Using Mockito’s generic “any()” method

I have an interface with a method that expects an array of Foo:

public interface IBar {
  void doStuff(Foo[] arr);
}

I am mocking this interface using Mockito, and I’d like to assert that doStuff() is called, but I don’t want to validate what argument are passed – “don’t care”.

How do I write the following code using any(), the generic method, instead of anyObject()?

IBar bar = mock(IBar.class);
...
verify(bar).doStuff((Foo[]) anyObject());

Answer

Since Java 8 you can use the argument-less any method and the type argument will get inferred by the compiler:

verify(bar).doStuff(any());

---

Explanation

The new thing in Java 8 is that the target type of an expression will be used to infer type parameters of its sub-expressions. Before Java 8 only arguments to methods where used for type parameter inference (most of the time).

In this case the parameter type of doStuff will be the target type for any(), and the return value type of any() will get chosen to match that argument type.

This mechanism was added mainly to be able to compile lambda expressions, but it improves type inferences generally.

---

Primitive types

This doesn’t work with primitive types, unfortunately:

public interface IBar {
    void doPrimitiveStuff(int i);
}

verify(bar).doPrimitiveStuff(any()); // Compiles but throws NullPointerException
verify(bar).doPrimitiveStuff(anyInt()); // This is what you have to do instead

The problem is that the compiler will infer Integer as the return value type of any(). Mockito will not be aware of this (due to type erasure) and return the default value for reference types, which is null. The runtime will try to unbox the null return value by calling the intValue method on it before passing it to doStuff, and the exception gets thrown.