为什么HashMap方法返回null对象引用

Question

我曾尝试创建一个HashMap方法来获取SQLite数据库数据，但当我调用该方法时，它会返回null引用，这使我的Android应用程序停止工作。

这是我的代码：

HashMap方法：

public HashMap<String, String> getUserDetails(){
    HashMap<String,String> user = new HashMap<String,String>();
    String selectQuery = "SELECT  * FROM " + TABLE_LOGIN;
    SQLiteDatabase db = this.getReadableDatabase();
    Cursor cursor = db.rawQuery(selectQuery, null);
    cursor.moveToFirst();
    if(cursor.getCount() > 0){
        user.put("name", cursor.getString(1));
        user.put("position", cursor.getString(2));
        user.put("level", cursor.getString(3));
        user.put("email", cursor.getString(4));
        user.put("uid", cursor.getString(5));
        user.put("created_at", cursor.getString(6));
    }
    cursor.close();
    db.close();
    return user;
}

这就是我如何调用其他类中的方法

DatabaseHandler db = new DatabaseHandler(getApplicationContext());
HashMap<String,String> userDetail= db.getUserDetails();
int userLevel = Integer.parseInt(userDetail.get("level").toString());

无论如何，我使用了一个教程参考来做这件事，并与我自己的需求相结合。

Answer 1

如果数据库中存在用户，则应返回user对象，否则返回null并检查user是否为null

public HashMap<String, String> getUserDetails(){
    HashMap<String,String> user = null;
    String selectQuery = "SELECT  * FROM " + TABLE_LOGIN;
    try {
    SQLiteDatabase db = this.getReadableDatabase();
    Cursor cursor = db.rawQuery(selectQuery, null);
    if (cursor != null){
    cursor.moveToFirst();
    if(cursor.getCount() > 0){
        user = new HashMap<String,String>();
        user.put("name", cursor.getString(1));
        user.put("position", cursor.getString(2));
        user.put("level", cursor.getString(3));
        user.put("email", cursor.getString(4));
        user.put("uid", cursor.getString(5));
        user.put("created_at", cursor.getString(6));
    }
}
} finally {
    cursor.close();
    db.close();
}
    return user;
}

然后

DatabaseHandler db = new DatabaseHandler(getApplicationContext());
HashMap<String,String> userDetail= db.getUserDetails();
if (userDetail != null) {
  String level = userDetail.get("level");
  if (level != null){
    try {
      int userLevel = Integer.parseInt(level);
    } catch (Exception e){
     e.printStackTrace();
    }
  }
}