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Class Design

CS 3331

Sec 6.1 & 6.3 of [Jia03]

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Outline

Organizing classes Design guidelines Canonical forms of classes

equals method hashCode method

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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

Client LinkedList Node<<use>>

public class helper class

0,20,2

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Organizing Helper Classes

Approaches Separate (standalone) classes in separate files Separate (standalone) classes in the same file Nested classes: static, non-static (inner), local inner,

anonymous inner

public class LinkedList { protected Node head, tail; /* … */ protected static class Node { private Object value; private Node next, prev; /* … */ }}

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Outline

Public vs helper classes Design guidelines Canonical forms of classes

equals method hashCode method

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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

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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) { /* … */ }}

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Separation through Interfaces

Client <<interface>>List

uses

LinkedList ArrayList

service provider

service user

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Outline

Public vs. helper classes Design guidelines Canonical forms of classes

equals method hashCode method

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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<Point> 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!”);}

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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

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The toString Method

Why?

public class Point {

private int x, y;

/* … */

public String toString() { return “Point(“ + x + “, “ + y + “)”; }}

Default behavior: Point@1a46e30

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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; }}

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Equality Identity equality vs. value quality

“==“ vs. equals()“==“ implies equals()

Is “p1 == p2” ?Is “p1.equals(p2)” ?Is “p1 == p3” ?Is “p1.equals(p3)” ?

: Point

x = 10y = 20

p1:

: Point

x = 10y = 20

p2:

p3:

Point p1 = new Point(10, 20);Point p2 = new Point(10, 20);Point p3 = p1;

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Equality (Cont.) Default implementation of equals method

Inherited from the class Object Test for object identity

Can be overridden by subclasses

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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

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Exercise Define equals method for the class Point.

Assume that the class has two fields x and y, both of type int.

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Template for equals MethodProposal 1: for class T

public boolean equals(Object other) { if (this == other) { return true; } if (other instanceof T) { T otherObj = (T) other; << compare each significant field and return false if not equals>> return true; } return false;}

Q: What is wrong?

For class Point:

public boolean equals(Object o) { if (this == o) { return true; } if (o instanceof Point) { Point p = (Point) o; return x == p.x && y == p.y; } return false;}

Point-x: int-y: int

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Perfect equals Method

public boolean equals(Object other) {

if (this == other) { return true; }

if (other == null) { return false; }

if (getClass() != other.getClass()) { return false; }

T otherObj = (T) other; << compare each significant field and return false if not equals>>

return true;}

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Comparing Fields

if (f != otherObj.f) { return false;}

Primitive types

if (f == null ? otherObj.f != null : !f.equals(otherObj.f)) { return false;}

Reference types

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Template for equals Method

When T has a superclass:

public boolean equals(Object other) {

if (!super.equals(other)) { return false; }

T otherObj = (T) other; << compare each significant field and return false if not equals>>

return true;}

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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.

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Outline

Public vs. helper classes Design guidelines Canonical forms of classes

equals method hashCode method

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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()

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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;}

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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

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How to Combine Hash Code? Bitwise-or (|)

result = result << n | h;where n is an arbitrary integer constant, e.g., 8.

Additionresult = result * p + h;where p is a prime number, e.g., 37.

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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;}

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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.