Find whether given number is a Prime Number or not

Here is a Java Program to Find whether given number is a Prime Number or not.

class FindPrime {
  public static void main(String args[]) {
    int num;
    boolean isPrime = true;

    num = 157;
    for(int i=2; i <= num/2; i++) {
      if((num % i) == 0) {
        isPrime = false;
        break;
      }
    }
    if(isPrime) System.out.println("Prime");
    else System.out.println("Not Prime");
  }
}

Output of Above Java Program
Prime

[ Read More ]

Program to Calculate Prime Numbers using Vector

Implement a version of the program to calculate prime numbers to use a Vector<> object instead of an array to store the primes.

import java.util.Vector;

public class MorePrimes {
  public static void main(String[] args) {
    Vector<Long> primes = new Vector<Long>();                          // Vector to store primes
    int primesRequired = 20;                                           // Number of primes required

    primes.add(new Long(2L));                                          // Seed the first prime
    primes.add(new Long(3L));                                          // and the second
    long number = 5L;                                                  // Next integer to be tested
    long divisor = 0L;                                                 // Stores current trial divisor

    outer:
    for( ; primes.size() < primesRequired; number += 2) {
      // The maximum divisor we need to try is square root of number
      long limit = (long)Math.ceil(Math.sqrt((double)number));

      // Divide by all the primes we have up to limit
      for(int i = 1; i < primes.size() && (divisor = primes.elementAt(i)) <= limit; ++i) {
        if(number%divisor == 0) {                                      // Is it an exact divisor?
          continue outer;                                              // Yes, try the next number
        }
      }
      primes.add(number);                                              // We got one!
    }
    // Output all the primes six to a line
    int count = 0;
    for(Long prime : primes) {
      System.out.printf("%15d", prime);
      if(++count%6 == 0) {
        System.out.println();
      }
    }
  }
}

Output of Above Java Program

              2              3              5              7             11             13
             17             19             23             29             31             37
             41             43             47             53             59             61
             67             71

[ Read More ]

Program to Calculate the Prime numbers until it reaches to Given Number

 When testing whether an integer is a prime, it is sufficient to try to divide by integers up to and including the square root of the number being tested.

public class Primes {
  public static void main(String args[]) {
    int maxValue = 50;                                                 // The maximum value to be checked.

    // Check all values from 2 to maxValue:
    OuterLoop:
    for(int i = 2 ; i <= maxValue ; ++i) {
      // Try dividing by all integers from 2 to square root of i:
      for(int j = 2 ; j <= Math.sqrt(i) ; ++j) {
        if(i%j == 0)                                                   // This is true if j divides exactly
          continue OuterLoop;                                          // so exit the loop.
      }
      // We only get here if we have a prime:
      System.out.println(i);                                           // so output the value.
    }
  }
}

Output of Above Java Program
2
3
5
7
11
13
17
19
23
29
31
37
41
43
47

[ Read More ]