Java program to print prime numbers upto n

                                    // Java program to print prime numbers upto n
import java.util.Scanner;
public class PrimeNumberUptoN
{
   public static void main(String[] args)
   {
      Scanner sc = new Scanner(System.in);
      int a = 0;
      int n = 0;
      String strPrime = "";
      System.out.println("Please enter the value of n: ");
      int number = sc.nextInt();
      for(a = 1; a <= number; a++)
      {
         int counter = 0;
         for(n = a; n >= 1; n--)
         {
            if(a % n == 0)
            {
               counter = counter + 1;
            }
         }
         if(counter == 2)
         {
            strPrime = strPrime + a + " ";
         }
      }
      System.out.println("prime numbers upto n are: ");
      System.out.println(strPrime);
      sc.close();
   }
}

                                    // Java program to print prime numbers from 2 to n
import java.util.Scanner;
public class PrimeNumberFrom2Ton
{
   public static void main(String[] args) 
   {
      int loop, max;
      System.out.print("Please enter highest (maximum) number: ");
      Scanner sc = new Scanner(System.in);
      max = sc.nextInt();
      for(loop = 2; loop < max; ++loop)
      {
         if(findPrime(loop))
         {
            System.out.println(loop);
         }
      }
      sc.close();
   }
   public static boolean findPrime(int number)
   {
      int a;
      boolean boolPrime = true;
      for(a = 2; a < number / 2; a++)
      {
         if(number%a == 0)
         {
            boolPrime = false;
            break;
         }
      }
      return boolPrime;
   }
}

                                    public class PrimeNumbersFrom1To100
{
   public static void main(String[] args)
   {
      int maxNumber = 100;
      boolean prime = true;
      String primeNumbers = "";
      for(int a = 1; a <= maxNumber; a++)
      {
         prime = checkPrime(a);
         if(prime)
         {
            primeNumbers = primeNumbers + a + " ";
         }
      }
      System.out.println("Display prime numbers from 1 to " + maxNumber + ": ");
      System.out.println(primeNumbers);
   }
   public static boolean checkPrime(int number)
   {
      int remainder;
      for(int a = 2; a <= number / 2; a++)
      {
         remainder = number % a;
         // if remainder is equal to 0 then number is not prime
         if(remainder == 0)
         {
            return false;
         }
      }
      return true;
   }
}

                                    class PrimeNumbers
{
   public static void main (String[] args)
   {		
       int i =0;
       int num =0;
       //Empty String
       String  primeNumbers = "";

       for (i = 1; i <= 100; i++)         
       { 		  	  
          int counter=0; 	  
          for(num =i; num>=1; num--)
	  {
             if(i%num==0)
	     {
 		counter = counter + 1;
	     }
	  }
	  if (counter ==2)
	  {
	     //Appended the Prime number to the String
	     primeNumbers = primeNumbers + i + " ";
	  }	
       }	
       System.out.println("Prime numbers from 1 to 100 are :");
       System.out.println(primeNumbers);
   }
}