import java.util.Scanner; public class Circle { private double r; public Circle() { } public Circle(double r) { this.r = r; } public double area() { return Math.PI * r * r; }} 将代码中的main 方法定义为: public static void main(String[] args)

import java.util.Scanner;

public class Circle {
private double r;

public Circle() {
}

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

public double area() {
return Math.PI * r * r;
}

public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
System.out.print("请输入圆的半径：");
double r = scanner.nextDouble();
Circle circle = new Circle(r);
System.out.println("圆的面积为：" + circle.area());
}
}