在程序左侧设置断点并启动debug就可以调试
不多做赘述

package codepractice;

import java.util.Scanner;

public class Main {

    public static void main(String args[])
    {
        Scanner input = new Scanner(System.in);
        System.out.println("Enter a number for radius: ");
        double radius =input.nextDouble();
        double area;
        area = radius * radius *3.14159;
        System.out.println("the area for the circle of radius "+radius + " is "+area);
    }
}
package codepractice;
import java.util.Scanner;
public class Main {
    public static void main(String args[])
    {
        double radius;
        double area;
        radius = 20;
        area = radius * radius *3.14159;
        System.out.println("the area for the circle of radius "+radius + " is "+area);
    }
}
package codepractice;

import java.util.Scanner;

public class Main {

    public static void main(String args[])
    {
        Scanner input = new Scanner(System.in);
        System.out.println("enter three numbers: ");
        double number1=input.nextDouble();
        double number2=input.nextDouble();
        double number3=input.nextDouble();
        double average=(number1+number2+number3)/3;
        System.out.println("the average of " +number1+" "+number2+" "+number3+" "+" is "+average);
    }
}
package codepractice;

import java.util.Scanner;

public class Main {

    public static void main(String args[])
    {
        final double pi =3.14159;
        Scanner input = new Scanner(System.in);
        System.out.print("enter a number for dadius: ");
        double radius = input.nextDouble();
        double area=radius*radius*pi;
        System.out.println("the area for the circle of radius "+radius+" is "+area);
    }
}