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

Consider a method signature like:

public String myFunction(String abc);

Can Mockito help return the same string that the method received?

You might want to use verify() in combination with the ArgumentCaptor to assure execution in the test and the ArgumentCaptor to evaluate the arguments:

ArgumentCaptor<String> argument = ArgumentCaptor.forClass(String.class);
verify(mock).myFunction(argument.capture());
assertEquals("the expected value here", argument.getValue());

The argument's value is obviously accessible via the argument.getValue() for further manipulation / checking /whatever.

With Java 8, Steve's answer can become

public void testMyFunction() throws Exception {
    Application mock = mock(Application.class);
    when(mock.myFunction(anyString())).thenAnswer(
    invocation -> {
        Object[] args = invocation.getArguments();
        return args[0];
    });

    assertEquals("someString", mock.myFunction("someString"));
    assertEquals("anotherString", mock.myFunction("anotherString"));
}

EDIT: Even shorter:

public void testMyFunction() throws Exception {
    Application mock = mock(Application.class);
    when(mock.myFunction(anyString())).thenAnswer(
        invocation -> invocation.getArgument(0));

    assertEquals("someString", mock.myFunction("someString"));
    assertEquals("anotherString", mock.myFunction("anotherString"));
}

This is a bit old, but I came here because I had the same issue. I'm using JUnit but this time in a Kotlin app with mockk. I'm posting a sample here for reference and comparison with the Java counterpart:

@Test
fun demo() {
  // mock a sample function
  val aMock: (String) -> (String) = mockk()

  // make it return the same as the argument on every invocation
  every {
    aMock.invoke(any())
  } answers {
    firstArg()
  }

  // test it
  assertEquals("senko", aMock.invoke("senko"))
  assertEquals("senko1", aMock.invoke("senko1"))
  assertNotEquals("not a senko", aMock.invoke("senko"))
}

I had a very similar problem. The goal was to mock a service that persists Objects and can return them by their name. The service looks like this:

public class RoomService {
    public Room findByName(String roomName) {...}
    public void persist(Room room) {...}
}

The service mock uses a map to store the Room instances.

RoomService roomService = mock(RoomService.class);
final Map<String, Room> roomMap = new HashMap<String, Room>();

// mock for method persist
doAnswer(new Answer<Void>() {
    @Override
    public Void answer(InvocationOnMock invocation) throws Throwable {
        Object[] arguments = invocation.getArguments();
        if (arguments != null && arguments.length > 0 && arguments[0] != null) {
            Room room = (Room) arguments[0];
            roomMap.put(room.getName(), room);
        }
        return null;
    }
}).when(roomService).persist(any(Room.class));

// mock for method findByName
when(roomService.findByName(anyString())).thenAnswer(new Answer<Room>() {
    @Override
    public Room answer(InvocationOnMock invocation) throws Throwable {
        Object[] arguments = invocation.getArguments();
        if (arguments != null && arguments.length > 0 && arguments[0] != null) {
            String key = (String) arguments[0];
            if (roomMap.containsKey(key)) {
                return roomMap.get(key);
            }
        }
        return null;
    }
});

We can now run our tests on this mock. For example:

String name = "room";
Room room = new Room(name);
roomService.persist(room);
assertThat(roomService.findByName(name), equalTo(room));
assertNull(roomService.findByName("none"));

I use something similar (basically it's the same approach). Sometimes it's useful to have a mock object return pre-defined output for certain inputs. That goes like this:

private Hashtable<InputObject,  OutputObject> table = new Hashtable<InputObject, OutputObject>();
table.put(input1, ouput1);
table.put(input2, ouput2);

...

when(mockObject.method(any(InputObject.class))).thenAnswer(
       new Answer<OutputObject>()
       {
           @Override
           public OutputObject answer(final InvocationOnMock invocation) throws Throwable
           {
               InputObject input = (InputObject) invocation.getArguments()[0];
               if (table.containsKey(input))
               {
                   return table.get(input);
               }
               else
               {
                   return null; // alternatively, you could throw an exception
               }
           }
       }
       );

You can achieve this by using ArgumentCaptor

Imagine you have bean function like so.

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

Then in your test class:

//Use ArgumentCaptor to capture the value
ArgumentCaptor<String> param = ArgumentCaptor.forClass(String.class);


when(mock.myFunction(param.capture())).thenAnswer(new Answer<String>() {
    @Override
    public String answer(InvocationOnMock invocation) throws Throwable {
      return param.getValue();//return the captured value.
    }
  });

OR if you fan of lambda simply do:

//Use ArgumentCaptor to capture the value
ArgumentCaptor<String> param = ArgumentCaptor.forClass(String.class);


when(mock.myFunction(param.capture()))
    .thenAnswer((invocation) -> param.getValue());

Summary: Use argumentcaptor, to capture the parameter passed. Later in answer return the value captured using getValue.

This is a pretty old question but i think still relevant. Also the accepted answer works only for String. Meanwhile there is Mockito 2.1 and some imports have changed, so i would like to share my current answer:

import static org.mockito.AdditionalAnswers.returnsFirstArg;
import static org.mockito.ArgumentMatchers.any;
import static org.mockito.Mockito.when;

@Mock
private MyClass myClass;

// this will return anything you pass, but it's pretty unrealistic
when(myClass.myFunction(any())).then(returnsFirstArg());
// it is more "life-like" to accept only the right type
when(myClass.myFunction(any(ClassOfArgument.class))).then(returnsFirstArg());

The myClass.myFunction would look like:

public class MyClass {
    public ClassOfArgument myFunction(ClassOfArgument argument){
        return argument;
    }  
}

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".