How can I mock a method in easymock that shall return one of its parameters?

Question

public Object doSomething(Object o); which I want to mock. It should just return its parameter. I tried:

Capture<Object> copyCaptcher = new Capture<Object>();
expect(mock.doSomething(capture(copyCaptcher)))
        .andReturn(copyCatcher.getValue());

but without success, I get just an AssertionError as java.lang.AssertionError: Nothing captured yet. Any ideas?

Answer 1

I was looking for the same behavior, and finally wrote the following :

import org.easymock.EasyMock;
import org.easymock.IAnswer;

/**
 * Enable a Captured argument to be answered to an Expectation.
 * For example, the Factory interface defines the following
 * <pre>
 *  CharSequence encode(final CharSequence data);
 * </pre>
 * For test purpose, we don't need to implement this method, thus it should be mocked.
 * <pre>
 * final Factory factory = mocks.createMock("factory", Factory.class);
 * final ArgumentAnswer<CharSequence> parrot = new ArgumentAnswer<CharSequence>();
 * EasyMock.expect(factory.encode(EasyMock.capture(new Capture<CharSequence>()))).andAnswer(parrot).anyTimes();
 * </pre>
 * Created on 22 juin 2010.
 * @author Remi Fouilloux
 *
 */
public class ArgumentAnswer<T> implements IAnswer<T> {

    private final int argumentOffset;

    public ArgumentAnswer() {
        this(0);
    }

    public ArgumentAnswer(int offset) {
        this.argumentOffset = offset;
    }

    /**
     * {@inheritDoc}
     */
    @SuppressWarnings("unchecked")
    public T answer() throws Throwable {
        final Object[] args = EasyMock.getCurrentArguments();
        if (args.length < (argumentOffset + 1)) {
            throw new IllegalArgumentException("There is no argument at offset " + argumentOffset);
        }
        return (T) args[argumentOffset];
    }

}

I wrote a quick "how to" in the javadoc of the class.

Hope this helps.

Answer 2

Well, the easiest way would be to just use the Capture in the IAnswer implementation... when doing this inline you have to declare it final of course.

MyService mock = createMock(MyService.class);

final Capture<ParamAndReturnType> myCapture = new Capture<ParamAndReturnType>();
expect(mock.someMethod(capture(myCapture))).andAnswer(
    new IAnswer<ParamAndReturnType>() {
        @Override
        public ParamAndReturnType answer() throws Throwable {
            return myCapture.getValue();
        }
    }
);
replay(mock)

This is probably the most exact way, without relying on some context information. This does the trick for me every time.

Answer 3

Captures are for testing the values passed to the mock afterwards. If you only need a mock to return a parameter (or some value calculated from the parameter), you just need to implement IAnswer.

See "Remi Fouilloux"s implementation if you want a reusable way of passing paramter X back, but ignore his use of Capture in the example.

If you just want to inline it like "does_the_trick"s example, again, the Capture is a red herring here. Here is the simplified version:

MyService mock = createMock(MyService.class);
expect(mock.someMethod(anyObject(), anyObject()).andAnswer(
    new IAnswer<ReturnType>() {
        @Override
        public ReturnType answer() throws Throwable {
            // you could do work here to return something different if you needed.
            return (ReturnType) EasyMock.getCurrentArguments()[0]; 
        }
    }
);
replay(mock)

Answer 4

Based on @does_the_trick and using lambdas, you can now write the following:

MyService mock = EasyMock.createMock(MyService.class);

final Capture<ParamAndReturnType> myCapture = EasyMock.newCapture();
expect(mock.someMethod(capture(myCapture))).andAnswer(() -> myCapture.getValue());

or without capture as @thetoolman suggested

expect(mock.someMethod(capture(myCapture)))
.andAnswer(() -> (ParamAndReturnType)EasyMock.getCurrentArguments()[0]);

Answer 5

Um, if I understand your question correctly I think you may be over complicating it.

Object someObject = .... ;
expect(mock.doSomething(someObject)).andReturn(someObject);

Should work just fine. Remember you are supplying both the expected parameter and returne value. So using the same object in both works.