In the Java programming language, every class implicitly or explicitly 
provides a hashCode() method, which digests the data stored in an 
instance of the class into a single hash value (a 32-bit signed 
integer).

public final native Class<?> getClass(); 
public native int hashCode(); 
public boolean equals(Object obj) { 
 return (this == obj); 
}  
public String toString() { 
 return getClass().getName() + "@" + Integer.toHexString(hashCode()); 
} 

public int hashCode() { 
  int h = hash; 
  if (h == 0) { 
    int off = offset; 
    char val[] = value; 
    int len = count; 

      for (int i = 0; i < len; i++) { 
        h = 31*h + val[off++]; 
      } 
      hash = h; 
    } 
    return h; 
  } 

s[0]*31^(n-1) + s[1]*31^(n-2) + … + s[n-1]

public class Main {
  public static void main(String[] args) {
    Main m = new Main();
    System.out.println(m);
    System.out.println(Integer.toHexString(m.hashCode()));
    String a = "Aa";
    String b = "BB";
    System.out.println(a.hashCode());
    System.out.println(b.hashCode());
  }
}

Main@2a139a55 
2a139a55 
2112 
2112

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;
  }
}

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));
  }
}

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());
}

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

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