java.util.HashMap cannot be cast to java.util.ArrayList

Question

I'm new to android and recently I was trying to extract data from firebase into recyclerview/cardview to display data in vertical layout, it is showing the error of converting Hashmap to Arraylist, where code is:

    protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    recyclerView = (RecyclerView) findViewById(R.id.recycler_view);
    recyclerView.setHasFixedSize(true);
    recyclerView.setLayoutManager(new LinearLayoutManager(this));
    new GetDataFromFirebase().executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);

// Read from the database
    FirebaseDatabase database = FirebaseDatabase.getInstance();
    DatabaseReference myRef = database.getReference("0XtzzDme");

    myRef.addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {

            ArrayList<String> values = (ArrayList<String>) dataSnapshot.getValue();
            recyclerView.setAdapter(new RecyclerViewAdapter(values));
        }

        @Override
        public void onCancelled(DatabaseError error) {
            // Failed to read value
            System.out.println("Failed to read value." + error.toException());
        }
    });
}

and the recyclerviewadapter

    class RecyclerViewAdapter extends RecyclerView.Adapter<RecyclerViewAdapter.ViewHolder> {
    private ArrayList<String> values;
    RecyclerViewAdapter(ArrayList<String> values) {
       this.values = values;
  }

  @Override
   public ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
      return new ViewHolder(LayoutInflater.from(parent.getContext()).inflate( R.layout.recycler_view_item,parent,false));
}

@Override
public void onBindViewHolder(ViewHolder holder, int position) {
    holder.name.setText(values.get(position));
}

@Override
public int getItemCount() {
    return values.size();
}

class ViewHolder extends RecyclerView.ViewHolder {
    private TextView name;
    ViewHolder(View itemView) {
        super(itemView);
        name = (TextView) itemView.findViewById(R.id.list_item_text);
    }
  }
}

Answer 1

As stated by the Firebase API, DataSnapshot.getValue() can return any of:

• Boolean
• String
• Long
• Double
• Map<String, Object>
• List<Object>

In case of your error it obviously returns a Map instead of the expected List.

To solve this problem you should rewind why you expect a List at that point and why a Map could possibly be returned. Maybe the fact that a Map is returned is an error already.

However, no matter to what conclusion you come, you will most probably have to write something like:

Object value = dataSnapshot.getValue();
if(value instanceof List) {
    List<Object> values = (List<Object>) value;
    // do your magic with values
}
else {
    // handle other possible types
}

Answer 2

Please use below Code to convert HashMap to ArrayList ,You can change String to File in your case.

//Creating a HashMap object

    HashMap<String, String> map = new HashMap<String, String>();

    //Getting Collection of values from HashMap

    Collection<String> values = map.values();

    //Creating an ArrayList of values

    ArrayList<String> listOfValues = new ArrayList<String>(values);

Answer 3

Convert you Hashmap to Array list like that..

List<String> list = new ArrayList<String>(theHashMap.values()); //Most likely dataSnapshot.getValues()

Answer 4

HashMap and ArrayList are two completely different data types.

HashMap is an Implementation of the Map interface which stores key:value pairs

ArrayList is an Implementation of the List interface which stores a list of items

There are tons of Threads on SO how to convert a Map to a list (e.g. here).

Answer 5

it is showing the error of converting Hashmap to Arraylist

There is no converting in your code, and that is exactly the problem. You're trying to cast an HashMap as an ArrayList, which it isn't. Possible reading Possible reading 2

Converting one would be done by:

ArrayList<String> values = new ArrayList<>((HashMap<String, Object>) dataSnapshot.getValue()).values();

Answer 6

The most likely answer is that the items stored in the database are objects with a public property, not basic strings or other base data types.

It's hard to know unless you can show what is getting put into the database.

For example, if you have a an object

class Item {
  private String text;
  public String getText();
  public void setText(String);
}

then you'll likely get out a map with a single key "text" and the corresponding value from getText()

If you have an item stored which has an array of strings

class Example {
  private list<String> items;
  public String getItem(int);
  public int getItemCount()
}

then your snapshot might contain a map with a key "item" and the value of that might be the list of strings that you're expecting.

have a Google of Java bean conventions it might help.

Answer 7

val arr = ArrayList(hashmap.values)