PrimeFactors.java

import java.util.LinkedList;
import java.util.List;

public class PrimeFactors {

    public static List<Integer> of(int n) {
		List<Integer> factors = new LinkedList<Integer>();
		for(int candidate = 2; candidate <= n; candidate++) {
			while (n % candidate == 0) {
				factors.add(candidate);
				n /= candidate;
			}			
		}
		return factors;
	}

}

PrimeFactorsTest.java

import static org.junit.Assert.*;
import static org.junit.Assert.assertEquals;

import java.util.LinkedList;
import java.util.List;

import org.junit.Test;

public class PrimeFactorsTest {
    	@Test
	public void itFactorsOne() {
		assertEquals(list(), PrimeFactors.of(1));
	}

	@Test
	public void itFactorsTwo() {
		assertEquals(list(2), PrimeFactors.of(2));
	}

	@Test
	public void itFactorsThree() {
		assertEquals(list(3), PrimeFactors.of(3));
	}

	@Test
	public void itFactorsFour() {
		assertEquals(list(2, 2), PrimeFactors.of(4));
	}
	
	@Test
	public void itFactorsEight() {
		assertEquals(list(2,2,2), PrimeFactors.of(8));
	}
	
	@Test
	public void itFactorsNine() {
		assertEquals(list(3,3), PrimeFactors.of(9));
	}

	private List<Integer> list(int... factors) {
		List<Integer> list = new LinkedList<Integer>();
		for (int factor : factors)
			list.add(factor);
		return list;
	}
}