Is a Number Prime in Java

The challenge

Define a function that takes one integer argument and returns a logical value true or false depending on if the integer is a prime.

Per Wikipedia, a prime number (or a prime) is a natural number greater than 1 that has no positive divisors other than 1 and itself.

Requirements:

  • You can assume you will be given an integer input.
  • You can not assume that the integer will be only positive. You may be given negative numbers as well (or 0).
  • NOTE on performance: There are no fancy optimizations required, but still the most trivial solutions might time out. Numbers go up to 2^31 (or similar, depends on language version). Looping all the way up to n, or n/2, will be too slow.

Examples:

is_prime(1) /* false */ is_prime(2) /* true */ is_prime(-1) /* false */
Code language: Java (java)

The solution in Java code

Option 1:

public class Prime { public static boolean isPrime(int num) { return num > 1 && java.math.BigInteger.valueOf(num).isProbablePrime(20); } }
Code language: Java (java)

Option 2:

public class Prime { public static boolean isPrime(int number) { if(number < 2) return false; for (int i=2; i<=Math.sqrt(number); i++){ if(number % i == 0) return false; } return true; } }
Code language: Java (java)

Option 3:

public class Prime { public static boolean isPrime(int num) { if (num == 2) return true; if (num < 2 || num % 2 == 0) return false; for (int i = 3; num >= i*i; i+=2) { if (num % i == 0) return false; } return true; } }
Code language: Java (java)

Test cases to validate our solution

import static org.junit.Assert.assertTrue; import static org.junit.Assert.assertFalse; import org.junit.Test; public class PrimeTest { @Test public void testBasic() { assertFalse("0 is not prime", Prime.isPrime(0)); assertFalse("1 is not prime", Prime.isPrime(1)); assertTrue ("2 is prime", Prime.isPrime(2)); assertTrue ("73 is prime", Prime.isPrime(73)); assertFalse("75 is not prime", Prime.isPrime(75)); assertFalse("-1 is not prime", Prime.isPrime(-1)); } @Test public void testPrime() { assertTrue("3 is prime", Prime.isPrime(3)); assertTrue("5 is prime", Prime.isPrime(5)); assertTrue("7 is prime", Prime.isPrime(7)); assertTrue("41 is prime", Prime.isPrime(41)); assertTrue("5099 is prime", Prime.isPrime(5099)); } @Test public void testNotPrime() { assertFalse("4 is not prime", Prime.isPrime(4)); assertFalse("6 is not prime", Prime.isPrime(6)); assertFalse("8 is not prime", Prime.isPrime(8)); assertFalse("9 is not prime", Prime.isPrime(9)); assertFalse("45 is not prime", Prime.isPrime(45)); assertFalse("-5 is not prime", Prime.isPrime(-5)); assertFalse("-8 is not prime", Prime.isPrime(-8)); assertFalse("-41 is not prime", Prime.isPrime(-41)); } }
Code language: Java (java)
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