Java Program to Check Whether a Number is Prime or Not

# Java Program to Check Whether a Number is Prime or Not

#### In this article, you'll learn to check whether a number is prime or not. This is done using a for loop and while loop in Java.

A prime number is a number which is divisible by only two numbers: 1 and itself. So, if any number is divisible by any other number, it is not a prime number.

## Example 1: Program to Check Prime Number using a for loop

``````public class Prime {

public static void main(String[] args) {

int num = 29;
boolean flag = false;
for(int i = 2; i <= num/2; ++i)
{
// condition for nonprime number
if(num % i == 0)
{
flag = true;
break;
}
}

if (!flag)
System.out.println(num + " is a prime number.");
else
System.out.println(num + " is not a prime number.");
}
}``````

Output

`29 is a prime number.`

In the above program, for loop is used to determine if the given number num is prime or not.

Here, note that we are looping from 2 to num/2. It is because a number is not divisible by more than its half.

Inside the `for` loop, we check if the number is divisible by any number in the given range `(2...num/2)`.

• If num is divisible, flag is set to `true` and we break out of the loop. This determines num is not a prime number.
• If num isn't divisible by any number, flag is false and num is a prime number.

## Example 2: Program to Check Prime Number using a while loop

``````public class Prime {

public static void main(String[] args) {

int num = 33, i = 2;
boolean flag = false;
while(i <= num/2)
{
// condition for nonprime number
if(num % i == 0)
{
flag = true;
break;
}

++i;
}

if (!flag)
System.out.println(num + " is a prime number.");
else
System.out.println(num + " is not a prime number.");
}
}``````

Output

`33 is not a prime number.`

In the above program, while loop is used instead of a for loop. The loop runs until `i <= num/2`. On each iteration, whether num is divisble by i is checked and the value of i is incremented by 1.

Visit this page to learn, how you can display all prime numbers between two intervals.