Course Introduction

CLASS II

1-2

Previous studies

• What is constructor?• How to use this()?

– Where I can call this()?– Can I call this() twice in one constructor?– What is the meaning between this() and

this.XX?

1-3

Previous studies

• How to use Inheritance?• How to use super()?

– Where can I call super()?– What is the meaning between super() and

super.XX?• What is polymorphism?

Synopsis of Polymorphism

• The polymorphism principle• Polymorphism means that an operation

may behave differently (in different classes).

• There are two kinds of polymorphism– static (overloading)– dynamic

• Example– GeomFigure (display(), remove(), position())

• Rectangle• Circle

1-4

再談 inheritance

• 矩型– 我們先擁有這個 class，並且進行了系統開發

• 正方形– 之後有了正方型的需求，再寫了一個正方形的 class– Square “IS A” kind of rectangle

• Reuse?– Q1: 你有沒有程式”砍掉重練”的經驗 ?– Q2: 你有沒有”再造輪子”的經驗 ?

1-5

Class Rectangle

class Rectangle {    double width;    double height;            public double getHeight() {        return height;    }    public void setHeight(double height) {        this.height = height;    }    public double getWidth() {        return width;    }    public void setWidth(double width) {        this.width = width;    }   }

1-6

Class Square

class Square extends Rectangle {    public void setHeight(double height) {        super.setHeight(height);        super.setWidth(height);    }        public void setWidth(double width) {        super.setHeight(width);        super.setWidth(width);    }}

1-7

void g(Rectangle r) {    r.setWidth(5);    r.setHeight(4);    if (r.getWidth() * r.getHeight() != 20) {        throw new RuntimeException();    }}

//some other placeRectangle square = new Square();g(square);

1-8

Liskov Substitution Principle (LSP)• Functions that use pointers or

references to base classes must be able to use objects of derived classes without knowing it

• Very difficult problem!!– Further study: OOAD– Software design principals

• Please google it if you are interested in it• One of the principals

– Prefer Composition over inheritnace– Why? Can you give an example?

1-9

1-10

類別（ Class ）架構

• 「類別架構」（ Class Hierarchy ）– 將整個類別關係的樹狀結構繪出來– 繼承的子類別可以有多層

• 如果父類別不只一個，即繼承多個類別，稱為「多重繼承」（ Multiple Inheritance ）。

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類別（ Class ）類別關係

• 類別關係是指不同類別間的關係，例如：– 繼承是一種 Is-a 的類別關係

• Nissan” 是”車子的一種 (Is-a)– 「成品和零件」（ Whole-Part ）關係，即 Part-of 和 Has-a 關係。• 輪胎是車子的”一部份” (Part-of)• 車子”有”輪胎 (Has-a)

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「 is a 」和「 has a 」

• 在描述類別時，常以「 is a 」和「 has a 」描述類別的父類別和類別中的資料成員。「 is a 」描述句通常會導出類別的父類別，而「 has a 」則是會導出類別中的資料成員。 – A man is a human that has a name, a father, and a

mother.

public class Man extends Human {Name theName;Father theFather;Mother theMother;

}

1-13

Wrong program

// 建立一個 Vehicle 的類別class Vehicle {

Vehicle(String x){

System.out.println(“Vehicle’s Constructor”);

}

public void drive(){

System.out.println(“I’m driving”);

}

}

1-14

Wrong program// 建立一個 Car 的類別 extend Vechicle

class Car extends Vehicle {

}

public class app{

public static void main(String[] args){

Car aCar=new Car();

}

}

Why this program is wrong??

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How to fix it!

// 建立一個 Vehicle 的類別class Vehicle { Vehicle(){

System.out.println(“Vehicle’s Constructor”); }

Vehicle(String x){

System.out.println(“Vehicle’s Constructor”);

}

public void drive(){

System.out.println(“I’m driving”);

}

}

1-16

How to fix it!!// 建立一個 Car 的類別 extend Vechicle

class Car extends Vehicle {

Car(){

Super(“X”);

}

}

public class app{

public static void main(String[] args){

Car aCar=new Car();

}

}

}

1-17

How to fix it!!// 建立一個 Car 的類別 extend Vechicle

class Car extends Vehicle {

Car(String x){

Super(x);

}

}

public class app{

public static void main(String[] args){

Car aCar=new Car(“x”);

}

}

}

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Example

class Circle{

double radius;

void setRadius(double radius){

this.radius=radius;

} void showRadius(){

System.out.println(“ 半徑為” +radius); } double showArea(){

return Math.PI*Math.pow(radius,2); }

}

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API search

• Search java api document• For example pow(a,b)

– Key in java api pow

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Example

public class Application{

public static void main(String argv[]){

Circle aCircle;

aCircle = new Circle();

aCircle.setRadius(12.0);

aCircle.showRadius();

System.out.println(“aCircle 的面積等於” + aCircle.showArea());

}

}

1-21

How to know new object’s hashCode?

public class RealWorld {

public static void main(String[] args){

Car aCar=new Car(); // 產生一個 Car 的實體String x=aCar.toString();

//Name+”@”+hashCode()

System.out.println(x);

Car bCar=aCar;

System.out.println(aCar.toString());

System.out.println(bCar.toString());

}

}

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Example detail

Circle aCircle;

aCircle = new Circle();

aCircle 的記憶體 location l-value:0x1111指向 Circle Type 的 reference 的 variable

aCircle l-value:0x1111Circle 某一個 instance其 l-value:0x3333

aCircle = new Circle();

aCircle l-value:0x1111aCircle r-value:0x3333

Circle 某一個 instance其 l-value:0x3333

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Example detail

bCircle=aCircle;

Circle 物件aCircle

bCircle

bCircle=null;

Circle 物件aCircle

bCircle

Problem: aCircle=null; 結果會如何 ?

1-24

物件的空間配置

class Vehicle{ // 定義的類別int wheel; // 定義一個實體變數

}Vehicle newCar1 = new Vehicle();// 實體化一個類別Vehicle newCar2 = new Vehicle();// 再實體化一個類別Vehicle rCar = newCar1; // 只是一個參考

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變數型態

• 種類– 基本型態 (Primitive Type)

• 8 種– 參考型態 (Reference Type)

• 基本型態– 整數、浮點數、字元、或布林值

• 參考型態– 由物件所組成的型態

• So, what is “static”?– We have not explained it, let us do it now.