Example of trying to mock a Class that has a static method is not working

I am trying to learn about using Mockito and in particular I am trying to understand how mocking of static methods can be made to work. I am using Eclipse for the IDE, and I found this page:

https://www.testim.io/blog/mocking-static-methods-mockito/

and trying to test the example in "Option #1: Create Wrapper Objects".

So I have created the classes and interface per that article and I have added the "mockito-extensions" directory and the "mockito-extensions/org.mockito.plugins.MockMaker" file with the string in it, but then I try to run a test in Eclipse (Runas Junit test), it appears that it is executing the code in the "StringCalculatorTest" class but it doesn't appear to actually going into the implementation class and the original class with the static method.

Here is the class with the static method:

package mil.nga.geoaxis.orchestrator_spring.rest;

public class StringCalculatorStatic {
	
	  public static int add(String numbers) {
		  System.out.println("In StringCalculatorStatic: Entering...");
		    String[] parts = numbers.split(",");
		    int sum = 0;
		    for (String part : parts) {
		      int number = Integer.parseInt(part);
		      sum += number;
		    }
		    System.out.println("In StringCalculatorStatic: sum=[" + sum + "]");
		    return sum;
	  } // end add()

}

and here is the interface:

package mil.nga.geoaxis.orchestrator_spring.rest;

public interface StringCalculator {
	
	public int add(String numbers);

}

and the implementation class:

package mil.nga.geoaxis.orchestrator_spring.rest;

public class StringCalculatorImpl implements StringCalculator{
    @Override
    public int add(String numbers) {
    	System.out.println("In StringCalculatorImp.add: Entering...");
        return StringCalculatorStatic.add(numbers);
    }

}

and finally the class with the Test:

package mil.nga.geoaxis.orchestrator_spring.rest;


import org.junit.jupiter.api.BeforeAll;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.junit.jupiter.MockitoExtension;
import org.mockito.Mockito;


public class StringCalculatorTest {
	
	
	@Test
	public void testAdd() {
		System.out.println("In StringCalculatorTest.testAdd: Entering...");
		System.out.println("In StringCalculatorTest.testAdd: About to create the mock of StringCalculator object...");
		StringCalculator calc = Mockito.mock(StringCalculator.class);
		System.out.println("In StringCalculatorTest.testAdd: Created the mock of StringCalculator object and reference stored in 'calc'");

		System.out.println("In StringCalculatorTest.testAdd: About to call calc() method...");
		int responseFromCallingCalc = calc.add("4,6");
		System.out.println("In StringCalculatorTest.testAdd: Returned from calling calc() method, responseFromCallingCalc=[" + responseFromCallingCalc + "]");
		
		Mockito.when(calc.add("4,6")).thenReturn(10);
		System.out.println("In StringCalculatorTest.testAdd FINISHED...");
	}

}

When I do RunAs Junit the output is:

In StringCalculatorTest.testAdd: Entering...
In StringCalculatorTest.testAdd: About to create the mock of StringCalculator object...
In StringCalculatorTest.testAdd: Created the mock of StringCalculator object and reference stored in 'calc'
In StringCalculatorTest.testAdd: About to call calc() method...
In StringCalculatorTest.testAdd: Returned from calling calc() method, responseFromCallingCalc=[0]
In StringCalculatorTest.testAdd FINISHED...

None of the output from the other classes is appearing, so I am guessing, that for some reason, it is not actually doing the mocking?

Can someone help/suggest what I am missing?

Thanks,
Jim

EDIT: I've edited the test class to add a call to 'calc' (per suggestion from Lesiak and also some additional debug printlns, but it looks like it is still only executing the Test class code and not execute the static method in the StringCalculatorStatic class.

> I found this page:
https://www.testim.io/blog/mocking-static-methods-mockito/
and trying to test the example in "Option #1: Create Wrapper Objects".

From that page for option 1 (emphasis mine)

> The first option consists of not mocking the static method at all.

So ...
> None of the output from the other classes is appearing, so I am guessing, that for some reason, it is not actually doing the mocking?

> Can someone help/suggest what I am missing?

You're missing the fact that the article you're following is suggesting a way that you do not mock a static object. It's working exactly as intended. Instead of mocking a static function, you create a wrapper class that just calls the static function. Then in your tests your mock instances of the wrapper class instead of mocking the static functions.

EDIT: Follow up question.

> But I think I AM doing what you said?

Yes, you are. That's the point. You are creating a wrapper around the static and never actually using or mocking the static functions.

> StringCalculatorImpl is the implementation of StringCalculator interface, and in StringCalculatorTest, I do "StringCalculator calc = Mockito.mock(StringCalculator.class)", i.e. create a mock instance of the StringCalculator class. Isn't that what the article (and what you suggested ("Then in your tests your mock instances of the wrapper class instead of mocking the static functions")?

Yes, it is. Going back to your original question:

> None of the output from the other classes is appearing, so I am guessing, that for some reason, it is not actually doing the mocking?

Based on your question and subsequent responses, I think you have a misunderstanding of how mocks work. You are not seeing output from the other classes precisely because it is doing the mocking.

Let's review:

StringCalculator calc = Mockito.mock(StringCalculator.class);

This is a mock of the interface you've defined. It has stubbed out methods that do nothing. Therefore, your real add method in StringCalculatorImpl will never be called and you will never see a print statement when you do:

int responseFromCallingCalc = calc.add("4,6");

And, because you've not told Mockito what to do when calling that method on that mock, you will get back a default value (0).

Next you instructed Mockito to return a specific value given specific inputs. However, you do nothing else with this mock after this, so this mocking is pointless in your example:

Mockito.when(calc.add("4,6")).thenReturn(10);

At this point, if you were to get the value of calc.add("4,6"), it would return 10, as you told it to. Again - none of your actual code gets executed because you're working with a mock.

If this is still unclear, then my best advice would be to Google around for and read through tutorials or examples of how Mockito works.