关于java抽象类的实现为什么这样写错误呢

package jj;
interface shape{
    abstract double computer(double a,double b);
}
class Triangle implements shape{
    double S;
    public double computer(double a,double b) {
        S=a*b/2.0;
        return S;
    }
class Rectangle implements shape{
    public double computer(double a,double b) {
        return a*b;
    }
}
}
public class Test3 {
   public static void main(String[] args) {
       Triangle p=new Triangle();
       Rectangle p1=new Rectangle();
       System.out.println("三角形的面积为"+p.computer(2, 1) );
       System.out.println("矩形的面积为"+p1.computer(2,2));
   }
}
 

什么错误？