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artemkorsakov's solution

to Collatz Conjecture in the Java Track

Published at Mar 16 2019 · 0 comments
Test suite

The Collatz Conjecture or 3x+1 problem can be summarized as follows:

Take any positive integer n. If n is even, divide n by 2 to get n / 2. If n is odd, multiply n by 3 and add 1 to get 3n + 1. Repeat the process indefinitely. The conjecture states that no matter which number you start with, you will always reach 1 eventually.

Given a number n, return the number of steps required to reach 1.


Starting with n = 12, the steps would be as follows:

  1. 12
  2. 6
  3. 3
  4. 10
  5. 5
  6. 16
  7. 8
  8. 4
  9. 2
  10. 1

Resulting in 9 steps. So for input n = 12, the return value would be 9.

Running the tests

You can run all the tests for an exercise by entering

$ gradle test

in your terminal.


An unsolved problem in mathematics named after mathematician Lothar Collatz https://en.wikipedia.org/wiki/3x_%2B_1_problem

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.


import org.junit.Before;
import org.junit.Ignore;
import org.junit.Rule;
import org.junit.Test;
import org.junit.rules.ExpectedException;

import static org.junit.Assert.assertEquals;

public class CollatzCalculatorTest {

    public ExpectedException expectedException = ExpectedException.none();

    private CollatzCalculator collatzCalculator;

    public void setUp() {
        collatzCalculator = new CollatzCalculator();

    public void testZeroStepsRequiredWhenStartingFrom1() {
        assertEquals(0, collatzCalculator.computeStepCount(1));

    @Ignore("Remove to run test")
    public void testCorrectNumberOfStepsWhenAllStepsAreDivisions() {
        assertEquals(4, collatzCalculator.computeStepCount(16));

    @Ignore("Remove to run test")
    public void testCorrectNumberOfStepsWhenBothStepTypesAreNeeded() {
        assertEquals(9, collatzCalculator.computeStepCount(12));

    @Ignore("Remove to run test")
    public void testAVeryLargeInput() {
        assertEquals(152, collatzCalculator.computeStepCount(1000000));

    @Ignore("Remove to run test")
    public void testZeroIsConsideredInvalidInput() {
        expectedException.expectMessage("Only natural numbers are allowed");


    @Ignore("Remove to run test")
    public void testNegativeIntegerIsConsideredInvalidInput() {
        expectedException.expectMessage("Only natural numbers are allowed");


class CollatzCalculator {

    int computeStepCount(int start) {
        if (start < 1) {
            throw new IllegalArgumentException("Only natural numbers are allowed");

        int count = 0;
        while (start != 1) {
            start = start % 2 == 0 ? start / 2 : 3 * start + 1;
        return count;


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