Java Object Oriented Design - Java Object Equals

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The following code shows how to implement equals() and hashCode() Methods

class Point {/*  w w  w  .j a  v  a2 s  . c o m*/
  private int x;
  private int y;

  public Point(int x, int y) {
    this.x = x;
    this.y = y;
  }

  /* implement the equals() method */
  public boolean equals(Object otherObject) {
    // Are the same?
    if (this == otherObject) {
      return true;
    }

    // Is otherObject a null reference?
    if (otherObject == null) {
      return false;
    }

    // Do they belong to the same class?
    if (this.getClass() != otherObject.getClass()) {
      return false;
    }

    // Get the reference of otherObject in a SmartPoint variable
    Point otherPoint = (Point) otherObject;

    // Do they have the same x and y co-ordinates
    boolean isSamePoint = (this.x == otherPoint.x && this.y == otherPoint.y);

    return isSamePoint;
  }

  /*
   * implement hashCode() method of the Object class, which is a requirement
   * when you implement equals() method
   */
  public int hashCode() {
    return (this.x + this.y);
  }
}

public class Main {
  public static void main(String[] args) {
    Point pt1 = new Point(10, 10);
    Point pt2 = new Point(10, 10);
    Point pt3 = new Point(12, 19);
    Point pt4 = pt1;

    System.out.println("pt1 == pt1: " + (pt1 == pt1));
    System.out.println("pt1.equals(pt1): " + pt1.equals(pt1));

    System.out.println("pt1 == pt2: " + (pt1 == pt2));
    System.out.println("pt1.equals(pt2): " + pt1.equals(pt2));

    System.out.println("pt1 == pt3: " + (pt1 == pt3));
    System.out.println("pt1.equals(pt3): " + pt1.equals(pt3));

    System.out.println("pt1 == pt4: " + (pt1 == pt4));
    System.out.println("pt1.equals(pt4): " + pt1.equals(pt4));
  }
}

The code above generates the following result.

Note

Here are specifications for the equals() method's implementation. Assume that x, y, and z are non-null references of three objects.

• Reflexivity. The expression x.equals(x) should return true.
• Symmetry. If x.equals(y) returns true, y.equals(x) must return true.
• Transitivity. If x.equals(y) returns true and y.equals(z) returns true, x.equals(z) must return true.
• Consistency. If x.equals(y) returns true, it should keep returning true until the state of x or y is modified. If x.equals(y) returns false, it should keep returning false until the state of x or y is modified.
• Comparison with null reference: An object of any class should not be equal to a null reference. The expression x.equals(null) should always return false.
• Relationship with hashCode() method: If x.equals(y) returns true, x.hashCode() must return the same value as y.hashCode().

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