需要帮助- Java简单的规则形状面积计算器与if语句

import java.util.Scanner;

public class ShapeAreaCalculator {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        
        System.out.println("请选择要计算的形状（1-矩形，2-圆形，3-三角形）：");
        int shape = scanner.nextInt();
        
        if (shape == 1) {
            System.out.println("请输入矩形的长度：");
            double length = scanner.nextDouble();
            
            System.out.println("请输入矩形的宽度：");
            double width = scanner.nextDouble();
            
            double area = length * width;
            System.out.println("矩形的面积为：" + area);
        } else if (shape == 2) {
            System.out.println("请输入圆形的半径：");
            double radius = scanner.nextDouble();
            
            double area = Math.PI * radius * radius;
            System.out.println("圆形的面积为：" + area);
        } else if (shape == 3) {
            System.out.println("请输入三角形的底边长度：");
            double base = scanner.nextDouble();
            
            System.out.println("请输入三角形的高度：");
            double height = scanner.nextDouble();
            
            double area = 0.5 * base * height;
            System.out.println("三角形的面积为：" + area);
        } else {
            System.out.println("输入无效的形状选择！");
        }
        
        scanner.close();
    }
}