How Can I Test Nondeterministic Responses with Mockito?

Testing Nondeterministic Responses with Mockito

When testing code that interacts with nondeterministic services, it can be difficult to ensure that the outcomes remain constant irrespective of the return order of methods. For instance, consider testing the following code, which uses an ExecutorCompletionService to group and process tasks:

<code class="java">ExecutorCompletionService<T> completionService = new ExecutorCompletionService<>(service);

for (Callable<T> t : ts)
    completionService.submit(request);

for (int i = 0; i < calls.size(); i ++) {
    try {
        T t = completionService.take().get();
        // do some stuff that I want to test
    } catch (...) { }        
}</code>

To address this challenge, Mockito provides a means to configure subsequent invocations of a method to return different objects. By using the thenReturn method, you can specify the objects to be returned in order:

<code class="java">when(method-call).thenReturn(value1, value2, value3);</code>

Each value will be returned sequentially, with the last value being used repeatedly once all other values have been exhausted. This allows you to test different scenarios and ensure that the outcome remains consistent regardless of the return order.

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