# Java program to check prime number

A number which is divided by one and itself is called prime number. For Example 7 is a prime number because it is divided by 1 and itself only  and 8 is not a prime number because it is divided by 2 and 4 also . In other words we can say a prime number is a number which have only two factors, that is 1 and the number itself. 2 is smallest prime numbers . because 1 is smallest non -zero number have only one factor 1*1 =1 and next greater number is 2 have two factor 2*1 =2 and a  prime number  is divided by 1 and itself . so  the smallest prime number is 2.

### Java Program to check whether input number is prime or not

Now we will write a program to check if a given number is prime or not in java . In this example we will take input from user using Scanner and program show output. Now we will write prime number program in java using for loop

``````import java.util.Scanner;

public static void main(String[] args) {
Scanner scan= new Scanner(System.in);
System.out.println("Enter any number:");
int num  =scan.nextInt();
if (flag) {
System.out.println("number " + num + " is prime number");
} else {
System.out.println("number " + num + " is  not prime number");
}
}

if (num <= 1) {
return false;
}
boolean flag = true;
for (int j = 2; j <= num / 2; j++) {
if (num % j == 0) {
flag = false;
break;
}
}
return flag;
}

}``````

Output :

``````First Input -->>
Enter any number:
10
number 10 is  not prime number```
Second Input --> Enter any number: 101 number 101 is prime number```

We have written prime number program in java using scanner . I have written prime number program in java using for loop now we write prime number program in java using while loopLet’s see code

``````import java.util.Scanner;

public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
System.out.println("Enter any number:");
int num = scan.nextInt();
if (flag) {
System.out.println("number " + num + " is prime number");
} else {
System.out.println("number " + num + " is  not prime number");
}
}

private static boolean isPrimeNumber(int num) {
if (num <= 1) {
return false;
}
boolean flag = true;
int j = 2;
while(j <= num / 2) {
if (num % j == 0) {
flag = false;
break;
}
j++;
}
return flag;
}
}``````

Output :

``````Enter any number:
101
number 101 is prime number``````

We have show prime number program in java using for loop and while loop .These examples for prime number in java iterate loop till num/2 . we can improve it and iterate loop sqrt of given number .  Now we will improve for java program to check prime number and iterate loop till sqrt of num .

``````import java.util.Scanner;

public static void main(String[] args) {
Scanner scan= new Scanner(System.in);
System.out.println("Enter any number:");
int num  =scan.nextInt();
if (flag) {
System.out.println("number " + num + " is prime number");
} else {
System.out.println("number " + num + " is  not prime number");
}
}

if (num <= 1) {
return false;
}
boolean flag = true;
for (int j = 2; j <= Math.sqrt(num) ; j++) {
if (num % j == 0) {
flag = false;
break;
}
}
return flag;
}

}``````

output :

``````Enter any number:
101
number 101 is prime number``````

### Java program to check prime number using recursion

Now we will check prime number using recursion in java . we will write a method and it will return result

``````public class PrimeNumberRecursion {
static int i =2;

public static void main(String[] args) {
int num =10;
if (flag) {
System.out.println("number " + num + " is prime number");
} else {
System.out.println("number " + num + " is  not prime number");
}
}

{

// Corner cases
if (num == 0 || num == 1)
{
return false;
}

// Checking Prime
if (num == i)
return true;

// Base cases
if (num % i == 0)
{
return false;
}
i++;
````number 10 is  not prime number`