新手写的！不会改！太逊了！

[EdgerLee]

public class Point{
protected int x,y;
Point(int a,int b){setPoint(a,b);}
public void setPoint(int a,int b){
x=a;
y=b;
}
public int getX(){return x;}
public int getY(){return y;}
}

public class Line extends Point{
protected int x,y,endX,endY;
Line(int x1,int y1,int x2,int y2){setLine(x1,y1,x2,y2);}
public void setLine(int x1,int y1,int x2,int y2){
x=x1;
y=y1;
endX=x2;
endY=y2;
}
public int getX(){return x;}
public int getY(){return y;}
public int getEndX(){return endX;}
public int getEndY(){return endY;}
public double length(){
return Math.sqrt((endX-x)*(endX-x)+(endY-y)*(endY-y));
}
}

public class Circle extends Point{
protected int radius;
Circle(int a,int b,int r){
super(a,b);
setRadius(r);
}
public void setRadius(int r){radius=r;}
public int getRadius(){return radius;}
public double area(){return 3.14159*radius*radius;}
}
错误提示：
Point.java:12: class Line is public, should be declared in a file named Line.jav
a
public class Line extends Point{
^
Point.java:30: class Circle is public, should be declared in a file named Circle
.java
public class Circle extends Point{
^
Point.java:14: cannot resolve symbol
symbol : constructor Point ()
location: class Point
Line(int x1,int y1,int x2,int y2){setLine(x1,y1,x2,y2);}
^
3 errors

[jcq]

不要在一个文件中定义多个public的class

[ChDw]

你必须将这两个类分别保存成两个文件
一个文件中只是允许存在一个最外面的类（内部类是存在最外类的内部的）

[jcq]

第三个错的原因是：
你Line从Point继承，但是你的Point没有不带参数的构造函数就出错了，改的方法是：
定义一个不带参数的构造，还有就是在Line的构造中使用super(int x1,int y1,int x2,int y2);

[yzxasd]

一个文件里可以有多个类，但是只能有一个public

[jcq]

记住super这句一定要放在Line构造的第一句