Android Studio Fails to Build Without Cast, But App Crashes With Cast

Question

I'm currently building an app for Android which retrieves data from Firebase. It displays children from other children . Either it fails to build when I remove the cast or the app crashes with the cast. Here is the error:

java.lang.RuntimeException: Unable to start activity ComponentInfo{app.name/app.name.retrieve}: java.lang.ClassCastException: android.widget.ListView cannot be cast to java.util.List

This is my code:

package app.name;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.widget.ArrayAdapter;
import android.widget.ListView;

import com.google.firebase.database.ChildEventListener;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;

import java.util.ArrayList;
import java.util.List;

public class datab extends AppCompatActivity {
        private ArrayList<String> child = new ArrayList<>();

        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
        }
        @Override
        protected void onStart() {
            super.onStart();
            setContentView(R.layout.activity_datab);
            DatabaseReference database = FirebaseDatabase.getInstance().getReference().child("child");
            final ListView view = findViewById(R.id.l);


            final ArrayAdapter adapter = new ArrayAdapter(this, android.R.layout.list_content, (List) view);
//Here is the cast
            view.setAdapter(child);

            database.addChildEventListener(new ChildEventListener() {
                @Override
                public void onChildAdded(DataSnapshot dataSnapshot, String s) {
                    String value = dataSnapshot.getValue(String.class);
                    adapter.add(value);
                    adapter.notifyDataSetChanged();
                }

                @Override
                public void onChildChanged(DataSnapshot dataSnapshot, String s) {

                }

                @Override
                public void onChildRemoved(DataSnapshot dataSnapshot) {

                }

                @Override
                public void onChildMoved(DataSnapshot dataSnapshot, String s) {

                }

                @Override
                public void onCancelled(DatabaseError databaseError) {

                }
            });
        }
    }

How can I remove the cast without the build failing?

Answer 1

Change this:

final ArrayAdapter adapter = new ArrayAdapter(this, android.R.layout.list_content, (List) view);

to this:

final ArrayAdapter adapter = new ArrayAdapter(this, android.R.layout.simple_list_item_1, child);
view.setAdapter(adapter);

Change this inside your onChildAdded(..){ :

  adapter.add(value);

to this:

  child.add(value);

add() is a method in the ArrayList.

Check this for more info: https://docs.oracle.com/javase/8/docs/api/java/util/ArrayList.html

Answer 2

you can't cast ListView to List , ArrayAdapter constructor takes Context instance and resourceId and List or array

you should pass the list to your adapter , and set the adapter to the listview

final ArrayAdapter adapter = new ArrayAdapter(this, android.R.layout.simple_list_item_1 , child);
view.setAdapter(adapter)