尝试在空对象引用上调用虚拟方法“int com.krish.recreatemybank.Models.User.get_id（）”（异步任务）

问：

我正在创建一个银行应用程序，并使用 AsyncTask 来获取用户进行的所有交易。

在这里，我只是在执行异步任务，但由于某种原因出现空指针异常：

Attempt to invoke virtual method 'int com.krish.recreatemybank.Models.User.get_id()' on a null object reference

这是我的utils代码：

Utils utils = new Utils(this);
        User user = utils.isUserLoggedIn();
        Log.d("user_id", user.get_id()+"");
        getTransactions = new GetTransactions();
        getTransactions.execute(user.get_id());

空指针异常发生在行 上。.execute

这是我的两个 Util 方法：

public void addUserToSharedPreferences(User user){
        SharedPreferences sharedPreferences = context.getSharedPreferences("logged_in_user", Context.MODE_PRIVATE);

        SharedPreferences.Editor editor = sharedPreferences.edit();

        Gson gson = new Gson();

        editor.putString("user", gson.toJson(user));

        editor.apply();
    }

    public User isUserLoggedIn(){

        SharedPreferences sharedPreferences = context.getSharedPreferences("logged_in_user", Context.MODE_PRIVATE);

        Gson gson = new Gson();
        Type type = new TypeToken<User>(){}.getType();

        User user = gson.fromJson(sharedPreferences.getString("user", null), type);
        return user;
    }

...然后，我在用户注册后将用户添加到共享首选项中：

utils.addUserToSharedPreferences(user);

此外，我尝试多次记录用户，并且用户不是 null。

这里可能出了什么问题？

全栈跟踪：

2021-05-25 19:14:32.106 5575-5604/com.krish.recreatemybank E/AndroidRuntime: FATAL EXCEPTION: AsyncTask #2
    Process: com.krish.recreatemybank, PID: 5575
    java.lang.RuntimeException: An error occurred while executing doInBackground()
        at android.os.AsyncTask$4.done(AsyncTask.java:415)
        at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:383)
        at java.util.concurrent.FutureTask.setException(FutureTask.java:252)
        at java.util.concurrent.FutureTask.run(FutureTask.java:271)
        at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:305)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
        at java.lang.Thread.run(Thread.java:923)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'int com.krish.recreatemybank.Models.User.get_id()' on a null object reference
        at com.krish.recreatemybank.HelperClasses.DatabaseHelper.addSampleTransaction(DatabaseHelper.java:67)
        at com.krish.recreatemybank.HelperClasses.DatabaseHelper.onCreate(DatabaseHelper.java:60)
        at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:411)
        at android.database.sqlite.SQLiteOpenHelper.getReadableDatabase(SQLiteOpenHelper.java:340)
        at com.krish.recreatemybank.Auth.RegisterActivity$DoesUserExist.doInBackground(RegisterActivity.java:73)
        at com.krish.recreatemybank.Auth.RegisterActivity$DoesUserExist.doInBackground(RegisterActivity.java:68)
        at android.os.AsyncTask$3.call(AsyncTask.java:394)
        at java.util.concurrent.FutureTask.run(FutureTask.java:266)
        at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:305) 
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167) 
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641) 
        at java.lang.Thread.run(Thread.java:923) 

谢谢！

异步任务的代码：

公共类 GetTransactions 扩展了 AsyncTask<Integer， Void， ArrayList>{

    @Override
    protected ArrayList<Transaction> doInBackground(Integer... integers) {
        try{
            SQLiteDatabase db = databaseHelper.getReadableDatabase();

            Cursor cursor = db.query("transactions", null, "user_id=?", new String[]{String.valueOf(integers[0])}, null, null, null);

            if(cursor != null){
                if(cursor.moveToFirst()){

                    ArrayList<Transaction> transactions = new ArrayList<>();

                    for(int i = 0; i<cursor.getCount(); i++){
                        Transaction transaction = new Transaction();
                        transaction.setDate(cursor.getString(cursor.getColumnIndex("date")));
                        transaction.setDescription(cursor.getString(cursor.getColumnIndex("description")));
                        transaction.setPrice(cursor.getDouble(cursor.getColumnIndex("amount")));
                        transaction.setTitle(cursor.getString(cursor.getColumnIndex("title")));
                        transaction.setTransactionId(cursor.getInt(cursor.getColumnIndex("_id")));

                        transactions.add(transaction);
                        cursor.moveToNext();

                        Log.d("transactions", String.valueOf(transactions));
                    }

                    cursor.close();
                    db.close();
                    return transactions;


                }else{
                    cursor.close();
                    db.close();
                    return null;
                }
            }else{
                db.close();
                return null;
            }

        }catch (SQLException e){
            e.printStackTrace();

            return null;
        }

    }

    @Override
    protected void onPostExecute(ArrayList<Transaction> transactions) {
        super.onPostExecute(transactions);

        if(transactions != null){
            initAdapter(transactions);

            Log.d("transactions", "Sucess!");
        }else{
            Log.d("transactions", "Failure!");
        }
    }
}