用Java实现用自定义方法， 分别求三角形、矩形和圆的面积。在主函数中调用自定义方法， 并返回面积

在Java中，你可以创建一个名为Shape的抽象基类，然后为具体的形状如Triangle（三角形）、Rectangle（矩形）和Circle（圆）分别创建子类，并重写计算面积的方法。下面是一个简单的示例：

// 创建抽象基类 Shape
abstract class Shape {
    abstract double getArea();
}

// 三角形类 Triangle
class Triangle extends Shape {
    private double base;
    private double height;

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

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

// 矩形类 Rectangle
class Rectangle extends Shape {
    private double length;
    private double width;

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

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

// 圆类 Circle
class Circle extends Shape {
    private double radius;

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

    @Override
    double getArea() {
        return Math.PI * Math.pow(radius, 2);
    }
}

public class Main {
    public static void main(String[] args) {
        // 创建并初始化每个形状的对象
        Triangle triangle = new Triangle(4, 6);
        Rectangle rectangle = new Rectangle(5, 7);
        Circle circle = new Circle(3);

        // 调用各自的面积计算方法
        double triangleArea = triangle.getArea();
        double rectangleArea = rectangle.getArea();
        double circleArea = circle.getArea();

        System.out.println("三角形的面积: " + triangleArea);
        System.out.println("矩形的面积: " + rectangleArea);
        System.out.println("圆的面积: " + circleArea);