声明一个Shape接口，其中有计算面积（方法名area）、周长（方法名perimeter）的方法，有以下几个实现：Circle(圆)，Rectangle(矩形)，Triangle(三角形)，都有计算面积、周长的方法。 编写TestShape类，用于测试

声明Shape接口可以这样设计：

public interface Shape {
    double area(); // 计算面积
    double perimeter(); // 计算周长
}

接着，我们为每个形状实现这个接口：

public class Circle implements Shape {
    private double radius;

    public Circle(double radius) {
        this.radius = radius;
    }

    @Override
    public double area() {
        return Math.PI * radius * radius;
    }

    @Override
    public double perimeter() {
        return 2 * Math.PI * radius;
    }
}

public class Rectangle implements Shape {
    private double width;
    private double height;

    public Rectangle(double width, double height) {
        this.width = width;
        this.height = height;
    }

    @Override
    public double area() {
        return width * height;
    }

    @Override
    public double perimeter() {
        return 2 * (width + height);
    }
}

public class Triangle implements Shape {
    private double base;
    private double height;

    public Triangle(double base, double height) {
        this.base = base;
        this.height = height;
    }

    @Override
    public double area() {
        return 0.5 * base * height;
    }

    @Override
    public double perimeter() {
        // 假设这是一个等腰三角形，仅计算底边+两腰
        // 实际情况可能需要提供具体的三边长度
        double side = base / 2;
        return base + 2 * side;
    }
}

最后，我们创建TestShape类来进行测试：

public class TestShape {
    public static void main(String[] args) {
        Shape circle = new Circle(5);
        Shape rectangle = new Rectangle(4, 6);
        Shape triangle = new Triangle(3, 4);

        System.out.println("Circle area: " + circle.area());
        System.out.println("Circle perimeter: " + circle.perimeter());

        System.out.println("Rectangle area: " + rectangle.area());
        System.out.println("Rectangle perimeter: " + rectangle.perimeter());

        System.out.println("Triangle area: " + triangle.area());
        System.out.println("Triangle perimeter: " + triangle.perimeter());
    }
}

在这个例子中，`TestShape`类通过Shape接口对各种形状的对象进行了操作，体现了多态性，即不关心具体的形状，只关注共同的行为。