尝试在空对象引用上调用直接方法 'void答案

【问题标题】：Attempt to invoke direct method 'void on a null object reference尝试在空对象引用上调用直接方法 'void
【发布时间】：2020-04-03 09:09:08
【问题描述】：

为什么我会收到错误 Attempt to invoke direct method 'void asyncFinished(com.ats.atsver2.ui.db.classes.User)' on a null object reference。在 onPostExecute 上，我得到了一个结果，但是当我尝试通过委托发送它时，我得到了错误。

这是我的代码：

public class UserRepository {
    private UserDao userDao;
    private User user;
    private LiveData<List<User>> allUsers;

    public UserRepository(Application application) {
        AtsDatabase database = AtsDatabase.getInstance(application);
        userDao = database.userDao();
        allUsers = userDao.getUsers();
    }

    private MutableLiveData<User> userSearched = new MutableLiveData<>();
    private void asyncFinished(User userResult) {
        if(userResult == null) return;
        Log.i("TAG", "asyncFinished: " + userResult.getLastName());
        userSearched.setValue(userResult);
    }

    public LiveData<User> getUserByUserName(String userName) {
        new GetUserByUserNameAsyncTask(userDao).execute(userName);
    }

    public LiveData<List<User>> getAllUsers() { return allUsers; }

    private static class InsertUserAsyncTask extends AsyncTask<User, Void, Void> {
        private UserDao userDao;

        private InsertUserAsyncTask(UserDao userDao) { this.userDao = userDao; }

        @Override
        protected Void doInBackground(User... users) {
            userDao.insert(users[0]);
            return null;
        }
    }

    private static class GetUserByUserNameAsyncTask extends AsyncTask<String, Void, User> {
        private LiveData<User> user;
        private UserDao userDao;
        private UserRepository delegate = null;

        private GetUserByUserNameAsyncTask(UserDao userDao) { this.userDao = userDao; }

        @Override
        protected User doInBackground(String... strings) {
             user = userDao.getUserByUserName(strings[0]); ======> I GET RESULT HERE 
             return null;
        }
        
        @Override
        protected void onPostExecute(User user) {
            super.onPostExecute(user);
            Log.i("TAG", "onPostExecute: "+user.getLastName());
            delegate.asyncFinished(user);
        }
    }
}

除了匿名之外，这个委托在同一个类中，为什么它是一个空对象？我怎样才能做到这一点？

【问题讨论】：

因为你没有初始化委托对象，private UserRepository delegate = null;

标签： android android-asynctask

【解决方案1】：

当您创建一个新的GetUserByUserNameAsyncTask 时，您永远不会将delegate 分配给外部类实例。因为当你调用asyncFinished() 时它是空的，它会抛出一个 NPE。

private static class GetUserByUserNameAsyncTask extends AsyncTask<String, Void, User> {
    private LiveData<User> user;
    private UserDao userDao;
    private UserRepository delegate;

    private GetUserByUserNameAsyncTask(UserDao userDao, UserRepository delegate) { 
        this.userDao = userDao; 
        this.delegate = delegate;
    }

    @Override
    protected User doInBackground(String... strings) {
         user = userDao.getUserByUserName(strings[0]); ======> I GET RESULT HERE 
         return null;
    }

    @Override
    protected void onPostExecute(User user) {
        super.onPostExecute(user);
        Log.i("TAG", "onPostExecute: "+user.getLastName());
        delegate.asyncFinished(user);
    }
}

您现在可以使用您的 UserRepository 实例 (this) 创建它：

public LiveData<User> getUserByUserName(String userName) {
    new GetUserByUserNameAsyncTask(userDao, this).execute(userName);
}

【讨论】：

【解决方案2】：

你需要更新这样的代码才能让它工作

private GetUserByUserNameAsyncTask(UserDao userDao , UserRepository delegate) {
 this.userDao = userDao; 
this.delegate = delegate ;
}

【讨论】：

【解决方案3】：

您已经创建了具有null 值的delegate 对象：

private UserRepository delegate = null;

在你为它赋值之后永远不会。

【讨论】：