OO的五大原則：SOLID

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import java.util.ArrayList;
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class Square {
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    private double width;
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    Square(double width){
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        this.width = width;
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    }
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    public double getWidth() { return width; }
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}
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class Circle {
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    private double radius;
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    Circle(double radius){
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        this.radius = radius;
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    }
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    public double getRadius() { return radius; }
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}
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class Traingle {
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    private double width;
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    private double height;
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    Traingle(double width, double height){
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        this.width = width;
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        this.height = height;
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    }
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    public double getWidth() { return width; }
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    public double getHeight() { return height; }
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}
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public class AreaExample {
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    public static void main(String[] args){
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        ArrayList shapes = new ArrayList<>();
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        shapes.add(new Square(10));
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        shapes.add(new Circle(5));
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        shapes.add(new Traingle(2,3));
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        for(Object shape: shapes) {
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            double area = 0;
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            if(shape instanceof Square){
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                area = Math.pow(((Square) shape).getWidth(), 2);
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            } else if (shape instanceof Circle) {
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                area = Math.pow(((Circle) shape).getRadius(), 2) * Math.PI;
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            } else if (shape instanceof Traingle) {
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                area = ((Traingle) shape).getWidth() * ((Traingle) shape).getHeight();
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            }
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            System.out.println("面積是：" + area);
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        }
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    }
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}

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import java.util.ArrayList;
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interface Shape {
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    double getArea();
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}
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class Square implements Shape{
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    private double width;
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    Square(double width){
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        this.width = width;
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    }
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    public double getArea() { return width * width; }
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}
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class Circle implements Shape{
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    private double radius;
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    Circle(double radius){
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        this.radius = radius;
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    }
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    public double getArea() { return radius * radius * Math.PI; }
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}
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class Traingle implements Shape{
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    private double width;
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    private double height;
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    Traingle(double width, double height){
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        this.width = width;
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        this.height = height;
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    }
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    public double getArea() { return width * height; }
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}
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public class AreaExample {
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    public static void main(String[] args){
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        ArrayList shapes = new ArrayList<>();
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        shapes.add(new Square(10));
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        shapes.add(new Circle(5));
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        shapes.add(new Traingle(2,3));
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        for(Shape shape: shapes) {
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            System.out.println("面積是：" + shape.getArea());
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        }
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    }
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}

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class Rectangle{
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    private int width;
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    private int height;
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    Rectangle(){}
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    public void setWidth(int width){ this.width = width; }
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    public void setHeight(int height){ this.height = height; }
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    public int area(){
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        return width * height;
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    }
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}
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class Square extends Rectangle{
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    Square(){}
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    public void setWidth(int width){
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        this.width = width;
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        this.height= width;
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    }
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    public void setHeight(int height){
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        this.width = height;
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        this.height= height;
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    }
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}
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public class ShapeLSP {
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    public static void main(String[] args) {
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        Rectangle rectangle = new Rectangle());
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        rectangle.setWidth(5);
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        rectangle.setHeight(10);
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        Rectangle square = new Square();
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        square.setWidth(5);
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        square.setHeight(10);
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        System.out.println(rectangle.area());
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        System.out.println(square.area());
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    }
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}

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interface Shape{
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    public int area();
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}
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class Square implements Shape{
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    private int edge;
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    Square(){}
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    public void setEdge(){ this.edge = edge; }
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    public int area(){
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        return edge * edge;
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    }
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}
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class Rectangle implements Shape{
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    private int width;
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    private int height;
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    Rectangle(){}
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    public void setWidth(int width){ this.width = width; }
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    public void setHeight(int height){ this.height = height; }
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    public int area(){
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        return width * height;
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    }
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}
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public class ShapeLSP {
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    public static void main(String[] args) {
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        Shape rectangle = new Rectangle());
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        rectangle.setWidth(5);
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        rectangle.setHeight(10);
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        Shape square = new Square();
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        square.setEdge(5);
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        System.out.println(rectangle.area());
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        System.out.println(square.area());
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    }
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}

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// 介面隔離
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interface SuperHero {
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    void fightVillains();
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}
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interface GrowingLargeSkill {
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    void grwoLarge();
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}
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interface ShinkingSkill {
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    void shrink();
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}
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interface FlyingSkill {
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    void fly();
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}
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// 實作需要的部分
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class FlyingSuperHero implements SuperHero, FlyingSkill {
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    void fightVillains() {
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        System.out.println("打擊壞人！");
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    }
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    void fly() {
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        System.out.println("我是會飛的英雄");
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    }
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}