How to Retrieve a List object from the firebase in

问题:

I am having trouble retrieving a List from the Firebase. I have no trouble storing it, but as soon as I try to cast dataSnapshot.getValue() to ArrayList my app crashes, giving an exception:

HashMap cannot be casted to ArrayList

But when I tried to cast it to a HashMap, it also crashes, giving exception:

ArrayList can't be casted to hashmap

Need help please! Here is the code that is creating the problem:

Fire.addValueEventListener(new ValueEventListener() {
    @Override
    public void onDataChange(DataSnapshot dataSnapshot) {
        List<TaskDes> td = (ArrayList<TaskDes>) dataSnapshot.getValue()
        notifyDataSetChanged();
    }

    @Override
    public void onCancelled(FirebaseError firebaseError) {

    }
});

I want to retrieve all the data in the Firebase as one List. The class TaskDes contains three fields:

class TaskDes { // definition
    boolean done
    String taskDescription
    String taskTitle
}

回答1:

You need to create a GenericTypeIndicator object to pass as DataSnapshot.getValue() parameter.

Code:

GenericTypeIndicator<List<String>> t = new GenericTypeIndicator<List<String>>() {};

List<String> yourStringArray = dataSnapshot.getValue(t);

回答2:

Your Model

public class TaskDes {
    private boolean done;
    private String taskDescription;
    private String taskTitle;

    public TaskDes() {
    }

    public boolean isDone() {
        return done;
    }

    public void setDone(boolean done) {
        this.done = done;
    }

    public String getTaskDescription() {
        return taskDescription;
    }

    public void setTaskDescription(String taskDescription) {
        this.taskDescription = taskDescription;
    }

    public String getTaskTitle() {
        return taskTitle;
    }

    public void setTaskTitle(String taskTitle) {
        this.taskTitle = taskTitle;
    }

}

You need to create a GenericTypeIndicator object to pass as DataSnapshot.getValue() parameter.

In Activity

 private static final String TAG=MainActivity.class.getSimpleName();
 private FirebaseDatabase database;
 private DatabaseReference myRef=null;

 @Override
 protected void onCreate(Bundle savedInstanceState){
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    database=FirebaseDatabase.getInstance();
    myRef=database.getReference("ADD_YOUR_REFERECE");


    myRef.addValueEventListener(new ValueEventListener(){
       @Override
       public void onDataChange(DataSnapshot dataSnapshot){
          /* This method is called once with the initial value and again whenever data at this location is updated.*/
          long value=dataSnapshot.getChildrenCount();
          Log.d(TAG,"no of children: "+value);

          GenericTypeIndicator<List<TaskDes>> genericTypeIndicator =new GenericTypeIndicator<List<TaskDes>>(){};

          List<TaskDes> taskDesList=dataSnapshot.getValue(genericTypeIndicator);

          for(int i=0;i<taskDesList.size();i++){
             Toast.makeText(MainActivity.this,"TaskTitle = "+taskDesList.get(i).getTaskTitle(),Toast.LENGTH_LONG).show();
          }
       }

       @Override
       public void onCancelled(DatabaseError error){
          // Failed to read value
          Log.w(TAG,"Failed to read value.",error.toException());
       }
    });
 }

回答3:

Make another item that contains a list for your item: This is your item:

class TaskDes { // definition
    boolean done
    String taskDescription
    String taskTitle
}

This is the list item

        class TaskDesList { // definition
              private  ArreyList<TaskDes> yourlist
            }

      public TaskDesList(){
         }

        public ArrayList<TaskDes> getYourlist() {
            return yourlist;
        }

and when calling an EventListener

ref.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                yourlist.clear();
             taskDesList=dataSnapshot.getValue(TaskDesList.class);
                if (taskDesList!=null) {
                    yourlist= taskDesList.getYourlist();
                }
}

and now "yourlist" is a list that contains all of your "TaskDes" items

回答4:

A bit late, but in case any one else needs this.

IF the list is inside another object.

The object

public class Question {

    public Date date;
    public String questionNumber;
    public String questionText;
    public QuestionType questionType;
    public String multipleSelection1;
    public String multipleSelection2;
    public String multipleSelection3;

    public Question() {
        // Default constructor required for calls to DataSnapshot.getValue(User.class)
    }
}

Then to get your array of question objects

GenericTypeIndicator<List<Question>> t = new GenericTypeIndicator<List<Question>>() {};
List<Question> questionList = dataSnapshot.getValue(t);