public class RoundArea extends Area{
    double roundarea , width;
    double area(double width){
    roundarea=Math.PI*width*width;
    return roundarea;    
   }
}


.......


RoundArea c=new RoundArea ();
c.area(1.0);//根据你在RoundArea定义的方法需要你传入一个double 类型的参数
System.out.println("圆的面积是"+c); //这样就出来结果了

//矩形类似的

------解决方案--------------------
Area.java

	public abstract class Area {
		public  double area(double x){
		
			return Math.PI * x * x ;
		}
		public  double area(double x, double y ){
		
			return x * y ;
		}
	}

RoundArea.java

	public class RoundArea extends Area{
			//private double roundarea , width;
			/**
			public double area(double width){
				roundarea=Math.PI*width*width;
				return roundarea;
			}
			*/
	}

RectArea.java

	  public class RectArea extends Area{
			//private double rectarea ,length, width;
			
			/**
			public double area(double length,double width){
				rectarea=length*width;
				return rectarea;
			}
			*/
		}


ImpleArea.java
	  public class ImpleArea{
				private double length,width;
				
				//保留默认构造函数
				public ImpleArea(){
				}
				//定义一个构造函数 ，初始化私有变量
				public ImpleArea(double length, double width){
					this.length = length ;
					this.width  = width ;
				}
				
				public double getx(){
					System.out.println("length="+length);
					System.out.println("length="+width);
					
					//到底是想返回 length 还是 width 还是 length * width ,看你的方法名定义getx()应该是返回 length  
					return length;
					//return width;
				}
		}


Test.java

public class Test{
		public static void main(String[] args) {
		// TODO Auto-generated method stub
		
		    Area c = new RoundArea() ;
		    double dc = c.area(10.1) ; // 你定义的area方法是有一个double参数的，调用的时候需要放入参数啊