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Is a Quantity Prime in Java

The problem

Outline a operate that takes one integer argument and returns a logical worth true or false relying on if the integer is a first-rate.

Per Wikipedia, a first-rate quantity (or a first-rate) is a pure quantity higher than 1 that has no constructive divisors apart from 1 and itself.


  • You may assume you may be given an integer enter.
  • You cannot assume that the integer can be solely constructive. It’s possible you’ll be given destructive numbers as nicely (or “).
  • NOTE on efficiency: There are not any fancy optimizations required, however nonetheless the most trivial options would possibly outing. Numbers go as much as 2^31 (or comparable, is dependent upon language model). Looping all the way in which as much as n, or n/2, can be too sluggish.


is_prime(1)  /* false */
is_prime(2)  /* true  */
is_prime(-1) /* false */

The answer in Java code

Choice 1:

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

Choice 2:

public class Prime {
  public static boolean isPrime(int quantity) {
    if(quantity < 2)
      return false;
    for (int i=2; i<=Math.sqrt(quantity); i++){
      if(quantity % i == 0)
        return false;
    return true;

Choice 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;

Check circumstances to validate our answer

import static org.junit.Assert.assertTrue;
import static org.junit.Assert.assertFalse;
import org.junit.Check;

public class PrimeTest {
	public void testBasic() {
		assertFalse("0  just isn't prime", Prime.isPrime(0));
		assertFalse("1  just isn't prime", Prime.isPrime(1));
		assertTrue ("2  is prime",     Prime.isPrime(2));
		assertTrue ("73 is prime",     Prime.isPrime(73));
		assertFalse("75 just isn't prime", Prime.isPrime(75));
		assertFalse("-1 just isn't prime", Prime.isPrime(-1));
	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));
	public void testNotPrime() {
		assertFalse("4 just isn't prime", Prime.isPrime(4));
		assertFalse("6 just isn't prime", Prime.isPrime(6));
		assertFalse("8 just isn't prime", Prime.isPrime(8));
		assertFalse("9 just isn't prime", Prime.isPrime(9));
		assertFalse("45 just isn't prime", Prime.isPrime(45));
		assertFalse("-5 just isn't prime", Prime.isPrime(-5));
		assertFalse("-8 just isn't prime", Prime.isPrime(-8));
		assertFalse("-41 just isn't prime", Prime.isPrime(-41));


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