Making a mocked method return an argument that was passed to it

You can create an Answer in Mockito. Let's assume, we have an interface named Application with a method myFunction.

public interface Application {  public String myFunction(String abc);}

Here is the test method with a Mockito answer:

public void testMyFunction() throws Exception {  Application mock = mock(Application.class);  when(mock.myFunction(anyString())).thenAnswer(new Answer<String>() {    @Override    public String answer(InvocationOnMock invocation) throws Throwable {      Object[] args = invocation.getArguments();      return (String) args[0];    }  });  assertEquals("someString",mock.myFunction("someString"));  assertEquals("anotherString",mock.myFunction("anotherString"));}

Since Mockito 1.9.5 and Java 8, you can also use a lambda expression:

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

If you have Mockito 1.9.5 or higher, there is a new static method that can make the Answer object for you. You need to write something like

import static org.mockito.Mockito.when;import static org.mockito.AdditionalAnswers.returnsFirstArg;when(myMock.myFunction(anyString())).then(returnsFirstArg());

or alternatively

doAnswer(returnsFirstArg()).when(myMock).myFunction(anyString());

Note that the returnsFirstArg() method is static in the AdditionalAnswers class, which is new to Mockito 1.9.5; so you'll need the right static import.

With Java 8 it is possible to create a one-line answer even with older version of Mockito:

when(myMock.myFunction(anyString()).then(i -> i.getArgumentAt(0, String.class));

Of course this is not as useful as using AdditionalAnswers suggested by David Wallace, but might be useful if you want to transform argument "on the fly".