CS 151 - Lecture 20

Cay S. Horstmann

Lab

• Work with a buddy
• One of you codes, the other types up the answers.
• One of you submits the code in the lab20 subdirectory of your personal repo, the other submits a file report.txt in the lab20 subdirectory of your personal repo.
• Put the name of the coding buddy into the report

Equals and no Hash Code

• Define a class Point like this:

  public class Point
  {
     private int x;
     private int y;
     public Point(int x, int y) { this.x = x; this.y = y; }
     public String toString() { return getClass().getName() + "[x=" + x + ",y=" + y + "]"; }
     public boolean equals(Object otherObject)
     {
        if (!(otherObject instanceof Point)) return false;
        Point other = (Point) otherObject;
        return x == other.x && y == other.y;
     }
  }

• Now run this program:

  import java.util.*;
  
  public class Prog1
  {
     public static void main(String[] args)
     {
        Set<Point> set = new HashSet<>();
        int n = 10;
        for (int i = 1; i <= n; i++)
           for (int j = 1; j <= n; j++)
              set.add(new Point(i % 2, j % 2));
        System.out.println(set.size());
     }
  }
  

• What does it print?
• Change n to 1000. What does it print?
• How many different (i.e. not equal to another) Point objects are inserted in the set?
• Adding an equal object to a set should not increase the size. Explain why you are getting the wrong answer. (And don't just say "because we didn't implement the ... method".)

Hash Code and No Equals

• Define a class Point2 like this:

  public class Point2
  {
     private int x;
     private int y;
     public Point2(int x, int y) { this.x = x; this.y = y; }
     public String toString() { return getClass().getName() + "[x=" + x + ",y=" + y + "]"; }
     public int hashCode()
     {
        return x + y;
     }
  }

• Now run this program:

  import java.util.*;
  
  public class Prog2
  {
     public static void main(String[] args)
     {
        Set<Point2> set = new HashSet<>();
        int n = 10;
        for (int i = 1; i <= n; i++)
           for (int j = 1; j <= n; j++)
              set.add(new Point2(i % 2, j % 2));
        System.out.println(set.size());
     }
  }
  

• What does it print?
• How many different (i.e. not equal to another) Point2 objects are inserted in the set?
• Explain why you are getting the wrong answer. (And don't just say "because we didn't implement the ... method".)
• Change n to 1000 and run the program again. Why does it seem to hang?
• Fix Point2 to works correctly. Insert objects into the hash set as before. What count do you now get?

Equals and Subclasses

• Make a subclass LabeledPoint of Point:

  public class LabeledPoint extends Point
  {
     private String label;
     public LabeledPoint(int x, int y, String label) { super(x, y); this.label = label; }
     public String toString() { return super.toString() + "[label=" + label + "]"; }
  }

• Run this program:

  public class Prog3
  {
     public static void main(String[] args)
     {
        LabeledPoint p1 = new LabeledPoint(3, 4, "Fred");
        LabeledPoint p2 = new LabeledPoint(3, 4, "Wilma");
        
        System.out.println(p1.equals(p2));
     }
  }

  What do you get? Why?
• How do you fix that?
• Go ahead and implement LabeledPoint.equals. Then add this code:

  Point p3 = new Point(3, 4);
  System.out.println(p1.equals(p3));
  System.out.println(p3.equals(p1));
  

  What do you get? Why?
• How can you fix it so that equals is symmetrical?
• Is your fix transitive? That is, look at

  System.out.println(p1.equals(p3));
  System.out.println(p3.equals(p2));
  System.out.println(p1.equals(p2));
  

  If the first two are true, so should be the third.