### Question:
SubFormWidget extends Composite implements TakesValue<Object>,
                                                        IsNestedModel,
                                                        NeedsFlush { @Override public void addFieldChangeHandler(FieldChangeHandler handler) { formRenderer.addFieldChangeHandler(handler); }  void render(FormRenderingContext renderingContext); @Override Object getValue(); @Override void setValue(Object value); @Override void clear(); @Override void flush(); @Override void addFieldChangeHandler(FieldChangeHandler handler); void setReadOnly(boolean readOnly); boolean isValid();  }### Answer:
@Test public void testAddFieldChangeHandler() { FieldChangeHandler handler = mock(FieldChangeHandler.class); subFormWidget.addFieldChangeHandler(handler); verify(formRenderer).addFieldChangeHandler(same(handler)); }