Java Program to Check Prime Number

Write a Java Program to Check Prime Number using For Loop, While Loop, and Functions. Prime Numbers are any natural number that is not divisible by any other number except 1 and itself.

And they are: 2, 3, 5,7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, etc.

Java Program to Check Prime Number using For Loop

This program allows the user to enter any integer value. Next, this Java example program checks whether the given number is a Prime or Not using For Loop.

TIP: 2 is the only even.

import java.util.Scanner;

public class Example {	
	
	private static Scanner sc;
	
	public static void main(String[] args) 
	{	
		int i, number, count = 0; 
		sc = new Scanner(System.in);
		
		System.out.print(" Please Enter any Integer : ");
		number = sc.nextInt();		
		
		for (i = 2; i <= number/2; i++)
		{
			if(number % i == 0)
		    {
				count++;
		        break;
		    }	
		}
		if(count == 0 && number != 1 )
		{
			System.out.println( number + " is a Prime");
		}
		else
		{
		   System.out.println(number + " is Not");
		}
	}
}

Java Program to Check Prime Number using for loop output

We initialized the integer i value to 2, and (i <= Number/2) condition to terminate when the condition fails.

Within the for loop, there is an If statement to check whether the Number divisible by i is exactly equal to 0 or not. If the condition is True, then Count is incremented, and then Break Statement is executed.

Next, we used another If statement to check whether Count is Zero, and the Number is Not equal to 1. If it is True then prime.

The user entered an integer in the above Java Program to Check the Prime Number example is 11

First Iteration: for(i = 2; i <= 11/2; i++)
Condition (2 <= 5.5) is True. So, it checks the If Statement.
if(number % i == 0) => if (11%2 == 0) – Condition is False
i++ means i becomes 3

Second Iteration: (i = 3; 3 <= 11/2; i++)
Condition (3 <= 5.5) is True.
if (11 % 3 == 0) – Condition is False
i become 4

Third Iteration: (i = 4; 4 <= 11/2; i++)
Condition (4 <= 5.5) is True.
if (11 % 4 == 0) – Condition is False
i become 5

Fourth Iteration: (i = 5; 5 <= 11/2; i++)
Condition (5 <= 5.5) is True.
if (11 % 5 == 0) – Condition is False
i become 6

Fifth Iteration: (i = 6; 6 <= 11/2; i++)

It means the condition inside the For loop (6 <= 5.5) is False. So, the compiler comes out of the For Loop.

Next, it will enter into the If statement. if(count == 0 && Number!= 1 ). 

In all five iterations, the Java If statement failed, so Count Value has not incremented from initialized 0, and Number= 11. So, the condition is True, and the number is Prime.

Java Program to Check Prime Number using While Loop

This example checks whether the given number is a Prime or not using While Loop.

import java.util.Scanner;

public class Example {	
	
	private static Scanner sc;
	
	public static void main(String[] args) 
	{	
		int i = 2, num, count = 0; 
		sc = new Scanner(System.in);
		
		System.out.print(" Please Enter any Value : ");
		num = sc.nextInt();		
		
		while(i <= num/2)
		{
			if(num % i == 0)
		    {
				count++;
		        break;
		    }
			i++;
		}
		if(count == 0 && num != 1 )
		{
			System.out.println( num + " is a Prime");
		}
		else
		{
		   System.out.println(num + " is Not");
		}
	}
}

We just replaced the For loop in the above example with the While loop. If you don’t know the While Loop, please refer to WHILE LOOP.

output

 Please Enter any Value : 193
193 is a Prime

Let me enter another

 Please Enter any Value : 32
32 is Not

Java Program to Check Prime Number Using Function

This program to find the prime number is the same as the first example. But we separated the logic and placed it in a separate method.

import java.util.Scanner;

public class Example {	
	
	private static Scanner sc;
	
	public static void main(String[] args) 
	{	
		int num, cnt = 0; 
		sc = new Scanner(System.in);
		
		System.out.print(" Please Enter any : ");
		num = sc.nextInt();
		
		cnt = PriNu(num);
		
		if(cnt == 0 && num != 1 )
		{
			System.out.println( num + " is a Prime");
		}
		else
		{
		   System.out.println(num + " is Not");
		}
	}
	
	public static int PriNu(int num)
	{
		int i, cnt = 0;
		
		for (i = 2; i <= num/2; i++)
		{
			if(num % i == 0)
		    {
				cnt++;
		        break;
		    }	
		}
		return cnt;
	}
}

 Please Enter any : 103
103 is a Prime

Output 2

 Please Enter any : 242
242 is Not