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Find the hidden change in six tests. Write a test that passes on the original code but fails on the mutant. After each attempt, feedback tells you whether you covered the mutated method or line.

How to play

Like Wordle or Mastermind, Mutle gives you feedback after each guess. Here, your guesses are Java tests, and your goal is to find and kill a hidden mutant: a small change to the code.

  1. Read the original class and its specification. The mutant is hidden until the game ends.
  2. Use the test editor to write a JUnit test that passes on the original class but fails on the mutant. This kills the mutant and solves the puzzle.
  3. If the mutant survives, use the feedback to narrow down the change: no relevant coverage, the mutated method covered, or the changed line covered. Select an attempt to see its test and coverage, then refine your next test. You have six attempts.
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Original class

                        

                        

                            
                        

                        

                            
                            Select an attempt to show its coverage.
                        

                    

                

                

                    

                        

                            
Your test

                        

                        

                            
                        

                        

                            
                        

                    

                

                

                    

                

            

            

                

                    
                    
                

                

                    

                        

















Test rules



    
        

            

                
            

            

                

                    
    
                        
                            
  • No new classes
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  • No new methods
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  • No loops
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  • No conditional statements (like if, switch etc)
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  • No logical operators like '&&' or '||'
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  • No bitwise operators like &= or |=
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  • No "assert()"-Statements
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  • No calls to System.*
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  • No use of random number generators
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  • No multithreading
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  • No calls to Date classes
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  • No IO calls
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  • Only use the assertions of the correct test library
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  • Only 2 assertion per test
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Your attempts

                            
    
                            
    
                                No relevant coverage
                                Mutated method covered
                                Changed line covered, mutant survived
                                Mutant killed