java oop

本文主要作为java笔记，方便以后查看，大部分内容都源于以下网站：

　　http://www.ntu.edu.sg/home/ehchua/programming/index.html#Game

主要关于java面向对象编程。

Circle.java

public class Circle {
    public static final double RADIUS = 1.0;　　　　//final 表示常量，只读
    public static final String COLOR  = "red";

    private double radius;
    private String color;
　　//构造函数 constructor，无返回值
    public Circle() {
        this.radius = RADIUS;
        this.color = COLOR;
    }   
　　//constructor
    public Circle(double radius) {
        this.radius = radius;
        this.color = COLOR;
    }   
　　//constructor
    public Circle(double radius, String color) {
        this.radius = radius;
        this.color = color;
    }   
　　//对private变量的获取和更改，通过下面的get和set函数来完成
    // Public getters and setters for private variables
    public double getRadius() {
        return this.radius;
    }   
    public void setRadius(double radius) {
        this.radius = radius;
    }   

    public String getColor() {
        return this.color;
    }   
    public void setColor(String color) {
        this.color = color;
    }   

    // toString() to provide a short description of this instance 
    public String toString() {
        return "Circle with radius = " + radius + " and color of " + color;
    }   
    public double getArea() {
        return radius * radius * Math.PI;
    }   
}

主函数：

TestCircle.java

public class TestCircle {
    public static void main(String[] args) {
        Circle c1 = new Circle(2.0, "blue");
        System.out.println("Radius is " + c1.getRadius()
                + " Color is " + c1.getColor()
                + " Area is " + c1.getArea());
    
        Circle c2 = new Circle(2.0);
        System.out.println("Radius is " + c2.getRadius()
                + " Color is " + c2.getColor()
                + " Area is " + c2.getArea());

        Circle c3 = new Circle();
        System.out.println("Radius is " + c3.getRadius()
                + " Color is " + c3.getColor()
                + " Area is " + c3.getArea());

        //invoke the toString() function
        System.out.println(c3);
        System.out.println(c3.toString());
    }   
}

运行结果：

Radius is 2.0 Color is blue Area is 12.566370614359172
Radius is 2.0 Color is red Area is 12.566370614359172
Radius is 1.0 Color is red Area is 3.141592653589793
Circle with radius = 1.0 and color of red
Circle with radius = 1.0 and color of red

另外一个例子：

MyPoint.java

public class MyPoint {
    private int x;
    private int y;

    public MyPoint() {
        x = 0;
        y = 0;
    }   
    public MyPoint(int x, int y) {
        this.x = x;
        this.y = y;
    }   
    public int getX() {
        return x;
    }   
    public void setX(int x) {
        this.x = x;
    }   
    public int getY() {
        return x;
    }   
    public void setY(int x) {
        this.y = y;
    }   
    public void setXY(int x,int y) {
        this.x = x;
        this.y = y;
    }   

    public String toString() {
        return "("+ x + ", " + y + ")";
    }   
    public double distance(int x, int y) {
        double xDiff = this.x - x;
        double yDiff = this.y - y;
        return Math.sqrt(xDiff*xDiff + yDiff*yDiff);
    }   
    public double distance(MyPoint another) {
        double xDiff = this.x - another.x;
        double yDiff = this.y - another.y;
        return Math.sqrt(xDiff*xDiff + yDiff*yDiff);
    }   
}

TestMyPoint.java

public class TestMyPoint {
    public static void main(String[] args) {
        MyPoint p1 = new MyPoint();
        System.out.println(p1);
        p1.setXY(0,3);
        System.out.println(p1);

        MyPoint p2 = new MyPoint(4,0);
        System.out.println(p2);

        System.out.println(p1.distance(4,0));
        System.out.println(p1.distance(p2));
    }   
}

运行结果：

(0, 0) 
(0, 3) 
(4, 0) 
5.0 
5.0

note：

Using the keyword "this", the constructor, getter and setter methods for a private variable called xxx of type T are as 

follows:

public class Aaa {
   // A private variable named xxx of type T
   private T xxx;
 
   // Constructor
   public Aaa(T xxx) {
      this.xxx = xxx;
   }
 
   // A getter for variable xxx of type T receives no argument and return a value of type T
   public T getXxx() {
      return xxx;
   }
 
   // A setter for variable xxx of type T receives a parameter of type T and return void
   public void setXxx(T xxx) {
      this.xxx = xxx;
   }
}

For a boolean variable xxx, the getter shall be named isXxx(), instead of getXxx(), as follows:

// Private boolean variable
private boolean xxx;
 
// Getter
public boolean isXxx() {
   return xxx;
}
 
// Setter
public void setXxx(boolean xxx) {
   this.xxx = xxx;
}