```
interface Shape{
    double area(double len);
}

class Square implements Shape{
    public double area(double len){
        if(len < 0){
            return 0;
        }
        return len*len;
    }
}

class Circle implements Shape{
    public double area(double r){
        if(r < 0){
            return 0;
        }
        return r*r*Math.PI;
    }
}

///测试

public class MainTest {
    @org.junit.Test
    public void testSquare() throws Exception {
        Shape square = new Square();
        assertEquals(4,square.area(2),0.001);

    }

    @org.junit.Test
    public void testCircle() throws Exception {
        Shape circle = new Circle();
        assertEquals(3*3*Math.PI,circle.area(3),0.001);
    }

}