这个问题在这里已经有答案了：

什么是nullpointerexception，如何修复它(12个答案）
上个月关门了。
基本上，我正在尝试为一个任务片段实现一个回收器视图，并且遵循了大多数人似乎做过的对他们有用的操作，但是遇到了一个问题，myadapter.startlistening（）方法会提示以下错误消息：
java.lang.nullpointerexception:尝试对空对象引用调用虚拟方法“void com.example.student\u productivity\u and\u reflection\u app.tasks.taskadapter.startlistening（）”
我不确定出了什么问题。这是我的任务类：

public class Task {
    private String title, description;
    private Date date;

    public Task() {
        //Empty constructor needed for firebase
    }

    public Task(String title, String description, Date date) {
        this.title = title;
        this.description = description;
        this.date = date;
    }

    public String getTitle() {
        return title;
    }

    public void setTitle(String title) {
        this.title = title;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    public Date getDate() {
        return date;
    }

    public void setDate(Date date) {
        this.date = date;
    }
}

任务适配器：

public class TaskAdapter extends FirestoreRecyclerAdapter <Task, TaskAdapter.TaskHolder>{

    public TaskAdapter(@NonNull FirestoreRecyclerOptions<Task> options) {
        super(options);
    }

    @Override
    protected void onBindViewHolder(@NonNull TaskHolder holder, int position, @NonNull Task model) {
        holder.textViewTitle.setText(model.getTitle());
        holder.textViewDescription.setText(model.getDescription());
        holder.textViewDate.setText(String.valueOf(model.getDate()));
    }

    @NonNull
    @Override
    public TaskHolder onCreateViewHolder(@NonNull ViewGroup viewGroup, int i) {

        View v = LayoutInflater.from(viewGroup.getContext()).inflate(R.layout.task_items, viewGroup, false);

        return new TaskHolder(v);
    }

    class TaskHolder extends RecyclerView.ViewHolder {

        TextView textViewTitle;
        TextView textViewDescription;
        TextView textViewDate;

        public TaskHolder(@NonNull View itemView) {
            super(itemView);

            textViewTitle = itemView.findViewById(R.id.task_title);
            textViewDescription = itemView.findViewById(R.id.task_description);
            textViewDate = itemView.findViewById(R.id.task_date);
        }
    }
}

和任务碎片：

public class TasksFragment extends Fragment{

  private FirebaseAuth mAuth = FirebaseAuth.getInstance();
  private FirebaseFirestore db = FirebaseFirestore.getInstance();

  private TaskAdapter myAdapter;
  private FloatingActionButton newTask;
  private RecyclerView recyclerView;

  @Nullable
  @Override
  public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
      return inflater.inflate(R.layout.fragment_tasks, container, false);
  }

    private void fetchTasks(View view){

      Query query = db.collection("users").document("coolguy").collection("task").orderBy("date", Query.Direction.DESCENDING);

      FirestoreRecyclerOptions<Task> options = new FirestoreRecyclerOptions.Builder<Task>().setQuery(query, Task.class).build();

      myAdapter = new TaskAdapter(options);

      recyclerView = view.findViewById(R.id.tasksList);
      recyclerView.setHasFixedSize(true);

      recyclerView.setLayoutManager(new LinearLayoutManager(getContext()));

      recyclerView.setAdapter(myAdapter);

    }

    @Override
    public void onStart() {
        super.onStart();
        myAdapter.startListening();
    }

    @Override
    public void onStop() {
        super.onStop();
        myAdapter.stopListening();
    }
}

（我知道日期应该是一个具有适当格式的日期变量，但我计划在功能真正起作用后修复它。）