# Java – Check whether number is Prime or Not ?

In this article, we will write a simple Java program to check whether a number is Prime number or Not

# 1. Prime Number programs :

• A number is Prime number which is divisible by 1 or itself only
• 0 & 1 aren’t Prime Numbers
• 2 is the only Even Prime number
• We will check whether a number is Prime or Not using 2 different loops,
1. using for loop
2. using while loop

## 1.1 Using for-loop :

• Initially we will declare a flag isPrime and set to true assuming given number is prime
• Iterate using for loop,
• starting from 2
• up to given number/2
• increment by 1 in every iteration
• During iteration inside for-loop, we will check whether remainder is zero by modulo of given number by iterating numbers from 2 till given number/2
• If condition succeeds then set the flag to false and break the for-loop i.e., given number is not Prime number
• Finally, returning isPrime flag to calling method to print to console

### CheckNumberIsPrimeUsingForLoop.java

```package in.bench.resources.numbers;

public class CheckNumberIsPrimeUsingForLoop {

// main() method
public static void main(String[] args) {

System.out.println("Whether 7 is prime ? " + checkPrimeOrNot(7));
System.out.println("Whether 20 is prime ? " + checkPrimeOrNot(20));
System.out.println("Whether 97 is prime ? " + checkPrimeOrNot(97));
System.out.println("Whether 100 is prime ? " + checkPrimeOrNot(100));
}

/**
* This method checks whether input number is prime or Not using for loop
*
* - 0 & 1 aren't Prime Numbers
* - 2 is the only EVEN Prime number
* - All even numbers are Prime numbers
* - Prime number is divisible by itself or 1
*
* @param number
* @return
*/
public static boolean checkPrimeOrNot(int number) {

// local variable
boolean isPrime = true;

if(0 == number || 1 == number) {

System.out.println("Numbers 0 & 1 aren't Prime Numbers");
isPrime = false;
}
else {

for(int i=2; i <= number / 2; i++) {

if(number % i == 0) {
isPrime = false;
break;
}
}
}

// return
return isPrime;
}
}
```

### Output:

```Whether 7 is prime ? true
Whether 20 is prime ? false
Whether 97 is prime ? true
Whether 100 is prime ? false
```

## 1.2 Using while-loop :

• Initially we will declare a flag isPrime and set to true assuming given number is prime
• Declare another variable i of type integer with initial value of 2
• Iterate using while-loop, by checking whether the value of i is less-than or equal-to number/2
• During iteration inside while-loop, we will check whether remainder is zero by modulo of given number by iterating numbers from 2 till given number/2 and also increment variable i by 1 in every iteration at the end
• If condition succeeds then set the flag to false and break the while-loop i.e., given number is not Prime number
• Finally, returning isPrime flag to calling method to print to console

### CheckNumberIsPrimeUsingWhileLoop.java

```package in.bench.resources.numbers;

public class CheckNumberIsPrimeUsingWhileLoop {

// main() method
public static void main(String[] args) {

System.out.println("Whether 7 is prime ? " + checkPrimeOrNot(7));
System.out.println("Whether 20 is prime ? " + checkPrimeOrNot(20));
System.out.println("Whether 97 is prime ? " + checkPrimeOrNot(97));
System.out.println("Whether 100 is prime ? " + checkPrimeOrNot(100));
}

/**
* This method checks whether input number is prime or Not using while loop
*
* - 0 & 1 aren't Prime Numbers
* - 2 is the only EVEN Prime number
* - All even numbers are Prime numbers
* - Prime number is divisible by itself or 1
*
* @param number
* @return
*/
public static boolean checkPrimeOrNot(int number) {

// local variable
int i = 2;
boolean isPrime = true;

if(0 == number || 1 == number) {

System.out.println("Numbers 0 & 1 aren't Prime Numbers");
isPrime = false;
}
else {

while(i <= number / 2) {

if(number % i == 0) {
isPrime = false;
break;
}

// increment value by 1
i++;
}
}

// return
return isPrime;
}
}
```

### Output:

```Whether 7 is prime ? true
Whether 20 is prime ? false
Whether 97 is prime ? true
Whether 100 is prime ? false
```

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Happy Coding !!
Happy Learning !!