Java 8 Tutorial Through Katas: Berlin Clock (Easy)

A programming kata is an exercise which helps a programmer hone his skills through practice and repetition.

This article is part of the series Java Tutorial Through Katas.

Fizz Buzz (Easy) – Java 7
Berlin Clock (Easy) – Java 7 and 8
Tennis Game (Easy) – Java 7
Reverse Polish Notation (Medium) – Java 7 and 8

The article assumes that the reader already has experience with Java 7, that he is familiar with the basic usage of JUnit tests and that he knows how to run them from his favorite IDE (ours is IntelliJ IDEA).

Tests that prove that the solution is correct are displayed below. Recommended way to solve this kata is to use test-driven development approach (write the implementation for the first test, confirm that it passes and move to the next). Once all of the tests pass, the kata can be considered solved.

Since many of the new features added to Java 8 were inspired by Scala and other functional programming languages, short comparison of Java 7 (or earlier), Java 8 and Scala solutions is provided at the end of the article.

One possible solution is provided below the tests. Try to solve the kata by yourself first.

Berlin Clock

Create a representation of the Berlin Clock for a given time (hh::mm:ss).

The Berlin Uhr (Clock) is a rather strange way to show the time.
On the top of the clock there is a yellow lamp that blinks on/off every two seconds.
The time is calculated by adding rectangular lamps.

The top two rows of lamps are red. These indicate the hours of a day. In the top row there are 4 red lamps.
Every lamp represents 5 hours. In the lower row of red lamps every lamp represents 1 hour.
So if two lamps of the first row and three of the second row are switched on that indicates 5+5+3=13h or 1 pm.

The two rows of lamps at the bottom count the minutes. The first of these rows has 11 lamps, the second 4.
In the first row every lamp represents 5 minutes.
In this first row the 3rd, 6th and 9th lamp are red and indicate the first quarter, half and last quarter of an hour.
The other lamps are yellow. In the last row with 4 lamps every lamp represents 1 minute.

The lamps are switched on from left to right.

Y = Yellow
R = Red
O = Off


public class BerlinClockTest {

    BerlinClock berlinClock = new BerlinClock();

    // Yellow lamp should blink on/off every two seconds
    public void testYellowLampShouldBlinkOnOffEveryTwoSeconds() {
        Assert.assertEquals("Y", berlinClock.getSeconds(0));
        Assert.assertEquals("O", berlinClock.getSeconds(1));
        Assert.assertEquals("O", berlinClock.getSeconds(59));

    // Top hours should have 4 lamps
    public void testTopHoursShouldHave4Lamps() {
        Assert.assertEquals(4, berlinClock.getTopHours(7).length());

    // Top hours should light a red lamp for every 5 hours
    public void testTopHoursShouldLightRedLampForEvery5Hours() {
        Assert.assertEquals("OOOO", berlinClock.getTopHours(0));
        Assert.assertEquals("RROO", berlinClock.getTopHours(13));
        Assert.assertEquals("RRRR", berlinClock.getTopHours(23));
        Assert.assertEquals("RRRR", berlinClock.getTopHours(24));

    // Bottom hours should have 4 lamps
    public void testBottomHoursShouldHave4Lamps() {
        Assert.assertEquals(4, berlinClock.getBottomHours(5).length());

    // Bottom hours should light a red lamp for every hour left from top hours
    public void testBottomHoursShouldLightRedLampForEveryHourLeftFromTopHours() {
        Assert.assertEquals("OOOO", berlinClock.getBottomHours(0));
        Assert.assertEquals("RRRO", berlinClock.getBottomHours(13));
        Assert.assertEquals("RRRO", berlinClock.getBottomHours(23));
        Assert.assertEquals("RRRR", berlinClock.getBottomHours(24));

    // Top minutes should have 11 lamps
    public void testTopMinutesShouldHave11Lamps() {
        Assert.assertEquals(11, berlinClock.getTopMinutes(34).length());

    // Top minutes should have 3rd, 6th and 9th lamps in red to indicate first quarter, half and last quarter
    public void testTopMinutesShouldHave3rd6thAnd9thLampsInRedToIndicateFirstQuarterHalfAndLastQuarter() {
        String minutes32 = berlinClock.getTopMinutes(32);
        Assert.assertEquals("R", minutes32.substring(2, 3));
        Assert.assertEquals("R", minutes32.substring(5, 6));
        Assert.assertEquals("O", minutes32.substring(8, 9));

    // Top minutes should light a yellow lamp for every 5 minutes unless it's first quarter, half or last quarter
    public void testTopMinutesShouldLightYellowLampForEvery5MinutesUnlessItIsFirstQuarterHalfOrLastQuarter() {
        Assert.assertEquals("OOOOOOOOOOO", berlinClock.getTopMinutes(0));
        Assert.assertEquals("YYROOOOOOOO", berlinClock.getTopMinutes(17));
        Assert.assertEquals("YYRYYRYYRYY", berlinClock.getTopMinutes(59));

    // Bottom minutes should have 4 lamps
    public void testBottomMinutesShouldHave4Lamps() {
        Assert.assertEquals(4, berlinClock.getBottomMinutes(0).length());

    // Bottom minutes should light a yellow lamp for every minute left from top minutes
    public void testBottomMinutesShouldLightYellowLampForEveryMinuteLeftFromTopMinutes() {
        Assert.assertEquals("OOOO", berlinClock.getBottomMinutes(0));
        Assert.assertEquals("YYOO", berlinClock.getBottomMinutes(17));
        Assert.assertEquals("YYYY", berlinClock.getBottomMinutes(59));

    // Berlin Clock should result in array with 5 elements
    public void testBerlinClockShouldResultInArrayWith5Elements()  {
        Assert.assertEquals(5, berlinClock.convertToBerlinTime("13:17:01").length);

    // Berlin Clock it should "result in correct seconds, hours and minutes" in {
    public void testBerlinClockShouldResultInCorrectSecondsHoursAndMinutes() {
        String[] berlinTime = berlinClock.convertToBerlinTime("16:37:16");
        String[] expected = new String[] {"Y", "RRRO", "ROOO", "YYRYYRYOOOO", "YYOO"};
        Assert.assertEquals(expected.length, berlinTime.length);
        for (int index = 0; index < expected.length; index++) {
            Assert.assertEquals(expected[index], berlinTime[index]);


Test code can be found in the GitHub


public class BerlinClock {

    public String[] convertToBerlinTime(String time) {
        int[] parts = Stream.of(time.split(":")).mapToInt(Integer::parseInt).toArray();
        return new String[] {

    protected String getSeconds(int number) {
        if (number % 2 == 0) return "Y";
        else return "O";

    protected String getTopHours(int number) {
        return getOnOff(4, getTopNumberOfOnSigns(number));

    protected String getBottomHours(int number) {
        return getOnOff(4, number % 5);

    protected String getTopMinutes(int number) {
        return getOnOff(11, getTopNumberOfOnSigns(number), "Y").replaceAll("YYY", "YYR");

    protected String getBottomMinutes(int number) {
        return getOnOff(4, number % 5, "Y");

    private String getOnOff(int lamps, int onSigns) {
        return getOnOff(lamps, onSigns, "R");
    private String getOnOff(int lamps, int onSigns, String onSign) {
        String out = "";
        for (int i = 0; i < onSigns; i++) {
            out += onSign;
        for (int i = 0; i < (lamps - onSigns); i++) {
            out += "O";
        return out;

    private int getTopNumberOfOnSigns(int number) {
        return (number - (number % 5)) / 5;


Java 8 solution code can be found in the solution.
Java 7 (old earlier) equivalent can be found in the solution.
Scala solution can be found in the Scala Tutorial Through Katas: Berlin Clock (Easy) post.

Major difference between Java 8 and Java 7 solutions is in the usage of the Stream to create an array of integers.

[JAVA 7]

List<Integer> parts = new ArrayList<Integer>();
for (String part : time.split(":")) {

[JAVA 8]

int[] parts = Stream.of(time.split(":")).mapToInt(Integer::parseInt).toArray();


val parts = time.split(":").map(_.toInt)

Both Java 8 and Scala have the methods that can be used to transform a collection. While Java 8 requires a lot of boilerplate code, Scala provides a simple and clear solution.

Another difference between Java and Scala solutions that resulted in less code is the getOnOff method.

    private String getOnOff(int lamps, int onSigns, String onSign) {
        String out = "";
        for (int i = 0; i < onSigns; i++) {
            out += onSign;
        for (int i = 0; i < (lamps - onSigns); i++) {
            out += "O";
        return out;


  private def onOff(lamps: Int, onSigns: Int, onSign: String = "R") = {
    onSign * onSigns + "O" * (lamps - onSigns)

What was your solution? Post it as a comment so that we can compare different ways to solve this kata.

Test-Driven Java Development

7429OS_Test-Driven Java DevelopmentTest-Driven Java Development book wrote by Alex Garcia and me has been published by Packt Publishing. It was a long, demanding, but very rewarding journey that resulted in a very comprehensive hands-on material for all Java developers interested in learning or improving their TDD skills.

If you liked this article I am sure that you’ll find this book very useful. It contains extensive tutorials, guidelines and exercises for all Java developers eager to learn how to successfully apply TDD practices.

You can download a sample or purchase your own copy directly from Packt or Amazon.

4 thoughts on “Java 8 Tutorial Through Katas: Berlin Clock (Easy)

  1. Pingback: object oriented-ness, extensability and modularity for simple program

  2. Pingback: Top 10+ Java 8 Tutorials – Must Read | RAKESH KR

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