Study Note

Java 快速學習自我挑戰 Day10

Symbols count in article: 2.8k Reading time: 16 mins.

物件導向程式設計 (OOP) - Classes, Constructors 和 Inheritance

繼承(Inheritance) 挑戰

  1. Class 會自動從 Object Class 繼承，所以下面的程式碼是本來應該有的樣子，為了簡便，所以省略了 extends Object
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public class Main extends Object {

    public static void main(String[] args) {
	     
    }
}
  1. 挑戰
    Start with a base class of a Vehicle, then create a Car class that inherits from this base class.
    Finally, crate another class, a specific type of Car that inherits from the Car class.
    You should be able to hand steering, changing gears, and moving (speed in other words).
    You will want to decide where to put the appropriate state and behaviours (fields and methods).
    As mentioned above, changing gears, increasing/decreasing speed should be included.
    For you specific type of vehicle you will want to add something specific for that type of car.
  2. 新增 Vehicle.java
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public class Vehicle {

    private String name;
    private String size;

    private int currentVelocity;
    private int currentDirection;

    public Vehicle(String name, String size) {
        this.name = name;
        this.size = size;

        this.currentVelocity = 0;
        this.currentDirection = 0;
    }

    public void steer(int direction) {
        this.currentDirection += direction;
        System.out.println("Vehicle.steer(): Steering at " + currentDirection + " degrees.");
    }

    public void move(int velocity, int direction) {
        currentVelocity = velocity;
        currentDirection = direction;
        System.out.println("Vehicle.move(): Moving at " + currentVelocity + " in direction " + currentDirection);
    }

    public String getName() {
        return name;
    }

    public String getSize() {
        return size;
    }

    public int getCurrentVelocity() {
        return currentVelocity;
    }

    public int getCurrentDirection() {
        return currentDirection;
    }

    public void stop() {
        this.currentVelocity = 0;
    }
}
  1. 新增 Car.java
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public class Car extends Vehicle {

    private int wheels;
    private int doors;
    private int gears;
    private boolean isManual;

    private int currentGear;

    public Car(String name, String size, int wheels, int doors, int gears, boolean isManual) {
        super(name, size);
        this.wheels = wheels;
        this.doors = doors;
        this.gears = gears;
        this.isManual = isManual;
        this.currentGear = 1;
    }

    public void changeGear(int currentGear) {
        this.currentGear = currentGear;
        System.out.println("Car.setCurrentGear(): Change to " + this.currentGear + " gear.");
    }

    public void changeVelocity(int speed, int direction) {
        move(speed, direction);
        System.out.println("Car.changeVelocity(): Velocity " + speed + " direction " + direction);
    }
}
  1. 新增 Outlander.java
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public class Outlander extends Car {

    private int roadServiceMonths;

    public Outlander(int roadServiceMonths) {
        super("Outlander", "4WD", 5, 5, 6, false);
        this.roadServiceMonths = roadServiceMonths;
    }

    public void accelerate(int rate) {

        int newVelocity = getCurrentVelocity() + rate;
        if (newVelocity == 0) {
            stop();
            changeGear(1);
        } else if (newVelocity > 0 && newVelocity <= 10) {
            changeGear(1);
        } else if (newVelocity > 10 && newVelocity <= 20) {
            changeGear(2);
        } else if (newVelocity > 20 && newVelocity <= 30) {
            changeGear(3);
        } else {
            changeGear(4);
        }

        if (newVelocity > 0) {
            changeVelocity(newVelocity, getCurrentDirection());
        }
    }
}
  1. 修改 Main.java
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public class Main {

    public static void main(String[] args) {

        Outlander outlander = new Outlander(36);
        outlander.steer(45);
        outlander.accelerate(30);
        outlander.accelerate(20);
        outlander.accelerate(-42);
    }
}

練習：圓柱體

  1. 題目

    1. Write a class with the name Circle. The class needs one field (instance variable) with name radius of type double.
      The class needs to have one constructor with parameter radius of type double and it needs to initialize the fields.
      In case the radius parameter is less than 0 it needs to set the radius field value to 0.
      Write the following methods (instance methods):
      • Method named getRadius without any parameters, it needs to return the value of radius field.
      • Method named getArea without any parameters, it needs to return the calculated area (radius * radius * PI). For PI use Math.PI constant.
    2. Write a class with the name Cylinder that extends Circle class. The class needs one field (instance variable) with name height of type double.
      The class needs to have one constructor with two parameters radius and height both of type double. It needs to call parent constructor and initialize a height field.
      In case the height parameter is less than 0 it needs to set the height field value to 0.
      Write the following methods (instance methods):
      • Method named getHeight without any parameters, it needs to return the value of height field.
      • Method named getVolume without any parameters, it needs to return the calculated volume. To calculate volume multiply the area with height.

    TEST EXAMPLE
    → TEST CODE:

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    Circle circle = new Circle(3.75);
    System.out.println("circle.radius= " + circle.getRadius());
    System.out.println("circle.area= " + circle.getArea());
    Cylinder cylinder = new Cylinder(5.55, 7.25);
    System.out.println("cylinder.radius= " + cylinder.getRadius());
    System.out.println("cylinder.height= " + cylinder.getHeight());
    System.out.println("cylinder.area= " + cylinder.getArea());
    System.out.println("cylinder.volume= " + cylinder.getVolume());

    → OUTPUT

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    circle.radius= 3.75
    circle.area= 44.178646691106465
    cylinder.radius= 5.55
    cylinder.height= 7.25
    cylinder.area= 96.76890771219959
    cylinder.volume= 701.574580913447

    NOTE: All methods should be defined as public NOT public static.
    NOTE: In total, you have to write 2 classes.
    NOTE: Do not add a main method to the solution code.

  2. 答案

    1. 新增 Circle.java
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    public class Circle {
        private double radius;

        public Circle(double radius) {
            this.radius = (radius < 0) ? 0 : radius;
        }

        public double getRadius() {
            return radius;
        }

        public double getArea() {
            return radius * radius * Math.PI;
        }
    }
    1. 新增 Cylinder.java
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    public class Cylinder extends Circle {
        private double height;

        public Cylinder(double radius, double height) {
            super(radius);
            this.height = (height < 0) ? 0 : height;
        }

        public double getHeight() {
            return height;
        }

        public double getVolume() {
            return getArea() * height;
        }
    }

練習：游泳池面積

  1. 題目
    The Swimming Company has asked you to write an application that calculates the volume of cuboid shaped pools.
    1. Write a class with the name Rectangle. The class needs two fields (instance variable) with name width and length both of type double.
      The class needs to have one constructor with parameters width and length both of type double and it needs to initialize the fields.
      In case the width parameter is less than 0 it needs to set the width field value to 0.
      In case the length parameter is less than 0 it needs to set the length field value to 0.
      Write the following methods (instance methods):
      • Method named getWidth without any parameters, it needs to return the value of width field.
      • Method named getLength without any parameters, it needs to return the value of length field.
      • Method named getArea without any parameters, it needs to return the calculated area (width * length).
    2. Write a class with the name Cuboid that extends Rectangle class. The class needs one field (instance variable) with name height of type double.
      The class needs to have one constructor with three parameters width, length, and height all of type double. It needs to call parent constructor and initialize a height field.
      In case the height parameter is less than 0 it needs to set the height field value to 0.
      Write the following methods (instance methods):
      • Method named getHeight without any parameters, it needs to return the value of height field.
      • Method named getVolume without any parameters, it needs to return the calculated volume. To calculate volume multiply the area with height.
        TEST EXAMPLE
        → TEST CODE:
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    Rectangle rectangle = new Rectangle(5, 10);
    System.out.println("rectangle.width= " + rectangle.getWidth());
    System.out.println("rectangle.length= " + rectangle.getLength());
    System.out.println("rectangle.area= " + rectangle.getArea());
    Cuboid cuboid = new Cuboid(5,10,5);
    System.out.println("cuboid.width= " + cuboid.getWidth());
    System.out.println("cuboid.length= " + cuboid.getLength());
    System.out.println("cuboid.area= " + cuboid.getArea());
    System.out.println("cuboid.height= " + cuboid.getHeight());
    System.out.println("cuboid.volume= " + cuboid.getVolume());
    → OUTPUT 
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    rectangle.width= 5.0
    rectangle.length= 10.0
    rectangle.area= 50.0
    cuboid.width= 5.0
    cuboid.length= 10.0
    cuboid.area= 50.0
    cuboid.height= 5.0
    cuboid.volume= 250.0
    NOTE: All methods should be defined as public NOT public static.
    NOTE: In total, you have to write 2 classes.
    NOTE: Do not add a main method to the solution code.
  2. 答案
    1. 新增 Rectangle.java
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    public class Rectangle {
        private double width;
        private double length;

        public Rectangle(double width, double length) {
            this.width = (width < 0) ? 0 : width;
            this.length = (length < 0) ? 0 : length;
        }

        public double getWidth() {
            return width;
        }

        public double getLength() {
            return length;
        }

        public double getArea() {
            return width * length;
        }
    }
    1. 新增 Cuboid.java
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    public class Cuboid extends Rectangle {
        private double height;

        public Cuboid(double width, double length, double height) {
            super(width, length);
            this.height = (height < 0) ? 0 : height;
        }

        public double getHeight() {
            return height;
        }

        public double getVolume() {
            return getArea() * height;
        }
    }

物件導向程式設計 (OOP) - Composition, Encapsulation 和 Polymorphism

Composition

  1. 在 Inheritance 的地方，我們使用了 Car 和 Vehicle 的例子，可以理解成 Car 就是 Vehicle 的一種。但是在某些情況，例如：電腦有機殼、螢幕和主機板，機殼不等於電腦；螢幕也不等於電腦；主機板也不等於電腦，我們就不用 Inheritance，用 Composition 更適合這樣的情境，同時，電腦這個 Class 還可以繼承三種 Class，而不僅僅只能 extends 一種 Class。以下用 Java 呈現電腦的範例。
  2. 新增 Motherboard.java
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public class Motherboard {

    private String model;
    private String manufacturer;
    private int ramSlots;
    private int cardSlots;
    private String bios;

    public Motherboard(String model, String manufacturer, int ramSlots, int cardSlots, String bios) {
        this.model = model;
        this.manufacturer = manufacturer;
        this.ramSlots = ramSlots;
        this.cardSlots = cardSlots;
        this.bios = bios;
    }

    public void loadProgram(String programName) {
        System.out.println("Program " + programName + " is now loading...");
    }

    public String getModel() {
        return model;
    }

    public String getManufacturer() {
        return manufacturer;
    }

    public int getRamSlots() {
        return ramSlots;
    }

    public int getCardSlots() {
        return cardSlots;
    }

    public String getBios() {
        return bios;
    }
}
  1. 新增 Monitor.java
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public class Monitor {

    private String model;
    private String manufacturer;
    private int size;
    private Resolution nativeResolution;

    public Monitor(String model, String manufacturer, int size, Resolution nativeResolution) {
        this.model = model;
        this.manufacturer = manufacturer;
        this.size = size;
        this.nativeResolution = nativeResolution;
    }

    public void drawPixelAt(int x, int y, String color) {
        System.out.println("Drawing pixel at " + x + "," + y + " in colour " + color);
    }

    public String getModel() {
        return model;
    }

    public String getManufacturer() {
        return manufacturer;
    }

    public int getSize() {
        return size;
    }

    public Resolution getNativeResolution() {
        return nativeResolution;
    }
}
  1. 新增 Resolution.java
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public class Resolution {
    private int width;
    private int height;

    public Resolution(int width, int height) {
        this.width = width;
        this.height = height;
    }

    public int getWidth() {
        return width;
    }

    public int getHeight() {
        return height;
    }
}
  1. 新增 Case.java
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public class Case {

    private String model;
    private String manufacturer;
    private String powerSupply;
    private Dimensions dimensions;

    public Case(String model, String manufacturer, String powerSupply, Dimensions dimensions) {
        this.model = model;
        this.manufacturer = manufacturer;
        this.powerSupply = powerSupply;
        this.dimensions = dimensions;
    }

    public void pressPowerButton() {
        System.out.println("Power button pressed");
    }

    public String getModel() {
        return model;
    }

    public String getManufacturer() {
        return manufacturer;
    }

    public String getPowerSupply() {
        return powerSupply;
    }

    public Dimensions getDimensions() {
        return dimensions;
    }
}
  1. 新增 Dimensions.java
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public class Dimensions {

    private int width;
    private int height;
    private int depth;

    public Dimensions(int width, int height, int depth) {
        this.width = width;
        this.height = height;
        this.depth = depth;
    }

    public int getWidth() {
        return width;
    }

    public int getHeight() {
        return height;
    }

    public int getDepth() {
        return depth;
    }
}
  1. 修改 Main.java
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public class Main {

    public static void main(String[] args) {
        Dimensions dimensions = new Dimensions(20, 20, 5);
        Case theCase = new Case("220B", "Dell", "240", dimensions);

        // 我們也可以使用 new 的方式來新增物件
        Monitor theMonitor = new Monitor("27 inch Beast", "Acer", 27, new Resolution(2540, 1440));

        Motherboard theMotherboard = new Motherboard("BJ-200", "Asus", 4, 6, "v2.44");

        PC thePC = new PC(theCase, theMonitor, theMotherboard);
        thePC.getMonitor().drawPixelAt(1500, 1200, "red");
        thePC.getMotherboard().loadProgram("Windows 1.0");
        thePC.getTheCase().pressPowerButton();
    }
}
  1. 如何選擇 Composition 和 Inheritance？
    • 如果你使用 Java，推薦先使用 Composition，這會給你更多額外的優點和彈性，但是還是需要看你想要解決的問題來決定。

挑戰

  1. 題目
    Create a single room of a house using composition.
    Think about the things that should be included in the room.
    Maybe physical parts of the house but furniture as well
    Add at least one method to access an object via a getter and
    then that objects public method as you saw in the previous video
    then add at least one method to hide the object e.g. not using a getter
    but to access the object used in composition within the main class
    like you saw in this video.
  2. 答案
    1. 新增 Bedroom.java
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    public class Bedroom {

        private String name;
        private Wall wall1;
        private Wall wall2;
        private Wall wall3;
        private Wall wall4;
        private Ceiling ceiling;
        private Bed bed;
        private Lamp lamp;

        public Bedroom(String name, Wall wall1, Wall wall2, Wall wall3, Wall wall4, Ceiling ceiling, Bed bed, Lamp lamp) {
            this.name = name;
            this.wall1 = wall1;
            this.wall2 = wall2;
            this.wall3 = wall3;
            this.wall4 = wall4;
            this.ceiling = ceiling;
            this.bed = bed;
            this.lamp = lamp;
        }

        public Lamp getLamp() {
            return lamp;
        }

        public void makeBed() {
            System.out.println("Bedroom -> Making bed");
            bed.make();
        }
    }
    1. 新增 Wall.java
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    public class Wall {

        private String direction;

        public Wall(String direction) {
            this.direction = direction;
        }

        public String getDirection() {
            return direction;
        }
    }
    1. 新增 Ceiling.java
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    public class Ceiling {

        private int height;
        private int paintedColor;

        public Ceiling(int height, int paintedColor) {
            this.height = height;
            this.paintedColor = paintedColor;
        }

        public int getHeight() {
            return height;
        }

        public int getPaintedColor() {
            return paintedColor;
        }
    }
    1. 新增 Bed.java
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    public class Bed {

        private String style;
        private int pillows;
        private int height;
        private int sheets;
        private int quilt;

        public Bed(String style, int pillows, int height, int sheets, int quilt) {
            this.style = style;
            this.pillows = pillows;
            this.height = height;
            this.sheets = sheets;
            this.quilt = quilt;
        }

        public void make() {
            System.out.println("Bed -> Making");
        }

        public String getStyle() {
            return style;
        }

        public int getPillows() {
            return pillows;
        }

        public int getHeight() {
            return height;
        }

        public int getSheets() {
            return sheets;
        }

        public int getQuilt() {
            return quilt;
        }
    }
    1. 新增 Lamp.java
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    public class Lamp {

        private String style;
        private boolean battery;
        private int globRating;

        public Lamp(String style, boolean battery, int globRating) {
            this.style = style;
            this.battery = battery;
            this.globRating = globRating;
        }

        public void turnOn() {
            System.out.println("Lamp -> Turning on");
        }

        public String getStyle() {
            return style;
        }

        public boolean isBattery() {
            return battery;
        }

        public int getGlobRating() {
            return globRating;
        }
    }
    1. 修改 Main.java
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    Wall wall1 = new Wall("West");
    Wall wall2 = new Wall("East");
    Wall wall3 = new Wall("South");
    Wall wall4 = new Wall("North");

    Ceiling ceiling = new Ceiling(12, 55);

    Bed bed = new Bed("Modern", 4, 3, 2, 1);

    Lamp lamp = new Lamp("Classic", false, 75);

    Bedroom bedroom = new Bedroom("Vincent", wall1, wall2, wall3, wall4, ceiling, bed, lamp);
    bedroom.makeBed();
    bedroom.getLamp().turnOn();
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