Java定义一个图形接口，包含周长和面积方法。声明三角形、矩形和圆形类，实现上述接口。用Java代码实现

在Java中，我们可以首先定义一个`Shape`接口，它包含了计算周长`perimeter()`和面积`area()`的方法。然后，我们创建三个具体的形状类`Triangle`, `Rectangle`, 和 `Circle`，它们分别继承自`Shape`接口，并提供各自的实现。

// 定义Shape接口
public interface Shape {
    double perimeter();
    double area();
}

// 实现三角形类
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 perimeter() {
        return base + (base * 0.5); // 假设这是一个等腰三角形，只给出一个边长
    }

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

// 实现矩形类
public class Rectangle implements Shape {
    private double length;
    private double width;

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

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

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

// 实现圆形类
public class Circle implements Shape {
    private double radius;

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

    @Override
    public double perimeter() {
        return 2 * Math.PI * radius; // 使用Math.PI计算圆的周长
    }

    @Override
    public double area() {
        return Math.PI * Math.pow(radius, 2); // 计算圆的面积
    }
}

现在，你可以创建并使用这三个类的对象，因为它们都实现了`Shape`接口：

public class Main {
    public static void main(String[] args) {
        Triangle triangle = new Triangle(4, 6);
        Rectangle rectangle = new Rectangle(5, 7);
        Circle circle = new Circle(3);

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

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

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