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You mean, aside from giving the team that made `B` a stern talking to? ;-)

You could introduce another layer of indirection, and mock that. For instance like this:

    interface MockFriendlyB {
        public void add(T arg);
    }
            
    class UnmockedB implements MockFriendlyB {
        B b = ...;

        public void add(T arg) {
            b.add(arg);
        }
    }

    class MockedB implements MockFriendlyB {
        public void add(T arg) {
            // do whatever you like
        }
    }

and then change `A` to use a `MockFriendlyB` rather than `B`, and give `A` an `UnmockedB` in normal usage, but a `MockedB` when testing.