尝试对可扩展列表上的null对象引用调用虚拟方法

Question

这是我第一次在StackOverflow中提出问题，我想向您展示我的代码，如下所示：

SecondFragment类

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_second, container, false);
listView = (ExpandableListView) view.findViewById(R.id.expand_list);
final List<String> listGroup = new ArrayList<>();
listGroup.add("Menu 1");
listGroup.add("Menu 2");
listGroup.add("Menu 3");
List<Makanan> listItem1 = new ArrayList<>();
List<Makanan> listItem2 = new ArrayList<>();
List<Makanan> listItem3 = new ArrayList<>();
listItem1.add(new Makanan("Menu 1", "Ikan", "Telur dan Susu", "Panjang", "Bandung"));
listItem2.add(new Makanan("Menu 2", "Ikan", "Telur dan Susu", "Panjang", "Bandung"));
listItem3.add(new Makanan("Menu 3", "Ikan", "Telur dan Susu", "Panjang", "Bandung"));
final HashMap<String, List<Makanan>> lstItemsGroup = new HashMap<>();
this.mainPresenter.addList(listGroup.get(0),listItem1);
this.mainPresenter.addList(listGroup.get(1),listItem2);
this.mainPresenter.addList(listGroup.get(2),listItem3);
final ListMakanan listMakanan = new ListMakanan(getActivity(),listGroup,lstItemsGroup);
listView.setAdapter(listMakanan);
FloatingActionButton fab = view.findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
listener.changePage(3);
}
});
return view;
}

主演示者类

public class MainPresenter {
protected final HashMap<String , List<Makanan>> makananList;
protected IMainActivity ui;
public MainPresenter(IMainActivity active){
this.ui = active;
this.makananList = new HashMap<String, List<Makanan>>();
}
public void addList(String title,  List<Makanan> listMakanan){
makananList.put(title, listMakanan);
this.ui.updateList(this.makananList);
this.ui.resetAddForm();
}

}

问题是，为什么我不能在MainPresenter类中添加/放置方法为addList((的hashMap？

你们能解决这个问题吗？

非常感谢。

(错误显示在标题中(

Answer 1

您永远不应该这样做。makananList引起的零点异常是用addList方法初始化的。除非在提供MainPresenter实例之前调用该方法。如下所示：MainPresenter mp = new MainPresenter (active) mp.addList(parameters)