Mockito

method when()

public static <T> OngoingStubbing<T> when(T methodCall){

    return MOCKITO_CORE.when(methodCall);

}

public <T> OngoingStubbing<T> when(T methodCall) {
    MockingProgress mockingProgress = mockingProgress();
    mockingProgress.stubbingStarted();
    @SuppressWarnings("unchecked")
    OngoingStubbing<T> stubbing = (OngoingStubbing<T>) mockingProgress.pullOngoingStubbing();
    if (stubbing == null) {
        mockingProgress.reset();
        throw missingMethodInvocation();
    }
    return stubbing;
}

@Override
public OngoingStubbing<T> thenReturn(T value) {
    return thenAnswer(new Returns(value));
}

@Override
public OngoingStubbing<T> thenReturn(T value, T... values) {
    OngoingStubbing<T> stubbing = thenReturn(value);
    if (values == null) {
        // For no good reason we're configuring null answer here
        // This has been like that since forever, so let's keep it for compatibility (unless
        // users complain)
        return stubbing.thenReturn(null);
    }
    for (T v : values) {
        stubbing = stubbing.thenReturn(v);
    }
    return stubbing;
}

public class OngoingStubbingImpl<T> extends BaseStubbing<T> {

    private final InvocationContainerImpl invocationContainer;
    private Strictness strictness;

    public OngoingStubbingImpl(InvocationContainerImpl invocationContainer) {
        super(invocationContainer.invokedMock());
        this.invocationContainer = invocationContainer;

		@Override
		public OngoingStubbing<T> thenAnswer(Answer<?> answer) {
		    if (!invocationContainer.hasInvocationForPotentialStubbing()) {
		        throw incorrectUseOfApi();
		    }
		
		    invocationContainer.addAnswer(answer, strictness);
		    return new ConsecutiveStubbing<T>(invocationContainer);
		}

		......
}