How to use mockClass method of org.mockito.internal.creation.bytebuddy.InlineBytecodeGenerator class

You need to inject mock inside the class you're testing. At the moment you're interacting with the real object, not with the mock one. You can fix the code in a following way:

void testAbc(){
     myClass.myObj = myInteface;
     myClass.abc();
     verify(myInterface).myMethodToBeVerified(new String("a"), new String("b"));
}

although it would be a wiser choice to extract all initialization code into @Before

@Before
void setUp(){
     myClass = new myClass();
     myClass.myObj = myInteface;
}

@Test
void testAbc(){
     myClass.abc();
     verify(myInterface).myMethodToBeVerified(new String("a"), new String("b"));
}

There is a type-safe way: use ArgumentMatchers.any() and qualify it with the type:

ArgumentMatchers.<AsyncCallback<ResponseX>>any()

You seem to be after contradictory goals. On the one hand, you're trying to avoid writing data to disk, which isn't a bad goal in tests. On the other, you're trying to test your I/O-handling class, which means you'll be working with system utilities that assume that your File will work with native calls. As such, here's my guidance:

• Don't try to mock a File. Just don't. Too many native things depend on it.
• If you can, split your I/O-handling code into the half that opens a File and turns it into a Reader, and the half that parses HTML out of the Reader.
• At that point, you don't need a mock at all--just construct a StringReader to simulate the data source.
• While that handles your unit tests pretty well, you may also want to write an integration test that uses a temporary file and ensure that it reads right. (Thanks Brice for adding that tip!)

Don't be afraid to refactor your class to make testing easier, as here:

class YourClass {
  public int method(File file) {
    // do everything here, which is why it requires a mock
  }   
}   

class YourRefactoredClass {
  public int method(File file) {
    return methodForTest(file.getName(), file.isFile(),
        file.isAbsolute(), new FileReader(file));
  }   

  /** For testing only. */
  int methodForTest(
      String name, boolean isFile, boolean isAbsolute, Reader fileContents) {
    // actually do the calculation here
  }   
}   

class YourTest {
  @Test public int methodShouldParseBadHtml() {
    YourRefactoredClass yrc = new YourRefactoredClass();
    assertEquals(42, yrc.methodForTest(
        "bad.html", true, false, new StringReader(badHTMLText));
  }   
}   

At this point the logic in method is so straightforward it's not worth testing, and the logic in methodForTest is so easy to access that you can test it heavily.

Make sure you have the following dependency in your pom file

        <dependency>
            <groupId>org.mockito</groupId>
            <artifactId>mockito-core</artifactId>
            <version>3.5.15</version>
            <scope>test</scope>
        </dependency>

You're not passing the spy to when(). You're passing the original object.

Change the line

Mockito.spy(greeter);

to

greeter = Mockito.spy(greeter);

Mockito.spy() creates a spy object that is a copy of the original object. It doesn't modify the original object.

I've never used Mockito, but want to learn, so here goes. If someone less clueless than me answers, try their answer first!

Mockito.when(reader.document(anyInt())).thenAnswer(new Answer() {
 public Object answer(InvocationOnMock invocation) {
     Object[] args = invocation.getArguments();
     Object mock = invocation.getMock();
     return document(fakeIndex((int)(Integer)args[0]));
     }
 });

Mockito cannot mock final classes or methods. Try removing the final modifier from the hasInventory method. Or better yet, do not mock the Warehouse class, instead mock the IWarehouse interface, whose methods cannot be final, and presumably defines those used by Order.

In general, it's preferable to mock interfaces, but it's not mandatory.

Not being able to mock final classes or methods is briefly mentioned in the Mockito FAQ, it is due to the runtime class generation technique used for creating the mocks.

Mocking the iterator works for me. See below code sample:

import static org.junit.Assert.assertEquals;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.when;

import java.util.Collection;
import java.util.Iterator;

import org.junit.Before;
import org.junit.Test;

public class TestMockedIterator {

    private Collection<String> fruits;
    private Iterator<String> fruitIterator;

    @SuppressWarnings("unchecked")
    @Before
    public void setUp() {
        fruitIterator = mock(Iterator.class);
        when(fruitIterator.hasNext()).thenReturn(true, true, true, false);
            when(fruitIterator.next()).thenReturn("Apple")
            .thenReturn("Banana").thenReturn("Pear");

        fruits = mock(Collection.class);
        when(fruits.iterator()).thenReturn(fruitIterator);
    }

    @Test
    public void test() {
        int iterations = 0;
        for (String fruit : fruits) {
            iterations++;
        }
        assertEquals(3, iterations);
    }
}

According to docs, this is deprecated. You should use factory method forClass(Class) to create captors instead to avoid NullPointerExceptions. see here

Example:

  ArgumentCaptor<Person> argument = ArgumentCaptor.forClass(Person.class);
  verify(mock).doSomething(argument.capture());
  assertEquals("John", argument.getValue().getName());