Equals and Hashcode

/**
     * hashCode() is used for bucketing in hash data structures.
     * The value received from hashCode() is used as the bucket number for storing elements of the set/map.
     * This bucket number is the address of the element inside the set/map.
     *
     * When you do contains() it will take the hash code of the element, then look for the bucket where hash code points to.
     * If more than 1 element is found in the same bucket (multiple objects can have the same hash code), then it uses the equals() method to evaluate if the objects are equal,
     * and then decide if contains() is true or false, or decide if element could be added in the set or not.
     */

    // Even when emp1 and emp2 objects are the same, the JVM doesn't recognize
    // that is why we have to override equals/hashCode to be sure that the objects we want to compare are equal according to my preferences
    EmployeeEqualsAndHashcode emp1 = new EmployeeEqualsAndHashcode(101, "Jim", "Carrey");
    EmployeeEqualsAndHashcode emp2 = new EmployeeEqualsAndHashcode(101, "Jim", "Carrey");
    EmployeeEqualsAndHashcode emp3 = new EmployeeEqualsAndHashcode(102, "Buba", "Mara");
    EmployeeEqualsAndHashcode emp4 = new EmployeeEqualsAndHashcode(103, "Martin", "Scorzese");

    System.out.println(emp2.equals( emp1 ));

    /**
     * Hashcode is all different for all the objects ( no matter if there information is the same )
     */
    System.out.println("hashCode for emp1 ---> " + emp1.hashCode());
    System.out.println("hashCode for emp2 ---> " + emp2.hashCode());
    System.out.println("hashCode for emp3 ---> " + emp3.hashCode());
    System.out.println("hashCode for emp4 ---> " + emp4.hashCode());


    /**
     * In case of using collections like HashSet and HashMap
     * the hashCode is going to be used to put them in a particular bucket
     */
    Set<EmployeeEqualsAndHashcode> employees = new HashSet<>();
    employees.add( emp1 );
    employees.add( emp2 );
    employees.add( emp3 );
    employees.add( emp4 );

    System.out.println("\nEmployee set size: " + employees.size());


    Map<EmployeeEqualsAndHashcode, EmployeeEqualsAndHashcode> employeeMap = new HashMap<>();
    employeeMap.put( emp1, emp1 );
    employeeMap.put( emp2, emp2 );
    employeeMap.put( emp3, emp3 );
    employeeMap.put( emp4, emp4 );

    System.out.println("\nEmployee map size: " + employeeMap.size());
  }

  private int id;
  private String name;
  private String lastName;

  public EmployeeEqualsAndHashcode() {}

  public EmployeeEqualsAndHashcode(int id, String name, String lastName) {
    this.id = id;
    this.name = name;
    this.lastName = lastName;
  }

  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 getLastName() {
    return lastName;
  }

  public void setLastName( String lastName ) {
    this.lastName = lastName;
  }

  @Override public String toString() {
    final StringBuilder sb = new StringBuilder( "Employee{" );
    sb.append( "id=" ).append( id );
    sb.append( ", name='" ).append( name ).append( '\'' );
    sb.append( ", lastName='" ).append( lastName ).append( '\'' );
    sb.append( '}' );
    return sb.toString();
  }

  @Override public boolean equals( Object o ) {
    if( this == o ) {
      return true;
    }
    if( o == null || getClass() != o.getClass() ) {
      return false;
    }
    EmployeeEqualsAndHashcode employee = (EmployeeEqualsAndHashcode) o;
    return id == employee.id &&
        Objects.equals( name, employee.name ) &&
        Objects.equals( lastName, employee.lastName );
  }