程序在 main 函数内部调用时终止

Question

I am new to java programming, I have programmed in C and C++ but have moved to Java recently, So I am a bit confuse about how things are in Java. I am calling a function inside my main but the program gets terminated, I don't know why its happening and can't figure it out. Here is my program

package Prime;
import java.util.Scanner;
public class isprime
{
    public static boolean isPrime (int n)
    {
        int flag=0;
        for (int i=2;i<=n;i++)
        {
            if(i%n==0)
            {
                flag=1;
            }
        }   
        if(flag==1)
        {
            return false;
        }
        else
        {
            return true;
        } 
    }   
    public static void main(String[] args)
    {
    int n;
    System.out.println("Please enter a number you want to test");
    Scanner sc = new Scanner(System.in);
    sc.close();
    isPrime(n);
    }   
    n = sc.nextInt();
}

Answer 1

• Your n = sc.nextInt(); is outside the scope of main() function. Moreover your are closing the scanner first.

• You called isPrime(n); which returns boolean but you did not catch the return value.

You want to do something like:

public static void main(String[] args) {
    int n;
    System.out.println("Please enter a number you want to test");
    Scanner sc = new Scanner(System.in);
    n = sc.nextInt();

    if (isPrime(n)) {
        System.out.println("prime");
    } else {
        System.out.println("not prime");
    }
    sc.close();
}

Finally , your prime calculation is wrong. A prime number is divisible by itself, so change

for (int i=2;i<=n;i++)

to

for (int i=2;i < n;i++)
              ^^^