Quick ways to check for Prime and find next Prime in Java

2.8

Many programming contest problems are somehow related Prime Numbers. Either we are required to check Prime Numbers, or we are asked to perform certain functions for all prime number between 1 to N. Example: Calculate the sum of all prime numbers between 1 and 1000000.

Java provides two function under java.math.BigInteger to deal with Prime Numbers.

  • isProbablePrime(int certainty): A method in BigInteger class to check if a given number is prime. For certainty = 1, it return true if BigInteger is prime and false if BigInteger is composite.

    Below is Java program to demonstrate above function.

    // A Java program to check if a number is prime using
    // inbuilt function
    import java.util.*;
    import java.math.*;
    
    class CheckPrimeTest
    {
        //Function to check and return prime numbers
        static boolean checkPrime(long n)
        {
            // Converting long to BigInteger
            BigInteger b = new BigInteger(String.valueOf(n));
    
            return b.isProbablePrime(1);
        }
    
        // Driver method
        public static void main (String[] args)
                             throws java.lang.Exception
        {
           long n = 13;
           System.out.println(checkPrime(n));
        }
    }
    

    Output:

    true
  • nextProbablePrime() : Another method present in BigInteger class. This functions returns the next Prime Number greater than current BigInteger.

    Below is Java program to demonstrate above function.

    // Java program to find prime number greater than a
    // given number using built in method
    import java.util.*;
    import java.math.*;
    
    class NextPrimeTest
    {
        // Function to get nextPrimeNumber
        static long nextPrime(long n)
        {
            BigInteger b = new BigInteger(String.valueOf(n));
            return Long.parseLong(b.nextProbablePrime().toString());
        }
    
        // Driver method
        public static void main (String[] args)
                        throws java.lang.Exception
        {
            long n = 14;
            System.out.println(nextPrime(n));
        }
    }
    

    Output:

    17
  •  

    This article is contributed by Ayush Jain. If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

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