``` 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); } }