Using own method in Java to test a number is prime or not

Question

Hi!

I wrote a program to check a number is prime or not using a boolean method. Here is my code:

package prim;
import java.util.Scanner;

public class Prim {

public static boolean isPrime(int num)
{
if ( num % 2 == 0 && num > 2 && num % 1 == 0)
{
return false;
}
return true;
}

public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
System.out.print("Kérem a számot, amiről el kell döntenem, hogy prím-e: ");
int n = sc.nextInt();
while (n < 2) {
System.out.print("Kérem a számot, amiről el kell döntenem, hogy prím-e: ");
n = sc.nextInt();
}
boolean m = isPrime(n);
if (isPrime(n))
{
System.out.println("A szám prím.");
}
else
{
System.out.println("A szám nem prím.");
}

}
}
The problem is, it says to 45 and 93 are prime numbers, but they aren't. Where's the problem in the code? Thanks in advance.

Answer 1

A prime number is divisible by 1 and itself only.
You can check if a number is prime by confirming if it is divisible by any other number asides 1 and itself. For that you can use a for loop.
Something like
for(int num = 2; i < theNumberwetesting; i++){
//check if its divisible by i
}
loop started from 2 to avoid zero division error and because it is surely going to be divisible by 1.
For a faster performance you can set the loop limit to numberWeAreTesting/2

Answer 2

Nice one @Tibor Toth
🤔Sometimes i really wish i could upvote things https://www.sololearn.com/Discuss/1089145/?ref=app

Answer 3

I solved with while loop:

public static boolean isPrime(int num)
{

int i=2;
while(i<= num/2)
{
if(num % i == 0)
{
return false;
}
i++;
}
return true;
}