Unable to stub a static method call using PowerMockito

Question

I have a file Util.java:

public class Util  {
    public static int returnInt() {
        return 1;
    }

    public static String returnString() {
        return "string";
    }
}

Another class:

public class ClassToTest {
    public String methodToTest() {
        return Util.returnString();
    }
}

I want to test it using TestNg and PowerMockito:

@RunWith(PowerMockRunner.class)
@PrepareForTest(Util.class)
public class PharmacyConstantsTest {    
    ClassToTest classToTestSpy;

    @BeforeMethod
    public void beforeMethod() {
        classToTestSpy = spy(new ClassToTest());
    }

    @Test
    public void method() throws Exception {
        mockStatic(Util.class);
        when(Util.returnString()).thenReturn("xyz");
        classToTestSpy.methodToTest();
    }
}

However, it throws the following error:

FAILED: method org.mockito.exceptions.misusing.MissingMethodInvocationException: when() requires an argument which has to be 'a method call on a mock'. For example: when(mock.getArticles()).thenReturn(articles);

I tried this solution using various solutions from the web, but could not locate the mistake in my code. I need to stub the call for static method, since I need it for a legacy code. How do I mock a static method using PowerMockito?

Answer 1

Just for the record, adding making the test class a subclass of PowerMockTestCase worked for me.

@PrepareForTest(Util.class)
public class PharmacyConstantsTest extends PowerMockTestCase {    
    ClassToTest classToTestSpy;

    @BeforeMethod
    public void beforeMethod() {
        classToTestSpy = spy(new ClassToTest());
    }

    @Test
    public void method() throws Exception {
        mockStatic(Util.class);
        when(Util.returnString()).thenReturn("xyz");
        classToTestSpy.methodToTest();
     }
 }