程序代码：
class Trangle
{
double sideA,sideB,sideC,area,length;
boolean boo;
public Trangle(double a,double b,double c)
{ sideA=a;
   sideB=b;
   sideC=c;
   if(a+b>c||a+c>b||b+c>a)
   { boo=ture;
   }
   else
   { boo=false;
   }
  }

double getLength()
{ length=sideA+sideB+sideC;
   return length;
}

public double getArea()
{ if(boo)
    { double p=(sideA+sideB+sideC)/2.0;
      area=Math.sqrt(p*(p-sideA)*(p-sideB)*(p-sideC));
      return area;
    }
    else
    { System.out.println("不是一个三角形，不能计算面积");
      return 0;
    }
}

public void setABC(double a,double b,double c)
{ sideA=a;
   sideB=b;
   sideC=c;
   if(a+b>c||a+c>b||b+c>a)
   { boo=ture;
   }
   else
   { boo=false;
   }
}
}
class Lader
{ double above,bottom,height,area;
  Lader(double a,double b,double h)
  { above=a;
    bottom=b;
    height=h;
  }
  double getArea()
  { area=(above+bottom)*height/2.0;
    return area;
  }
}

class Circle
{ double radius,area;
  Circle(double r)
  { radius=r;
  }
  double getArea()
  { area=3.14*radius*radius;
    return area;
  }
  double getLength()
  { length=2.0*3.14*radius;
    return length;
  }
  void setRadius(double newRadius)
  { radius=newRadius;
  }
  double getRadius()
  { return radius;
  }
}

public class AreaAndLength
{ public static void main(String args[])
  { double length,area;
    Circle circle=null;
    Trangle trangle;
    Lader lader;
    circle=new Circle(double r);
    trangle=new Trangle(double a,double b,double c);
    lader=new Lader(double a,double b,double h);
    length=circle.getLength();
    System.out.println("圆的周长:"+length);
    area=circle.getArea();
    System.out.println("圆的面积:"+area);
    length=trangle.getLength();
    System.out.println("三角形的周长:"+length);
    area=trangle.getArea();
    System.out.println("三角形的面积:"+area);
    area=lader.getArea();
    System.out.println("梯形的面积:"+area);
    trangle.Trangle(12.0,34.0,1.0);
    area=trangle.getArea();
    System.out.println("三角形的面积:"+area);
    length=trangle.getLength();
    System.out.println("三角形的周长:"+length);
  }
}

实在是非常感谢啊