Object-Oriented Programming: Polymorphism

Transcription

1 1 10 Object-Oriented Programming: Polymorphism 10.3 Demonstrating Polymorphic Behavior 10.4 Abstract Classes and Methods 10.5 Case Study: Payroll System Using Polymorphism 10.6 final Methods and Classes 10.7 Case Study: Creating and Using Interfaces 2 1

2 10.3 Demonstrating Polymorphic Behavior 3 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 a superclass 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) Concept of Polymorphism 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..."); Vehicle Jet Is-A 2

3 5 class App232 public static void main(string[] args) Vehicle j = new Jet(); j.start(); // aims to method of subclass flexibility //j.zoom(); error: cann't find symbol > 2nd Starting... Exercise 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() Is-A Vehicle Whirlybird a1 a2 Aircraft b1 Jet c1 c2 c3 3

4 2nd Polymorphism Example Fig CommissionEmploy3 See Fig 9.12, tostring(): double BasePlusCommissionEmploy4 - basesalary + tostring(): double 10.4 Abstract Classes and Methods 8 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 4

5 9 Extract common members -firstname - lastname - securitynumber Differ in earning() and tostring() - weeklysalary - grosssales - commissionrate - wage - hours For Fig 10.4 ~ basesalary Fig Employee hierarchy UML class diagram. Exercise 1 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 protected int width; 05 protected int height; public void show() System.out.println("width="+width); 10 System.out.println("height="+height); 11 System.out.println("area="+area()); public abstract int area(); // 計 算 面 積 14 5

6 11 15 class CWin extends CShape // 請 完 成 這 個 部 分 的 程 式 碼 public class hw11_ public static void main(string args[]) CWin win=new CWin(5,7); // 建 立 CWin 類 別 的 物 件 25 win.show(); Case Study: Payroll System Using Polymorphism 12 Fig

7 13 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) 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 Downcasting can get around this error 14 7

8 10.6 final Methods and Classes 15 final methods Cannot be overridden in a subclass private and static methods are implicitly final final methods are resolved at compile time, this is known as static binding final classes Cannot be extended by a subclass All methods in a final class are implicitly final Final (1) 16 class animal final void breathe() System.out.println("Breathing..."); class fish extends animal public void breathe() System.out.println("Bubbling..."); 8

9 Final (2) 17 final class animal public void breathe() System.out.println("Breathing..."); class fish extends animal public void breathe() System.out.println("Bubbling..."); Final (3) 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 9

10 10.7 Case Study: Creating and Using Interfaces 19 Interfaces Keyword interface Contains only constants and abstract methods All fields are implicitly public, static and final All methods are implicitly public abstract methods Typically used when disparate ( 不 同 的 ) classes need to share common methods and constants Interface (1) 20 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 interface 來 達 到 多 重 繼 承 的 目 的 10

11 extends class-a implements class-b 21 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; 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

12 Interface (2) 23 interface ishape final double PI=3.14; abstract void setcolor(string str); interface ishape2d extends ishape abstract void area(); 24 class CCircle implements ishape2d 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() System.out.println("area="+PI*radius*radius); 12

13 25 public class app11_7 public static void main(string args[]) CCircle cir; cir=new CCircle(2.0); cir.setcolor("blue"); cir.area(); Exercise 2 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 public void show(); 05 public int area(); // 計 算 面 積 06 13

14 27 07 class CWin implements ishape protected int width; 10 protected int height; // 請 完 成 這 個 部 分 的 程 式 碼 public class hw11_ public static void main(string args[]) CWin win=new CWin(5,7); // 建 立 CWin 類 別 的 物 件 20 win.show(); Exercise 3 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 public void show(); 05 public int area(); interface icolor public void showcolor(); 10 14

15 29 11 class CWin implements ishape,icolor // 請 完 成 這 個 部 分 的 程 式 碼 public class hw11_ public static void main(string args[]) CWin win=new CWin(5,7,"Green"); 21 win.show(); Fig realization 發 票 15

16 Declaring Constants with Interfaces 31 Interfaces can be used to declare constants used in many class declarations These constants are implicitly public, static and final Using a static import declaration allows clients to use these constants with just their names A reference to a subclass object can be assigned to an interface variable if the superclass implements that interface 16