Class Design

1. Class Design CS 3331 Fall 2009

2. Outline • Organizing classes • Design guidelines • Canonical forms of classes • equals method • hashCode method

3. Client LinkedList Node Public vs. Helper Classes • Motivation • To hide implementation decisions and details from clients • Approach • Separate helper classes and make them not visible to clients • Public classes: for general use by clients • Helper (or auxiliary) classes: used solely to implement public classes 0,2 <<use>> 0,2 helper class public class

4. Organizing Helper Classes • Approaches • Separate (standalone) classes in separate files • Separate (standalone) classes in the same file • Nested classes public class LinkedList { protected Node head, tail; /* … */ protected static class Node { private Object value; private Node next, prev; /* … */ } }

5. Outline • Public vs helper classes • Design guidelines • Canonical forms of classes • equals method • hashCode method

6. Design Guidelines • G1: Avoid public fields • To minimize accessibility of fields • Make fields non-public (e.g., private attributes) • Provide accessors (e.g., getAttr() or isAttr()) • Provide mutators (e.g., setAttr()) • G2: Completeness of public interfaces • G3: Separate interface from implementation

7. Question • Which is better and why? public class StudentBody1 { private ArrayList students; /* … */ public void addAll(ArrayList students) { /* … */ } } public class StudentBody2 { private List students; /* … */ public void addAll(List students) { /* … */ } }

8. service user LinkedList ArrayList <<interface>>List service provider Separation through Interfaces uses Client

9. Outline • Public vs. helper classes • Design guidelines • Canonical forms of classes • equals method • hashCode method

10. Canonical Form of Classes • Objective • To ensure that objects behave “well” when manipulated by Java runtime environment and other classes. // E.g., what will be printed? List points = new LinkedList(); points.add(new Point(10, 20)); If (points.contains(new Point(10, 20))) { System.out.println(“Found!”); } else { System.out.println(“Not found!”); }

11. Canonical Form (Cont.) • Canonical forms of public classes • String representation • toString method • No-argument constructor • Object equality • equals and hashCode methods • Cloning • Cloneable interface and clone method • Serialization • Serializable interface and readObject and writeObject methods

12. The toString Method • Why? public class Point { private int x, y; /* … */ public String toString() { return “Point(“ + x + “, “ + y + “)”; } }

13. No-argument Constructor • Why? public class Point { private int x, y; /* … */ public Point() { this(0, 0); } public Point(int x, int y) { this.x = x; this.y = y; } }

14. : Point : Point x = 10 y = 20 x = 10 y = 20 p1: p2: p3: Equality • Identity equality vs. value quality “==“ vs. equals() “==“ implies equals() Point p1 = new Point(10, 20); Point p2 = new Point(10, 20); Point p3 = p1; Is “p1 == p2” ? Is “p1.equals(p2)” ? Is “p1 == p3” ? Is “p1.equals(p3)” ?

15. Equality (Cont.) • Default implementation of equals method • Inherited from the class Object • Test for object identity • Can be overridden by subclasses

16. Contract of Equality • Equivalence relation • Reflective: x.equals(x) • Symmetric: x.equals(y)  y.equals(x) • Transitive: x.equals(y)  y.equals(z)  x.equals(z) • Consistency • x.equals(y) consistently returns true or false • Nonnullity • x.equals(null) always returns false

17. Exercise • Define equals method for the class Point. Assume that the class has two fields x and y, both of type int.

18. Template for equals Method Proposal 1: for class T

19. Perfect equals Method

20. Comparing Fields • Primitive types if (f != otherObj.f) { return false; } • Reference types if (f == null ? otherObj.f != null : !f.equals(otherObj.f)) { return false; }

21. Template for equals Method When T has a superclass:

22. Exercise • Define equals method for the class ColoredPoint, a subclass of the class Point. Assume that the class has an additional field named color of type Color.

23. Outline • Public vs. helper classes • Design guidelines • Canonical forms of classes • equals method • hashCode method

24. The hashCode Method • Purpose • Returns the hash code of an object • Used by hash-table-based collection classes (e.g., HashMap, HashSet) • Contract for hashCode • x.equals(y)  x.hashCode() == y.hashCode()

25. Defining hashCode Methods • General scheme • Compute hash code for each significant field • Combine hash code of all significant fields public int hashCode() { int result = 0; // accumulative hash code int h; // hash code for a field <<for each field compute and combine the hash code>> return result; }

26. Computing hash code for fields • boolean fields • f ? 0: 1 • byte, char, short, and int fields • (int) f • long fields • (int)(f ^ (f >>> 32)) • float fields • Float.floatToIntBits(f) • double fields • Double.doubleToLongBits(f) and then to int • References • If null, then 0 (or some fixed value), • Recursive equals  recursive hash on fields, or • Hash on canonical representation

27. How to Combine Hash Code? • Bitwise-or (|) result = result << n | h; where n is an arbitrary integer constant, e.g., 8. • Addition result = result * p + h; where p is a prime number, e.g., 37.

28. Example • Hashcode method for class Point public int hashCode() { int result = x; result = result << 8 | y; return result; } public int hashCode() { int result = x; result = result * 37 + y; return result; }

29. Exercise • Write a hashCode method for the class ColoredPoint, a subclass of the class Point. Assume that the class has an additional field color of type Color.