Display Prime Numbers Less Than a Given Number in Java

Let’s say the value you have set is 20 and you have to display a prime number less than this value i.e. 19 in this case.

The following is an example that displays a prime number less than the given number −

Example

 Live Demo

public class Demo {
   public static void main(String[] args) {
      int val = 20;
      boolean[] isprime = new boolean[val + 1];
      for (int i = 0; i<= val; i++)
         isprime[i] = true;
         // 0 and 1 is not prime
         isprime[0] = false;
         isprime[1] = false;
         int n = (int) Math.ceil(Math.sqrt(val));
      for (int i = 0; i<= n; i++) {
         if (isprime[i])
            for (int j = 2 * i; j<= val; j = j + i)
               // not prime
               isprime[j] = false;
      }
      int myPrime;
      for (myPrime = val; !isprime[myPrime]; myPrime--); // empty loop body
         System.out.println("Largest prime less than or equal to " + val + " = " + myPrime);
   }
}

Output

Largest prime less than or equal to 20 = 19

Above, we have set the following as non-prime since both 0 and 1 are non-prime −

isprime[0] = false;
isprime[1] = false;