overriding hashcode and equals method internal work flow

[jyothsna ananthula]

hi,
any body explain to me overriding hashcode and equals methods
in collections especially.

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

public class Employee {

int empid;
String empname;
static Employee e1, e2;

Employee(int id, String name) {
empid = id;
empname = name;
}

public static void main(String[] args) {
e1 = new Employee(12, "lod");
e2 = new Employee(12, "lod");


boolean a;
Set hs = new HashSet();
a = hs.add(e1);
System.out.println(a);
a = hs.add(e2);


System.out.println(a);
System.out.println(hs);

}
@Override
public int hashCode() {
//System.out.println("Hi");

return empid;
}


public boolean equals(Object o) {

//System.out.println("hi1");
if (e1.empid == e2.empid) {
return true;
} else {
return false;
}
}


}

please any one explain this hashcode ovveriding and equals method internally how it will excute. and also which method executes first, here i used only 2 elements if i want for more than 3 elements how to write.

hashcode ovveridden means unique hashcodes are providing based on body implemention and improves retriving the elements.

here i written in hashcode body return empid means based on empid it will return hashcode for that object or some other else.
if writing instead of empid, 55 what is the meaning of this.and getting output same. and also what is the workflow behind these.

please let me know as soon as possible

[jagadeesh manne]

when you call set.add() method it internally calls first equals method and compare present object hashcode with previous objects hashcode. by default for every object hashcode is different.so it will accept duplicates.

Java.util.HashSet

now if you dont want to allow duplicates in your hashset you have to over ride equals and hashcode. what it means duplicate object?
if object contains same values then it is duplicate.
in your example you override hashcode with id. but if id and name is equals then it is duplicate. so you have to override hashcode with every value of that object.
for your example

if you want to add more objects in set better to create separate bean class with setter and getter methods. and create object for that class and add values and add each object to set. and over ride hashcode and equals method

 Set<Employee> set = new HashSet<Employee>();
        Employee employee = null;
        Scanner scanner = new Scanner(System.in);
        do {
                employee = new Employee();
                System.out.println("Enter Employee id");
                int id = Scanner.nextInt();
                employee.setId(id);
                System.out.println("Enter Employee Name");
                String name = scanner.next();
                employee.setName(name);
                System.out.println("Enter Employee Designation");
                String designation = scanner.next();
                employee.setDesignation(designation);
                set.add(employee);
                System.out.println("Do you want to add more records(Y/N)");
                option = scanner.next();
        }while ("Y".equalsIgnoreCase(option));

and java bean class

    public class  Employee   {
    private int id;
    private String name;
    private String designation;

    public int getId() {
        return id;
    }
    
    
    public void setId(int id) {
        this.id = id;
    }

   
    public String getName() {
        return name;
    }
    
    
    public void setName(String name) {
        this.name = name;
    }

      
    public String getDesignation() {
        return designation;
    }

    
    public void setDesignation(String designation) {
       this.designation = designation;
    }
  
    
    @Override
    public int hashCode(){
        StringBuffer buffer = new StringBuffer();
        buffer.append(this.name.hashCode());
        buffer.append(String.valueOf(this.id).hashCode());
        buffer.append(this.designation.hashCode());
        return Integer.parseInt(buffer.toString()); 
    }
    @Override
    public boolean equals(Object object){
        if (object == null) return false;
        if (object == this) return true;
        if (this.getClass() != object.getClass())return false;
        Employee employee = (Employee)object;
        if(this.hashCode()== employee.hashCode())return true;
       return false;
    }    
}

in this case i validated if object contains same values for id and name and designation that is duplicate.if you want you can validate using only name or id or designation

please visit my java site for more info