类继承，定义了一个点类point，然后线条类line继承了point类，正方形类square继承line类

  1 /**
  2  *
  3 java继承实例。
  4 
  5 
  6 定义了一个点类point，然后线条类line继承了point类，正方形类square继承line类。
  7 
  8  */
  9 
 10 //点类
 11 class PointDemo
 12 {
 13     private int x;
 14     private int y;
 15     private static int pCount=0;
 16 
 17     //默认初始化坐标点（0,0），坐标点数计数+1
 18     PointDemo(){
 19         this.x=0;
 20         this.y=0;
 21         pCount++;
 22     }
 23 
 24     //带有x,y坐标参数构造函数，坐标点数计数+1
 25     PointDemo(int x, int y){
 26         this.x=x;
 27         this.y=y;
 28         pCount++;
 29     }
 30   
 31     //设置坐标x,y值
 32     public void setPoint(int x,int y){
 33         this.x=x;
 34         this.y=y;
 35     }
 36 
 37     //返回获取X,Y坐标值
 38     public int getX(){return x;}
 39     public int getY(){return y;}
 40 
 41     //返回程序累积坐标计数
 42     public static int getPCount(){return pCount;}
 43     
 44 }
 45 
 46 //行类继承点类
 47 class LineDemo extends PointDemo
 48 {
 49     private PointDemo po1;
 50     private PointDemo po2;
 51     private float range;
 52     private static int lCount=0;
 53     
 54     //返回程序累积线数计数
 55     public static int getLCount(){return lCount;}
 56 
 57     //默认空参构造函数，默认坐标(0,0),(4,3),初始化两坐标之间的长度，线数计数+1
 58     LineDemo(){
 59         po1=new PointDemo();
 60         po2=new PointDemo(4,3);
 61         setRange();
 62         lCount++;
 63     }
 64 
 65     //两坐标参数构造函数，初始化两坐标和坐标之间的长度，线数计数+1
 66     LineDemo(PointDemo po1,PointDemo po2){
 67         this.po1=po1;
 68         this.po2=po2;
 69         setRange();
 70         lCount++;
 71     }
 72 
 73     //返回获取两点坐标值
 74     public PointDemo getPo1(){return po1;}
 75     public PointDemo getPo2(){return po2;}
 76 
 77     //设置两点之间的长度
 78     public void setRange(){ 
 79         range=(float)Math.sqrt(Math.pow(Math.abs(po2.getX()-po1.getX()),2)+Math.pow(Math.abs(po2.getY()-po1.getY()),2));;
 80     }
 81     public float getRange(){return range;}
 82 
 83     //输出两点坐标及两点之间的距离
 84     public void out(){
 85         System.out.println("___________________________");
 86         System.out.println("("+po1.getX()+","+po1.getY()+")");
 87         System.out.println("("+po2.getX()+","+po2.getY()+")");
 88         System.out.println("range="+range);
 89         System.out.println("===========================");
 90     }
 91 }
 92 
 93 //正方型类
 94 /*
 95 (X1,Y1),(X2,Y2),(X3,Y3),(X4,Y4)
 96 A = X2-X1,     B =  Y2-Y1;
 97 X3  =   X1+(A-B)     =   X1+(X2-X1-Y2+Y1)   =    X2-Y2+Y1;
 98 Y3  =   Y1+A+B       =   Y1+X2-X1+Y2-Y1     =    X2-X1+Y2;
 99 X4  =   -(X2-(A-B))  =   -(X2+Y2-Y1-X2+X1)  =    Y1-X1-Y2;
100 Y4  =   Y2+(A-B)     =   Y2+X2-X1-Y2+Y1     =    X2-X1+Y1;
101 
102 */
103 class Square extends LineDemo
104 {
105     private LineDemo[] line = new LineDemo[6];
106     private PointDemo[] point = new PointDemo[4];
107     private static int sCount=0;
108 
109     //返回累积正方形个数计数
110     public static int getSCount(){return sCount;}
111 
112     //空参构造函数，默认初始化正方形四个角坐标及边长和周长的长度，正方形个数计数+1
113     Square(){
114         super();
115         setSquarePoint();
116         setSquareLine();
117         sCount++;
118     }
119 
120     //两坐标点参数构造函数，初始化正方形四个角坐标及边长和周长的长度，正方形个数计数+1
121     Square(PointDemo po1,PointDemo po2){
122         super(po1,po2);
123         point[0]=po1;
124         point[1]=po2;
125         setSquarePoint();
126         setSquareLine();
127         sCount++;
128     }
129 
130     //一条边长做参数的构造函数，初始化正方形四个角坐标及边长和周长的长度，正方形个数计数+1
131     Square(LineDemo line){
132         super(line.getPo1(),line.getPo2());
133         point[0]=line.getPo1();
134         point[1]=line.getPo2();
135         setSquarePoint();
136         setSquareLine();
137         sCount++;
138     }
139 
140     //设置正方形四个角坐标
141     private void setSquarePoint(){
142         int x1=point[0].getX();
143         int y1=point[0].getY();
144         int x2=point[1].getX();
145         int y2=point[1].getY();
146         point[2] = new PointDemo(x2-y2+y1,x2-x1+y2);
147         point[3] = new PointDemo(y1-x1-y2,x2-x1+y1);
148     }
149 
150     //设置正方形所有边长和对角线坐标及长度
151     private void setSquareLine(){
152         for (int i=0,k=0;i<point.length ;i++ )
153         {
154             for (int j=i+1;j<point.length ;j++,k++ )
155             {
156                 line[k] =new LineDemo(point[i],point[j]);
157             }
158         }
159 
160     }
161 
162     //输出所有边长和对角线坐标及长度
163     public void outAllLine(){
164         for (LineDemo li : line)
165         {
166             li.out();
167         }
168     }
169 
170     //输出正方形四个角坐标
171     public void outPoint(){
172         for (int i=0;i<point.length ;i++ )
173         {
174             System.out.println("point["+i+"] = ("+point[i].getX()+","+point[i].getY()+")");
175         }
176     }
177 }
178 
179 class ExtendsDemo 
180 {
181     public static void main(String[] args) 
182     {
183         //测试点
184         PointDemo[] pArr = new PointDemo[10];
185         for (int i=0,j=10;i<pArr.length ;i++,j-- )
186         {
187             pArr[i]=new PointDemo(i,j);
188         }
189         for (PointDemo po: pArr )
190         {
191             System.out.println("("+po.getX()+","+po.getY()+")");
192         }
193         System.out.println("pointCount="+PointDemo.getPCount()+"\n");
194 
195         //测试线
196         LineDemo line = new LineDemo();
197         line.out();
198         System.out.println("lineCount="+LineDemo.getLCount()+"\n");
199         LineDemo line1 = new LineDemo(pArr[0],pArr[9]);
200         line1.out();
201         System.out.println("lineCount="+LineDemo.getLCount()+"\n");
202         System.out.println("pointCount="+LineDemo.getPCount()+"\n");
203 
204         //测试正方形
205         Square su = new Square(line1);
206         System.out.println("显示所有坐标");
207         su.outPoint();      //显示所有坐标
208         System.out.println("显示所有边坐标及长度");
209         su.outAllLine();    //显示所有边坐标及长度
210         System.out.println("SquareCount="+Square.getSCount()+"\n");
211     }
212 }