There are many ways to check whether a number is prime or not, and today we will discuss in detail each way with examples.

*What is Prime Number? *

*What is Prime Number?*

Prime number is a number that is only divisible by 1 or itself. Alternatively you can say that prime number is a natural number which is greater than 1 and only divided by 1 or itself, just like - 2, 3, 5, 7, 11, 13, 17, 19, 23, 29 ....etc. But always remember that 0 and 1 are not prime number, and 2 is the only even prime number. Because all other even numbers can be divided by 2.

So let's program....

**Example 1: **

*Prime number program in java using for loop**.*

*.*

In this program we will take a number and with the help of 'for loop' we will check whether this number is Prime or Not.

**In this example we have taken a number 17, as we know that no number is divisible by more than half of itself, so the loop will iterate through the values of 2 to 8(17/2=8.5), If the number 17 is completely divisible by a number from 2 to 8, then set the variable, isPrime = 1 and exit the loop.**

*Program Logic:*```
public class CheckPrimeNumber{
public static void main(String args[]){
int number = 17; // This is the number which we have to checked.
int a = number/2; //No number is divisible by more than half of itself.
int isPrime = 0;
for(int i=2;i<=a;i++)
{
if(number%i==0)
{
System.out.println(number+" is not prime number.");
isPrime=1;
break;
}
}
if(isPrime==0)
{
System.out.println(number+" is prime number.");
}
}
}
```

**Example 2: **

*Prime number program in java using while loop**.*

*.*

**In the below program, we have used while loop instead of for loop. In this program we have taken a number 41 and The loop runs until (k <= m), where k is first divisor number and m is half of taken number (41). On each iteration, whether number(41) is divisible by 'k' is checked and the value of 'k' is incremented by 1.**

*Program Logic:*```
public class PrimeNumber {
public static void main(String[] args) {
int number = 41;
int k = 2;
int m = number/2; ///No number is divisible by more than half of itself.
boolean flag = true;
while (k <= m)
{
if (number % k == 0) {
flag = false;
break;
}
k++;
}
if (flag)
System.out.println(number + " is a Prime Number.");
else
System.out.println(number + " is not a Prime Number.");
}
}
```

**Example 3: **

*Prime number program in java using scanner(User Input)*

```
import java.util.Scanner;
public class PrimeNumber
{
public static void main(String[] args)
{
boolean flage=true;
Scanner scan = new Scanner(System.in);
System.out.print("Please enter the number: ");
int number = scan.nextInt();
if(number==0 || number==1)
{
System.out.println(number+" is not Prime Number.");
flage=false;
}
else{
for(int i=2;i<=number/2;i++)
{
if(number%i==0)
{
System.out.println(number+" is not Prime Number.");
flage=false;
break;
}
}
}
if(flage==true)
{
System.out.println(number+" is Prime Number.");
}
}
}
```

**Example 4: **

*Check Prime Number using Method in Java.*

```
class CheckPrimeTwo{
void CheckPrime(int k){
int i;
int flag = 0;
int a = k/2;
if(k==0||k==1)
{
System.out.println(k+" Is Not Prime Number.");
}
else
{
for(i=2;i<=a;i++){
if(k%i==0){
System.out.println(k+" Is Not Prime Number.");
flag=1;
break;
}
}
if(flag==0) { System.out.println(k+" Is Prime Number."); }
}
}
}
public class Main {
public static void main(String args[]){
CheckPrimeTwo cp = new CheckPrimeTwo();
cp.CheckPrime(3);
cp.CheckPrime(7);
cp.CheckPrime(10);
cp.CheckPrime(29);
cp.CheckPrime(25);
cp.CheckPrime(73);
cp.CheckPrime(87);
cp.CheckPrime(98);
}
}
```

**Example 5: **

*Print prime numbers from 1 to 100 in java**.*

```
import java.util.Scanner;
public class CheckPrime {
public static void main(String[] args) {
Scanner scan = new Scanner(System.in); \\ 'scan' is an object of scanner class;
System.out.print("Enter The First Number : ");
int begin_number = scan.nextInt(); \\ We are taking input from the user by the help of 'scan' object.
System.out.print("Enter The Second Number : ");
int end_number = scan.nextInt();
System.out.println("Below is the List of Prime Numbers Between " + begin_number + " and " + end_number);
int k=0;
for (int i = begin_number; i <= end_number; i++) {
if (isPrime(i)) //
{
System.out.print(i+" ");
k++; // This line is counting the total number of prime numbers.
}
}
System.out.println(" ");
System.out.println("The Total Number of Prime Number between " + begin_number + " and " + end_number+" are: "+k);
}
public static boolean isPrime(int n) {
if (n <= 1)
{
return false;
}
for (int i = 2; i <= Math.sqrt(n); i++) \\.Math.sqrt() returns the square root of vc 'n';
{
if (n % i == 0)
{
return false;
}
}
return true;
}
```

Note: So, there are 25 prime numbers from 1 - 100.

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