why wrote:
"this" is always of type Circle, unless you cast it as a Point.

class Point {
  protected int x,y;
  public Point(int a,int b) {
    x=a;
    y=b;
    System.out.println("1: " + this);  // print this
  }
  public String toString() {
      return "[" + x + "," + y + "]";
  }  
}  
class Circle extends Point {
  protected double radius;
  public Circle(double r,int a,int b) {
    super(a,b);
    radius=r;
    System.out.println("2: " + this); // print this
  }
  public String toString(){
    return "Center = " + super.toString() + ", Radius = " + radius;
  }
}
public class Test {
  public static void main(String[] args) {
    Point p1,p2;
    // from Circle to Point, we don't need to cast here
    p1 = new Circle(4.5,72,29);
    p2 = new Circle(10.0,5,5);
  }
}

C:\>javac Test.java

C:\>java -cp . Test
1: Center = [72,29], Radius = 0.0
2: Center = [72,29], Radius = 4.5
1: Center = [5,5], Radius = 0.0
2: Center = [5,5], Radius = 10.0

科大梦之队 wrote:
我一直在研究这个题,以上各位的解释我都不能认同,这应该是个
方法的覆盖问题.

class Point{
protected int x,y;
public Point(int a,int b){
x=a;
y=b;
System.out.println("Point constructor:"+this.toString());
}
public String toString(){//父类的方法
return"["+x+","+y+"]";
}
}
class Circle extends Point{
protected double radius;
public Circle(double r,int a,int b){
super(a,b);
radius=r;
System.out.println("Circle constructor:"+this.toString());
}
public String toString(){// 子类重新定义与父类同名的方法,实现对父类方法的覆盖!
return "Center="+super.toString()+"Radius="+radius;
}
}
public class Test{
public static void main(String[] args){
Circle circle1,circle2;
circle1=new Circle(4.5,72,29);
circle2=new Circle(10.0,5,5);
}
}
所以两次调用的都是子类的方法,不过父类缺少一个域,就采用了默认值!