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