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Code Kata: RPN Calculator

A RPN calculator program computes expressions written in RPN (Reverse Polish Notation).

A RPN expression (or a postfix expression) is one of the following:

  • a number X, in which case the value of the expression is that of X;
  • a sequence of the form E1 E2 O, where E1 and E2 are postfix expressions and O is an arithmetic operation; in this case, the value of the expression is that of E1 O E2.

The following are RPN expressions:

  • 1 => 1
  • 1 2 + => (1 + 2) = 3
  • 20 5 / => (20 / 5) = 4
  • 4 2 + 3 - => (4 + 2) - 3 = 3
  • 3 5 8 * 7 + * => 3*((5*8) + 7) = 141

Once this calculator works, add the SQRT operator:

  • 9 SQRT => √9 = 3

Once the SQRT operator has been added, add the MAX operator:

  • 5 8 1 4 2 MAX => 8
  • 4 5 MAX 1 2 MAX * => MAX(4, 5) * MAX(1, 2) = 10

Code Kata Boilerplate for Java 8

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Getting started

Firstly, either you fork this repository to put all your katas in the same repository, either you create a new repository per kata and push the kata boilerplate into it. Then replace the artifact coordinates used by the build system. Name the code kata too.
Make changes only in the "kata" branch.
You can use this shortened link if needed:


You may want to commit with another identifier to publish your code later. Nothing's easier:

git config ""
git config "Your Pseudo" # if needed

Take advantage of your IDE shortcuts while resolving any code kata. Download and print the keymap of IntelliJ IDEA. The table below should be very useful to learn, or to learn again, the most important shortcuts.

Theme Must Know Should Know Could Know
General Ctrl + Alt + S to open settings
Ctrl + Shift + A to find action
Editing Ctrl + Space to call basic code completion
Shift + Enter to start new line
Ctrl + Alt + L to auto-indent lines
Ctrl + Y to delete line at caret
Ctrl + Numpad +/- to expand/collapse code block
Ctrl + X to cut current line to clipboard
Ctrl + Q to call quick documentation lookup
Alt + Inser to generate code...
Refactoring Shift + F6 to rename
Ctrl + Alt + M to extract method
Ctrl + Alt + V to extract variable
Ctrl + Alt + F to extract field
Ctrl + Alt + C to extract constant
Ctrl + Alt + P to extract parameter
Navigation Ctrl + N to go to class
Ctrl + Shift + N to go to file
Alt + PageUp/PageDown* to navigate back / forward
Ctrl + B to go to declaration
Ctrl + Alt + B to go to implementation(s)
Ctrl + G to go to line
Compile and Run Shift + F10 / F9 to run / debug
Ctrl + Shift + F10 to run contexte configuration from editor
Usage Search Alt + F7 to find usages
Ctrl + Alt + F7 to show usages

If a shortcut is marked with a '*', it is to configure in the keymap settings of the IDE.

Command Line Interface as a GUI

You can invoke a standalone JAR after building it with the "standalone" Maven profile.

mvn package -Pstandalone # to build it
java -jar target/kata-standalone.jar # to use it

Updating your fork

Update your fork as follows. It is assumed you did not commit in the "master" branch. Otherwise please send me a pull request.

git fetch origin master && git rebase -p --onto origin/master master develop && git branch -f master origin/master

Definition of Done (DoD)

What should be a DoD for the code katas?

  • The kata is done by applying Test-Driven Development (TDD).
  • The code is versioned by applying TDDflow.
  • The path to the solution is discovered according to the Transformation Priority Premise.
  • The path of successive transformations is as simple as possible.
  • The code coverage is greater than 95%.
  • The problem to solve is described in the kata documentation.


This code kata boilerplate is distributed under the GNU GPLv3 license. The GPLv3 license is included in the LICENSE.txt file. More information about this license is available at

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