1992-2007 Pearson Education, Inc. All rights reserved.
10
10
Object-Oriented
Programming:
Polymorphism
2 10.3
Demonstrating Polymorphic
Behavior
10.4
Abstract Classes
and Methods
10.5
Case Study: Payroll System Using
Polymorphism10.6
final
Methods and Classes
1992-2007 Pearson Education, Inc. All rights reserved.
10.3
Demonstrating Polymorphic
Behavior
•
A superclass reference can be aimed at a subclass
object (i.e., 調用不同子類別的method)
– This is possible because a subclass object is asuperclass object as well
– When invoking a method from that reference, the type of the actual referenced object, not the type of the reference, determines which method is called
– The same method name and signature can cause different actions to occur, depending on the type of object on which the method is invoked (to write a more general program)
4
class Vehicle {
public void start()
{
System.out.println("Starting ..."); }
}
class Jet extends Vehicle {
public void start()
{
System.out.println("2nd Starting ..."); }
public void zoom() { System.out.println("zooming ...");
Concept of Polymorphism
Vehicle Jet Is-A1992-2007 Pearson Education, Inc. All rights reserved. class App232
{
public static void main(String[] args) {
Vehiclej = new Jet();
j.start(); // aims to method of subclassflexibility
//j.zoom(); error: cann't find symbol
} }
> 2nd Starting ... 6 –設計方法display(obj)，其參數obj可以是Whierlybird, Aircraft, or Jet
類別的物件，並顯示物件obj所有field的值 • Alternative 1. entirely 3 versions of display()
• Alternative 2. only 1 versions of display()
Exercise
Vehicle Aircraft Whirlybird Jet a1 a2 b1 c1 c2 c3 Is-A1992-2007 Pearson Education, Inc. All rights reserved. CommissionEmploy3 - baseSalary BasePlusCommissionEmploy4 See Fig 9.12, 9.13 + toString(): double + toString(): double
2nd Polymorphism Example
Fig 10.18
10.4
Abstract Classes and Methods
•
Abstract classes
– Used only as abstract superclasses for concrete subclasses and to declare reference variables
– Keyword abstract (see Fig. 10-4) • Use to declare a class abstract
• Also use to declare a method abstract
– Abstract classes normally contain one or more abstract methods
– All concrete subclasses must override all inherited abstract methods
1992-2007 Pearson Education, Inc. All rights reserved. Fig. 10.2 | Employeehierarchy UML class diagram.
Extract common members
Differ in earning() and toString() For Fig 10.4 ~ 10.8 - weeklySalary - firstName - lastName - securityNumber - wage - hours - grossSales - commissionRate - baseSalary 10
下面的程式中，在抽象類別CShape裡已定義好一個show()，以及一個abstract method。 請在CWin類別裡撰寫area() method的定義，使得我們可以利用CWin類別來顯示物件的
width、height與面積。例如，在第24行建立CWin類別的物件win後，便可利用它來執行 第25行的呼叫。
01 // hw11_2, 抽象類別
02 abstract class CShape
03 {
04 protected int width;
05 protected int height;
06
07 public void show()
08 {
09 System.out.println("width="+width);
10 System.out.println("height="+height);
11 System.out.println("area="+area());
12 }
13 public abstract int area(); // 計算面積
14 }
1992-2007 Pearson Education, Inc. All rights reserved.
15 class CWin extends CShape 16 { 17 // 請完成這個部分的程式碼 18 } 19 20 public class hw11_2 21 {
22 public static void main(String args[])
23 {
24 CWin win=new CWin(5,7); // 建立CWin類別的物件
25 win.show();
26 }
27 }
12
10.5 Case Study: Payroll System
Using Polymorphism
1992-2007 Pearson Education, Inc. All rights reserved.
•
Dynamic binding
– Calls to overridden methods are resolved at execution time, based on the type of object referenced
•
Downcasting
– Convert a reference to a superclass to a reference to a subclass
– Allowed only if the object has an is-a relationship with the subclass
•
The same method name and signature can cause
different actions to occur, depending on the type
of object on which the method is invoked (to write
a more general program)
14
Summary of the Allowed
Assignments
•
Superclass and subclass assignment rules
– Assigning a superclass reference (i.e., object instance) to a superclass variable (i.e., object reference) is
straightforward
– Assigning a subclass reference to a subclass variable is straightforward
– Assigning a subclass reference to a superclass variable is safe because of the is-a relationship
• Referring to subclass-only members through superclass variables is a compilation error
– Assigning a superclass reference to a subclass variable is a compilation error
1992-2007 Pearson Education, Inc. All rights reserved.
10.6
final
Methods and Classes
•
final
methods
– Cannot be overriddenin a subclass
– privateand staticmethods are implicitly final – finalmethods are resolved at compile time, this is known
as static binding
•
final
classes
– Cannot be extendedby a subclass
– All methods in a finalclass are implicitly final
16
class animal {
final void breathe() {
System.out.println("Breathing ..."); }
}
class fish extends animal {
public void breathe() {
System.out.println("Bubbling ..."); }
}
1992-2007 Pearson Education, Inc. All rights reserved. final class animal
{
public void breathe() {
System.out.println("Breathing ..."); }
}
class fish extends animal {
public void breathe() { System.out.println("Bubbling ..."); } }
Final (2)
18 class app242 {public static void main(String[] args) { final int a = 5; a = 6; } } > 1 error
Cannot assign a value to final variable a
Final (3)
1992-2007 Pearson Education, Inc. All rights reserved.
10.7
Case Study: Creating and Using
Interfaces
•
Interfaces
– Keyword interface
– Contains only constants and abstractmethods • All fields are implicitly public, staticand final
• All methods are implicitly public abstractmethods – Typically used when disparate (不同的) classes need to
share common methods and constants
20
Interface (1)
import java.io.*;
interface Vehicle // No method-definitions
{
public void startEngine(); public void stopEngine(); }
interface Asset // No method-definitions
{
public int initialCost(); public int currentCost(); }
• 在C++中可以使用多重繼承，但在
Java中只能單一繼承，也就是一次 只能擴充一個類別，Java使用
1992-2007 Pearson Education, Inc. All rights reserved. public class int01 implements Vehicle, Asset
{
public void startEngine() // no overriding among interfaces
{
System.out.println("aaa"); }
public void stopEngine() {
System.out.println("bbb"); }
public int initialCost() {
return 2; }
public int currentCost() {
return 3; }
extends class-A implements class-B
22
public static void main(String args[]) {
int01 a = new int01(); a.startEngine(); a.stopEngine(); System.out.println(a.initialCost()); System.out.println(a.currentCost()); } } aaa bbb 2 3
1992-2007 Pearson Education, Inc. All rights reserved.
Interface (2)
interface iShape {
final double PI=3.14;
abstract void setColor(String str); }
interface iShape2D extends iShape {
abstract void area(); }
24
class CCircleimplementsiShape2D { double radius; String color; public CCircle(double r) { radius=r; }
public void setColor(String str) { color=str;
System.out.println("color="+color); }
public void area() {
1992-2007 Pearson Education, Inc. All rights reserved. public class app11_7
{
public static void main(String args[]) { CCircle cir; cir=new CCircle(2.0); cir.setColor("Blue"); cir.area(); } } 26 下面的程式碼是修改自習題1，其中把抽象類別CShape改以介面來宣告。
請在CWin類別裡撰寫show() 及area() method的定義，使得我們可以利用CWin類別 來顯示物件的width、height與面積。
01 // hw11_5, 介面實作
02 interface iShape 03 {
04 public void show();
05 public int area(); // 計算面積
06 }
1992-2007 Pearson Education, Inc. All rights reserved.
07 class CWin implements iShape 08 {
09 protected int width; 10 protected int height; 11 12 // 請完成這個部分的程式碼 13 } 14 15 public class hw11_5 16 {
17 public static void main(String args[]) 18 {
19 CWin win=new CWin(5,7); // 建立CWin類別的物件 20 win.show();
21 } 22 }
28
下面的程式碼是修改自習題3，其中增加了一個iColor介面。iColor介面裡定義了1個method：
public void showColor();// 顯示顏色
請在CWin類別裡撰寫showColor() method的定義，使得我們可以利用CWin類別來顯示 物件的顏色、width、height與面積。
01 // hw11_9, 多重繼承的練習
02 interface iShape 03 {
04 public void show();
05 public int area();
06 }
07 interface iColor 08 {
09 public void showColor();
10 }
1992-2007 Pearson Education, Inc. All rights reserved.
11 class CWin implements iShape,iColor 12 { 13 // 請完成這個部分的程式碼 14 } 15 16 public class hw11_9 17 {
18 public static void main(String args[]) 19 {
20 CWin win=new CWin(5,7,"Green"); 21 win.show(); 22 } 23 } 30 realization Fig 10.11 – 10.15 發票
1992-2007 Pearson Education, Inc. All rights reserved.
10.7.7 Declaring Constants with
Interfaces
•
Interfaces can be used to declare constants used
in many class declarations
– These constants are implicitly public, staticand final
– Using a static importdeclaration allows clients to use these constants with just their names
