These are some of the useful methods and classes provided by Mockito

1. Mockito.mock()

Creates a mock of any class. By default, it will return default value for methods and fields when not mocked.

For Example: Primitive data types such as byte, short, int, long, float, double, boolean and char will return default value i.e 0 or false

Non-primitive data types such as String, Arrays and Classes return null when they are not mocked.

// with annotation
@Mock
private ClassName mock;

// Or with static method
final ClassName mock = Mockito.mock(ClassName.class);

// mock methods that returns something
Mockito.when(mock.sampleMethod()).thenReturn(sampleVal);

// mock method to throw Exception
Mockito.when(mock.sampleMethod()).thenThrow(SampleException.class);

// Mock void methods
Mockito.doNothing().when(mock).method();

// you can get invocation parameters with .thenAnswer()
Mockito.when(mock.sampleMethod(Mockito.any(OtherClass.class))).thenAnswer((Answer<ThirdClass>) invocation -> {
      final OtherClass parameter1 = invocation.getArgument(0);
      return thirdClassObject; 
});

// Verify interactions
Mockito.verify(mock, Mockito.times(1)).sampleMethod();

// Verify, method should not be invoked even once
Mockito.verify(mock, Mockito.never()).sampleMethod2();

// Verify, no methods should be called on given mock
Mockito.verifyNoMoreInteractions(otherMockedClass);

2. Mockito.spy()

It is similar to mock() but instead of return default value it will call the actual implementation when not mocked.

// with annotation
@Spy
private ClassName spy;

// Or with static method
final ClassName spy = Mockito.spy(ClassName.class);

// mock methods that returns something
Mockito.doReturn(value).when(spy).method();

// mock method to throw exception
Mockito.doThrow(new ExceptionClass()).when(spy).metohd();

// mock void methods
Mockito.doNothing().when(spy).method();

3. Mockito.mockStatic()

Now with Mockito 3, you can mock Classes with static methods.

try (MockedStatic<Foo> dummyStatic = Mockito.mockStatic(ClassName.class)) {
    dummyStatic.when(() -> ClassName.method("param1"))
               .thenReturn("someValue");
    // when
    System.out.println(ClassName.method("param1"));
    //then
    dummyStatic.verify(
            () -> ClassName.method("param1"),
            times(1), 
    );
}

4. ArgumentMatcher

Basic Rule to match any statement with Mockito is to correctly match its parameters. If any of the parameter did not match then instruction will not be mocked.

// example statement
final int addition = calculator.getAddition(2, 3);

// you have to match exact parameters, in this case (2,3)
Mockito.when(mckCalculator.getAddition(2, 3)).thenReturn(5);

// If you mock for (3,4), you mocked result will not be returned
Mockito.when(mckCalculator.getAddition(3, 4)).thenReturn(5);

// For cases, where you do not know exact parameter then you can use Mockito.anyInt()
Mockito.when(mckCalculator.getAddition(Mockito.anyInt(), Mockito.anyInt())).thenReturn(5);

Similarly we have Mockito.any(), Mockito.anyLong(), Mockito.anyString(), Mockito.anyBoolean(), Mockito.anyByte(), Mockito.anyChar(), Mockito.anyFloat(), Mockito.anyDouble(), Mockito.anyShort(), Mockito.anyList(), Mockito.anySet() and Mockito.anyMap()

5. ArgumentCaptor

You can also get value of parameters of mocked methods, you can then Assert for their value.

Below is the same example of calculator class.

// example statement
final int addition = calculator.getAddition(2, 3);

// Create captor
ArgumentCaptor<Integer> argumentCaptor = ArgumentCaptor.forClass(Integer.class);

Mockito.when(mckCalculator.getAddition(argumentCaptor.capture(), argumentCaptor.capture())).thenReturn(5);

final int firstNum = argumentCaptor.getValue();
Assert.assertEquals(2, firstNum);

// You can get second num by calling getAllValues()
final int secondNum = argumentCaptor.getAllValues()[1];
Assert.assertEquals(3, secondNum);