoEasy.com */

public class EmployeeOverride {
   public static void main(String...a){
         
          HashMap<Employee, String> hm=new HashMap<Employee, String>();
          hm.put(new Employee(1,"sam"), "employee1 data");
          hm.put(new Employee(2,"amy"), "employee2 data");
         
          System.out.println("HashMap's data> "+hm);
          System.out.println(hm.get(new Employee(1,"sam")));
         
          hm.put(new Employee(1,"sam"), "employee1 data OVERRIDDEN");
          System.out.println("\nAgain display HashMap after overriding data "+
                       "of Employee with id=1 and name=’sam’\n");
         
          System.out.println("HashMap's data> "+hm);
          System.out.println(hm.get(new Employee(1,"sam")));
         
   }
}
/*OUTPUT
HashMap's data> {Employee[id=1, name=sam] =employee1 data, Employee[id=2, name=amy] =employee2 data}
employee1 data
Again display HashMap after overriding data of Employee with id=1 and name=’sam’
HashMap's data> {Employee[id=1, name=sam] =employee1 data OVERRIDDEN, Employee[id=2, name=amy] =employee2 data}
employee1 data OVERRIDDEN
*/
If we note output >
Earlier, value corresponding to Employee with id=1 and name=’sam’ was employee1 data   
Later, value corresponding to Employee with id=1 and name=’sam’ was employee1 data   OVERRIDDEN



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Collection - List, Set and Map all properties in tabular form


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