Ignore code while testing a function using junit

Question

I am trying to write test case for my java function. My function is something of the following type

public static String parse33_05_x_Data(String message, Transaction transaction){
    String returnValue="";
    /**
    *Some Java code which I actually want to test.
    *
    **/
    //Writing the transaction to the file.
    FileManager.getInstance().WriteDataToFile(transaction);
    return returnValue;
}

But in the above code I want to somehow ignore/mock the line FileManager.getInstance().WriteDataToFile(transaction); as it is giving an exception. without making any code changes to the above function itself.

Whatever changes are required should happen inside my Test class .

Is it possible to do something like this?

Thanks in advance!

Answer 1

You should put the fileManager instance as a param. Then it is possible to mock this instance.

public static String parse33_05_x_Data(String message, FileManager fileManager, Transaction transaction){
    String returnValue="";
    /**
    *Some Java code which I actually want to test.
    *
    **/
    // mock doesnt write
    fileManager.WriteDataToFile(transaction);
    return returnValue;
}

Alternatively, (you write you dont want to change the method) you can use PowerMockito to mock the static method FileManager.getInstance(), but I would not recommend this, because in my opinion this is a code smell.

Answer 2

You can use a test framework called Mockito to "mock" parameters to a method. This will make it possible to create a FileManager that "doesn't do anything".

You should refactor the fileManager to a parameter to make it mockable

public static String parse33_05_x_Data(String message, Transaction transaction){
    String returnValue="";
    /**
    *Some Java code which I actually want to test.
    *
    **/
    //Writing the transaction to the file.
    FileManager.getInstance().WriteDataToFile(transaction);
    return returnValue;
}

to:

public static String parse33_05_x_Data(String message, Transaction transaction, FileManager fileManager){
    String returnValue="";
    /**
    *Some Java code which I actually want to test.
    *
    **/
    //Writing the transaction to the file.
    fileManager.WriteDataToFile(transaction);
    return returnValue;
}

You can then use Mockito to mock objects in your JUnit test cases like this:

@Test
public void test1()  {
        //  create mock
        FileManager mockedFileManager = Mockito.mock(FileManager.class);

        // define return value for method
        when(mockedFileManager.WriteDataToFile()).thenReturn("something");


        // use mock in test....
        assertEquals(MyObj.parse33_05_x_Data("message") new Transaction(), mockedFileManager), "expected value");
}