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

Since Mockito 1.9.5+ and Java 8+

You can use a lambda expression, like:

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

Where i is an instance of InvocationOnMock.

For older versions

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

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

Here is the test method with a Mockito answer:

public void testMyFunction() throws Exception {
    MyInterface mock = mock(MyInterface.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"));
}

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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.

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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".

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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"));

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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"));
}

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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;
    }  
}

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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.

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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"))
}

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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.

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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
               }
           }
       }
       );

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