How to Mock Void Methods in Mockito Effectively?

Mocking Void Methods with Mockito

Mocking void methods in Mockito can be achieved using methods from the doThrow(), doAnswer(), doNothing(), and doReturn() families. These methods allow you to specify behavior for a mocked method that has a void return type.

Example:

// Mock a void method that throws an exception
Mockito.doThrow(new Exception()).when(instance).methodName();

// Mock a void method that performs some action
Mockito.doAnswer(new Answer<Void>() {
    public Void answer(InvocationOnMock invocation) {
        Object[] args = invocation.getArguments();
        System.out.println("called with arguments: " + Arrays.toString(args));
        return null;
    }
}).when(instance).methodName();

In your specific example, you can mock the setState() method in the World class as follows:

// Mock the World class
World mockWorld = mock(World.class); 

// Mock the setState method to print the arguments passed to it
doAnswer(new Answer<Void>() {
    public Void answer(InvocationOnMock invocation) {
        Object[] args = invocation.getArguments();
        System.out.println("called with arguments: " + Arrays.toString(args));
        return null;
    }
}).when(mockWorld).setState(anyString());

Now, when you call mockWorld.setState() within your test, the provided Answer will be executed, allowing you to verify that the correct arguments were passed to the method.

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