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?

score 30 · Answer 1 · answered Oct 17 '11 at 16:41

30

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.

answered Oct 17 '11 at 16:41

does_the_trick

301
3
3

I like the post from Remi Fouilloux more and use it very often. It removes the need for a catpure object. – Jan Oct 26 '11 at 15:39
3

Great answer. Using Java 8 lambda's the entire IAnswer anonymous subclass can be rewritten as "myCapture::getValue". – Henno Vermeulen Feb 16 '17 at 13:47

score 19 · Accepted Answer · edited Jan 17 '12 at 03:18

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.

thanks! Although I changed the original unit test I'm sure I'll run into this issue again! (You maybe want to contribute it to EM direclty?) — Jan, Jun 24 '10 at 16:27
The Capture is a red herring in your javadoc example - it is not needed: EasyMock.expect(factory.encode(anyObject())).andAnswer(parrot).anyTimes(); — thetoolman, Jan 17 '12 at 20:09
Unfortunately, moderators have decided that an edit to remove the misleading & unused `Capture` is "either completely superfluous or actively harms readability" and that it does not "strive to preserve the goals of the post's owner". — AndrewF, Aug 13 '20 at 22:17

score 17 · Answer 3 · answered Jan 17 '12 at 20:22

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)

Not quite, my point is that in "Remi Fouilloux"s code, the Capture the javadoc example is unneeded. It is also unneeded in "does_the_trick"s example code, as improved above. — thetoolman, Feb 16 '12 at 21:06

score 10 · Answer 4 · answered Jan 16 '15 at 21:38

10

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]);

answered Jan 16 '15 at 21:38

Marco Baumeler

101
2
5

Thanks for EasyMock.newCapture(), it appears that now you cannot instantiate Capture with new as in other examples; it's a private constructor. – k-den Jun 27 '19 at 23:37

score 2 · Answer 5 · answered Apr 19 '10 at 12:14

2

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.

answered Apr 19 '10 at 12:14

drekka

20,957
14
79
135

I don't know "someObject". It is instantiated in the method I want to test. Think of a method "createImage(InputStream image)" (cut) which internally calls a "Image filter(Image imge)" (mock). – Jan Apr 24 '10 at 11:39
Ahhh. ok. Then you can do a couple of things. Firstly you can test that the object is a particular class using the isA() argument matcher. Second I would suggest writing your own argument capture. I haven't done that, but I have written my own argument matchers. Which is really useful if you, for example, want to check bean properties. Unfortunately I don't have that code anymore, but if you look at the example of writing a matcher, it's not hard. – drekka Apr 26 '10 at 04:00
Your code is valid but it isn't answering the question to use one of the parameters- it is using a known object. – thetoolman Jan 13 '12 at 01:47

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

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