### Question:
StackableStage extends Stage { public void push(Actor screen) { stack.push(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 push() { Actor screen = mock(Actor.class); stage.push(screen); verify(stack).push(eq(screen)); }