Question

I’m a .NET guy – and I mainly code in C#.

Since C# 3.0, we can leverage lambda expressions and expression trees to use static reflection. For example, it is possible to implement GetMethodName in the following snippet to return the name of the method passed in parameter:

string methodName = GetMethodName( o => o.DoSomething()); Console.WriteLine(methodName); // displays 'DoSomething' 

Now, when I look at Mockito samples (or EasyMock ones) in the java world, I see:

LinkedList mockedList = mock(LinkedList.class); when(mockedList.get(0)).thenReturn('first'); 

How does it work?

How does the when method work ? How does it interpret mockedList.get(0) as a call to the get method with 0 passed as parameter and not as a value?

Answer 1

Mocking libraries don’t typically work with expression trees. They build a type which implements the appropriate interface and responds to method calls either by recording them or validating them and returning the preprogrammed responses. This is usually done with either a proxy (e.g. RealProxy in .NET, Proxy in Java) or with dynamic code generation.

In the case of EasyMock, it uses Proxy (for interfaces, anyway), as you can see in the source code: look at org.easymock.internal.JavaProxyFactory.