Mocking Methods to Return Arguments Received

In object-oriented programming, mocking frameworks like Mockito are used to create a mock object that simulates the behavior of a real object for testing purposes. One common scenario in testing is the need to have a mocked method return the same argument that was passed to it.

Mockito and Lambda Expressions (Mockito 1.9.5 and Java 8 )

For versions of Mockito 1.9.5 and later in conjunction with Java 8 or above, you can leverage lambda expressions to achieve this behavior:

when(myMock.myFunction(anyString())).thenAnswer(i -> i.getArguments()[0]);

Here, i represents an instance of InvocationOnMock, and getArguments()[0] retrieves the first argument passed to the mocked method.

Mockito and Custom Answers (Older Versions)

For older versions of Mockito, you can use a custom Mockito Answer implementation:

MyInterface mock = mock(MyInterface.class);
when(mock.myFunction(anyString())).thenAnswer(new Answer() {
    @Override
    public String answer(InvocationOnMock invocation) throws Throwable {
        Object[] args = invocation.getArguments();
        return (String) args[0];
    }
});

This custom Answer retrieves the passed argument and returns it, allowing the mocked method to echo the received input.