Finding prime numbers in Java

Question

I have got a java code that checks for prime numbers and then it displays them. However, there are some instances when there are no prime numbers (e.g between 14 - 17). In those cases I want a single message to appear. For example "No primes found". I don't know how to add this to my code.

This is my whole class:

import java.io.*;

public class Prime {

    BufferedReader input = new BufferedReader(new InputStreamReader(System.in));

    public static int lowerBound;
    public static int higherBound;

    public void getInput() throws IOException {
        System.out.println("Please enter the lower and upper bound");
        String line1 = input.readLine();
        String line2 = input.readLine();
        int lowInput = Integer.parseInt(line1);
        int highInput = Integer.parseInt(line2);
        lowerBound = lowInput;
        higherBound = highInput;
    }

    public void validatedata() throws IOException {
        do {
            getInput();
            if (lowerBound < 2) {
                System.out.println("Finish");
                break;
            } else if (higherBound < lowerBound) {
                System.out
                        .println("The upper bound should be at least as big as the lower bound.");
            }

        } while (higherBound < lowerBound);

    }

    public void prime_calculation() throws IOException {
        while ((lowerBound >= 2) && (higherBound > lowerBound)) {

            int k;
            for (k = lowerBound; k < higherBound; k++) {
                boolean primecheck = true;
                for (int j = 2; j < k; j++) {
                    if (k % j == 0) {
                        primecheck = false;
                    }
                }
                if (primecheck)
                    System.out.println(k);
            }
            validatedata();
        }
    }
}

And this is my main void method:

import java.io.IOException;

public class PrimeUser extends Prime {

    public static void main(String argv[]) throws IOException {
        Prime isprime = new Prime();
        isprime.validatedata();
        isprime.prime_calculation();
    }
}

Answer 1

Have a look at http://rosettacode.org/wiki/Miller-Rabin_primality_test#Java which explains how to determine whether a number is a prime.

And then work over your array of values and keep a flag whether a number is a prime or not.