### Question:
StackableStage extends Stage { public void replace(Actor screen) { stack.replace(screen); } StackableStage(Viewport viewport, ScreenStack stack);  StackableStage(Viewport viewport); ScreenStack getScreenStack(); void replace(Actor screen); void push(Actor screen); void pop(int n);  }### Answer:
@Test public void replace() { Actor screen = mock(Actor.class); stage.replace(screen); verify(stack).replace(eq(screen)); }