JUnit Testing private variables

Question

I have been assigned the task of unit testing a class that I never worked directly on with JUnit  and am strictly forbidden to change the code in the package   This is usually no issue  since most of our unit testing is just for functionality and input output consistency  which can be done simply by running routines and checking their return values   However  occasionally there is a need to check a private variable within the class  or directly edit a private variable to check some internal behavior   Is there a way to gain access to these  whether through JUnit or any other way  for the purpose of unit testing without actually changing any of the code in the original source package   And if not  how do programmers handle this issue in the real world where a unit tester may not be the same person as the coder

User · Answer

First of all  you are in a bad position now - having the task of writing tests for the code you did not originally create and without any changes - nightmare  Talk to your boss and explain  it is not possible to test the code without making it  testable   To make code testable you usually do some important changes   Regarding private variables  You actually never should do that  Aiming to test private variables is the first sign that something wrong with the current design  Private variables are part of the implementation  tests should focus on behavior rather of implementation details   Sometimes  private field are exposed to public access with some getter  I do that  but try to avoid as much as possible  mark in comments  like  used for testing     Since you have no possibility to change the code  I don t see possibility  I mean real possibility  not like Reflection hacks etc   to check private variable

User · Answer

Reflection e g    public class PrivateObject      private String privateString   null     public PrivateObject String privateString        this privateString   privateString        PrivateObject privateObject   new PrivateObject  The Private Value     Field privateStringField   PrivateObject class              getDeclaredField  privateString     privateStringField setAccessible true    String fieldValue    String  privateStringField get privateObject   System out println  fieldValue       fieldValue

User · Answer

I can t tell if you ve found some special case code which requires you to test against private fields  But in my experience you never have to test something private - always public  Maybe you could give an example of some code where you need to test private

User · Answer

Yeah you can use reflections to access private variables  Altough not a good idea   Check this out   http   en wikibooks org wiki Java Programming Reflection Accessing Private Features with Reflection

User · Answer

If you create your test classes in a seperate folder which you then add to your build path    Then you could make the test class an inner class of the class under test by using package correctly to set the namespace  This gives it access to private fields and methods    But dont forget to remove the folder from the build path for your release build

User · Answer

Despite the danger of stating the obvious  With a unit test you want to test the correct behaviour of the object - and this is defined in terms of its public interface  You are not interested in how the object accomplishes this task - this is an implementation detail and not visible to the outside  This is one of the things why OO was invented  That implementation details are hidden  So there is no point in testing private members  You said you need 100  coverage  If there is a piece of code that cannot be tested by using the public interface of the object  then this piece of code is actually never called and hence not testable  Remove it

[java] JUnit Testing private variables?

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