写一个Java程序来求正方形、长方形和圆形的面积,用三种不同的方法来求面积。

好的，我可以回答您的问题。以下是Java程序来求正方形、长方形和圆形的面积，使用三种不同的方法：

方法1：使用公式直接求解

import java.util.Scanner;

public class Area {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println("请输入图形的类型：1.正方形 2.长方形 3.圆形");
        int choice = sc.nextInt();
        switch (choice) {
            case 1:
                System.out.println("请输入正方形的边长：");
                double side = sc.nextDouble();
                double area = side * side;
                System.out.println("正方形的面积为：" + area);
                break;
            case 2:
                System.out.println("请输入长方形的长：");
                double length = sc.nextDouble();
                System.out.println("请输入长方形的宽：");
                double width = sc.nextDouble();
                area = length * width;
                System.out.println("长方形的面积为：" + area);
                break;
            case 3:
                System.out.println("请输入圆形的半径：");
                double radius = sc.nextDouble();
                area = Math.PI * radius * radius;
                System.out.println("圆形的面积为：" + area);
                break;
            default:
                System.out.println("输入有误，请重新输入！");
        }
        sc.close();
    }
}

方法2：使用多态性和继承

abstract class Shape {
    public abstract double getArea();
}

class Square extends Shape {
    private final double side;

    public Square(double side) {
        this.side = side;
    }

    public double getArea() {
        return side * side;
    }
}

class Rectangle extends Shape {
    private final double length;
    private final double width;

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

    public double getArea() {
        return length * width;
    }
}

class Circle extends Shape {
    private final double radius;

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

    public double getArea() {
        return Math.PI * radius * radius;
    }
}

import java.util.Scanner;

public class Area {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        System.out.println("请输入图形的类型：1.正方形 2.长方形 3.圆形");
        int choice = sc.nextInt();
        switch (choice) {
            case 1:
                System.out.println("请输入正方形的边长：");
                double side = sc.nextDouble();
                Shape square = new Square(side);
                System.out.println("正方形的面积为：" + square.getArea());
                break;
            case 2:
                System.out.println("请输入长方形的长：");
                double length = sc.nextDouble();
                System.out.println("请输入长方形的宽：");
                double width = sc.nextDouble();
                Shape rectangle = new Rectangle(length, width);
                System.out.println("长方形的面积为：" + rectangle.getArea());
                break;
            case 3:
                System.out.println("请输入圆形的半径：");
                double radius = sc.nextDouble();
                Shape circle = new Circle(radius);
                System.out.println("圆形的面积为：" + circle.getArea());
                break;
            default:
                System.out.println("输入有误，请重新输入！");
        }
        sc.close();
    }
}

方法3：使用反射和注解

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import java.lang.reflect.Method;
import java.util.Scanner;

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
@interface AreaMethod {
    String value();
}

class AreaCalculator {
    @AreaMethod("正方形的面积为")
    public static double squareArea(double side) {
        return side * side;
    }

    @AreaMethod("长方形的面积为")
    public static double rectangleArea(double length, double width) {
        return length * width;
    }

    @AreaMethod("圆形的面积为")
    public static double circleArea(double radius) {
        return Math.PI * radius * radius;
    }
}

public class Area {
    public static void main(String[] args) throws Exception {
        Scanner sc = new Scanner(System.in);
        System.out.println("请输入图形的类型：1.正方形 2.长方形 3.圆形");
        int choice = sc.nextInt();
        switch (choice) {
            case 1:
                System.out.println("请输入正方形的边长：");
                double side = sc.nextDouble();
                Method squareMethod = AreaCalculator.class.getMethod("squareArea", double.class);
                double squareArea = (double) squareMethod.invoke(null, side);
                AreaMethod squareAnnotation = squareMethod.getAnnotation(AreaMethod.class);
                System.out.println(squareAnnotation.value() + " " + squareArea);
                break;
            case 2:
                System.out.println("请输入长方形的长：");
                double length = sc.nextDouble();
                System.out.println("请输入长方形的宽：");
                double width = sc.nextDouble();
                Method rectangleMethod = AreaCalculator.class.getMethod("rectangleArea", double.class, double.class);
                double rectangleArea = (double) rectangleMethod.invoke(null, length, width);
                AreaMethod rectangleAnnotation = rectangleMethod.getAnnotation(AreaMethod.class);
                System.out.println(rectangleAnnotation.value() + " " + rectangleArea);
                break;
            case 3:
                System.out.println("请输入圆形的半径：");
                double radius = sc.nextDouble();
                Method circleMethod = AreaCalculator.class.getMethod("circleArea", double.class);
                double circleArea = (double) circleMethod.invoke(null, radius);
                AreaMethod circleAnnotation = circleMethod.getAnnotation(AreaMethod.class);
                System.out.println(circleAnnotation.value() + " " + circleArea);
                break;
            default:
                System.out.println("输入有误，请重新输入！");
        }
        sc.close();
    }
}

以上三种方法都可以用来求解正方形、长方形和圆形的面积，具体使用哪种方法取决于场合和需求。