PowerPoint Slides. Object-Oriented Design Using JAVA. Chapter 2. by Dale Skrien

Transcription

1 PowerPoint Slides Object-Oriented Design Using JAVA by Dale Skrien Chapter 2

2 Object-oriented Programming Divides the program into a set of communicating objects Encapsulates in an object all the behavior and knowledge related to one concept Distributes control and data since the intelligence and knowledge is distributed among the objects

3 Object-oriented Languages Support OO style of programming Provide classes objects messages inheritance polymorphism

4 Sample Java class public class Person {! private String name;! private Date birthdate;! public Person(String name, Date birthdate) {! this.name = name;! this.birthdate = birthdate;! }! public String getname() {! return name;! }! }! public Date getbirthdate() {! return birthdate;! }!

5 Figure 2.1. Java primitive & reference types. The variable x stores the primitive integer value 3 whereas the variable s stores a reference to the String object hello.

6 Figure 2.2. The UML class diagram for the Person class.

7 Figure 2.3. The UML notation for subclass and superclass.

8 Implementation Inheritance Also called subclassing Indicated in Java by keyword extends Uses: Specialization by adding new behavior in a subclass Specialization by overriding existing behavior in a subclass Generalization by creating a superclass and moving code up

9 Figure 2.4. Specialization by adding new behavior to the subclass.

10 Inheritance Example public class EnhancedRectangle extends Rectangle { public EnhancedRectangle(int x, int y, int w, int h) //constructor { super(x, y, w, h); } public Point getcenter() { return new Point((int) getcenterx(), (int) getcentery()); } public void setcenter(int x, int y) { setlocation( x (int) getwidth()/2, y (int) getheight()/2); } }

11 FilledOval subclass of Oval Example of specialization by overriding existing methods FilledOval class overrides the draw method of the Oval class to draw a filled-in black oval instead of a white oval with black border

12 Figure 2.5. Two white ovals with black borders and a filled oval.

13 Public class Oval { } public int x, y, w, h; public Oval(int x, int y, int w, int h) { //constructor this.x = x; this.y = y; this.w = w; this.h = h; } public void draw(graphics g) { g.drawoval(x, y, w, h); } public int getwidth() {return w;} public int getheight() {return h;} public Point gettopleftpoint() {return new Point(x,y);} // other methods

14 Specialization Example Public class FilledOval extends Oval { } public FilledOval(int x, int y, int w, int h) { } super(x, y, w, h); public void draw(graphics g) { } g.filloval(x, y, w, h);

15 Figure 2.6. Generalization by moving common behavior to a superclass.

16 Generalization Example public abstract class Customer { } HumanCustomer human = new HumanCustomer( ); BusinessCustomer business = new BusinessCustomer( ); String humanname = human.getname(); String businessname = business.getname();

17 Figure 2.7. A Person class with two superclasses. This is not legal in Java.

18 Figure 2.8. Class C inherits two foo() methods.

19 Types and interfaces A type is a set of data values and the operations that can be performed on them. Example: The Java integer type has 2 32 values and has operations such as addition, subtraction, multiplication, and division.

20 Java classes as types Each Java class is a type The values of such a type are the objects of that class or any subclasses The operations are the methods of the class

21 Java interfaces as types Each Java interface is a type The values of such a type are the objects of all classes that implement the interface The operations are the methods declared in the interface

22 Subtypes A type S is a subtype of type T if the set of objects of S form a subset of the set of objects of T and the set of operations of S are a superset of the set of operations of T Example: Any subclass defines a subtype of its superclass

23 Figure 2.9. A SimpleRunner class that implements the Runnable interface and extends the Object class.

24 Subtype polymorphism You can use an object of a subtype wherever an object of a supertype is expected. Examples: List l = new ArrayList();! List l = new LinkedList();

25 Figure The List interface and two classes that implement it.

26 Abstract classes Abstract classes are denoted by the key word abstract. Objects of such classes cannot be created. Some of the methods can be declared abstract, in which case subclasses must implement those methods (or be abstract themselves).

27 Abstract classes vs. interfaces Abstract classes can include method implementations and non-final fields Abstract classes with no implementations are like interfaces except a class can extend only one superclass in Java.

28 Dynamic method invocation In a method call v.foo(), the Java runtime environment looks at the actual class of the value of v, not the declared type of v, to determine which implementation of foo to execute.

29 Dynamic method invocation example Suppose a class A has a method foo that prints A and a subclass B has a method foo that prints B. A v = new B();!v.foo(); The implementation of foo in class B is the one that is executed ( B is printed).

30 Figure The abstract Automobile class and its subclasses.

31 Dynamic method invocation for Automobiles Automobile[] fleet = new Automobile[]{! new Sedan(Color.black),! new Minivan(Color.blue),! new SportsCar(Color.red)};! int totalcapacity = 0;! for(automobile car : fleet) {! totalcapacity += car.getcapacity();! }! Three different getcapacity methods are executed in this loop (one in each subclass)

32 Overloading vs. Overriding Overloading refers to two methods in the same class with the same name, but different parameter lists. Overriding concerns two methods, one in a subclass and one in a superclass, with identical signatures (name and parameter list).

33 Figure The Automobile class has two equals methods, one inherited and one defined in Automobile.

34 Figure The Automobile class does not inherit the Object class equals method. Instead, it overrides that method.

35 Overloaded method example Object o = new Object();! Automobile auto = new Automobile();! Object autoobject = new Automobile();! auto.equals(o);! auto.equals(auto);! auto.equals(autoobject);! Which of the two equals methods in the Automobile class is executed in each of the last 3 lines above?!