How to write a Unit Test

Question

I have a Java class  How can I  unit test it     In my case  I have class does a binary sum  It takes two byte   arrays  sums them  and returns a new binary array

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Like  CoolBeans mentioned  take a look at jUnit  Here is a short tutorial to get you started as well with jUnit 4 x  Finally  if you really want to learn more about testing and test-driven development  TDD  I recommend you take a look at the following book by Kent Beck  Test-Driven Development By Example

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I provide this post for both IntelliJ and Eclipse   Eclipse   For making unit test for your project  please follow these steps  I am using Eclipse in order to write this test    1- Click on New -  Java Project     2- Write down your project name and click on finish     3- Right click on your project  Then  click on New -  Class     4- Write down your class name and click on finish     Then  complete the class like this    public class Math       int a  b      Math int a  int b            this a   a          this b   b            public int add             return a   b            5- Click on File -  New -  JUnit Test Case     6- Check setUp   and click on finish  SetUp   will be the place that you initialize your test     7- Click on OK     8- Here  I simply add 7 and 10  So  I expect the answer to be 17  Complete your test class like this   import org junit Assert  import org junit Before  import org junit Test  public class MathTest       Math math       Before     public void setUp   throws Exception           math   new Math 7  10              Test     public void testAdd             Assert assertEquals 17  math add               9- Write click on your test class in package explorer and click on Run as -  JUnit Test     10- This is the result of the test     IntelliJ  Note that I used IntelliJ IDEA community 2020 1 for the screenshots  Also  you need to set up your jre before these steps  I am using JDK 11 0 4   1- Right-click on the main folder of your project-  new -  directory  You should call this  test    2- Right-click on the test folder and create the proper package  I suggest creating the same packaging names as the original class  Then  you right-click on the test directory -  mark directory as -  test sources root   3- In the right package in the test directory  you need to create a Java class  I suggest to use Test java    4- In the created class  type   Test   Then  among the options that IntelliJ gives you  select Add  JUnitx  to classpath    5- Write your test method in your test class  The method signature is like    Test public void test lt name of original method gt             You can do your assertions like below   Assertions assertTrue f flipEquiv node1 1  node2 1      These are the imports that I added   import org junit jupiter api Assertions  import org junit jupiter api Test      This is the test that I wrote    You can check your methods like below   Assertions assertEquals  lt Expected gt   lt actual gt    Assertions assertTrue  lt actual gt          For running your unit tests  right-click on the test and click on Run     If your test passes  the result will be like below    I hope it helps  You can see the structure of the project in GitHub https   github com m-vahidalizadeh problem solving project

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Other answers have shown you how to use JUnit to set up test classes  JUnit is not the only Java test framework  Concentrating on the technical details of using a framework however detracts from the most important concepts that should be guiding your actions   so I will talk about those     Testing  of all kinds of all kinds of things  compares the actual behaviour of something  The System Under Test  SUT  with its expected behaviour  Automated testing can be done using a computer program  Because that comparison is being done by an inflexible and unintelligent computer program  the expected behaviour must be precisely and unambiguously known  What a program or part of a program  a class or method   is expected to do is its specification  Testing software therefore requires that you have a specification for the SUT  This might be an explicit description  or an implicit specification in your head of what is expected  Automated unit testing therefore requires a precise and unambiguous specification of the class or method you are testing  But you needed that specification when you set out to write that code  So part of what testing is about actually begins before you write even one line of the SUT  The testing technique of Test Driven Development  TDD  takes that idea to an extreme  and has you create the unit testing code before you write the code to be tested  Unit testing frameworks test your SUT using assertions  An assertion is a logical expression  an expression with a boolean result type  a predicate  that must be true if the SUT is behaving correctly  The specification must therefore be expressed  or re-expressed  as assertions  A useful technique for expressing a specification as assertions is programming by contract  These specifications are in terms of postconditions  A postcondition is an assertion about the publicly visible state of the SUT after return from a method or a constructor  Some methods have postconditions  that are invariants  which are predicates that are true before and after execution of the method  A class can also be said to have invariants   which are postconditions of every constructor and method of the class   and hence should always be true  Postconditions  And invariants  are expressed only in terms of publicity visible state  public and protected fields  the values returned by returned by public and protected methods  such as getters   and the publicly visible state of objects passed  by reference  to methods      Many beginners post questions here asking how they can test some code  presenting the code but without stating the specification for that code  As this discussion shows  it is impossible for anyone to give a good answer to such a question  because at best potential answereres must guess the specification  and might do so incorrectly  The asker of the question evidently does not understand the importance of a specification  and is thus a novice who needs to understand the fundamentals I ve described here before trying to write some test code

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This is a very generic question and there is a lot of ways it can be answered   If you want to use JUnit to create the tests  you need to create your testcase class  then create individual test methods that test specific functionality of your class module under tests  single testcase classes are usually associated with a single  production  class that is being tested  and inside these methods execute various operations and compare the results with what would be correct  It is especially important to try and cover as many corner cases as possible   In your specific example  you could for example test the following    A simple addition between two positive numbers  Add them  then verify the result is what you would expect  An addition between a positive and a negative number  which returns a result with the sign of the first argument   An addition between a positive and a negative number  which returns a result with the sign of the second argument   An addition between two negative numbers  An addition that results in an overflow    To verify the results  you can use various assertXXX methods from the org junit Assert class  for convenience  you can do  import static org junit Assert      These methods test a particular condition and fail the test if it does not validate  with a specific message  optionally    Example testcase class in your case  without the methods contents defined    import static org junit Assert     public class AdditionTests        Test     public void testSimpleAddition                  Test     public void testPositiveNegativeAddition                  Test     public void testNegativePositiveAddition                  Test     public void testNegativeAddition                  Test     public void testOverflow               If you are not used to writing unit tests but instead test your code by writing ad-hoc tests that you then validate  visually   for example  you write a simple main method that accepts arguments entered using the keyboard and then prints out the results - and then you keep entering values and validating yourself if the results are correct   then you can start by writing such tests in the format above and validating the results with the correct assertXXX method instead of doing it manually  This way  you can re-run the test much easier then if you had to do manual tests

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Define the expected and desired output for a normal case  with correct input   Now  implement the test by declaring a class  name it anything  Usually something like TestAddingModule   and add the testAdd method to it  i e  like the one below       Write a method  and above it add the  Test annotation  In the method  run your binary sum and assertEquals expectedVal calculatedVal   Test your method by running it  in Eclipse  right click  select Run as   JUnit test       for normal addition   Test public void testAdd1Plus1          int x    1   int y   1      assertEquals 2  myClass add x y        Add other cases as desired      Test that your binary sum does not throw a unexpected exception if there is an integer overflow  Test that your method handles Null inputs gracefully  example below       if you are using 0 as default for null  make sure your class works in that case   Test public void testAdd1Plus1          int y   1      assertEquals 0  myClass add null y

[java] How to write a Unit Test?

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