Android app crashing when trying to setText() from an arraylist

Question

I am a bit new to Android Studio and Java. I am trying to update a text by replacing it from an element of an ArrayList. However, when using setText(userList.get(0)), the app crashes. My guess is that the arraylist is empty or cannot be accessed. But I use the same code on a button with a setOnClickListener, the text is updated by the string found in the arrayList.

I want to make the app automatically update the Text onloading the activity.

Here is my MainActivity.java -- I commented the setText that is crashing the app.

package com.example.sampletest;

import androidx.appcompat.app.AppCompatActivity;

import android.os.AsyncTask;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.Statement;
import java.util.ArrayList;

public class MainActivity extends AppCompatActivity {

    ArrayList<String> userList = new ArrayList<String>();

    TextView text;
    Button swapText;

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

        new getUsers().execute();

        text = (TextView) findViewById(R.id.text);
        swapText = (Button) findViewById(R.id.button);

        //text.setText(userList.get(0));

        swapText.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                text.setText(userList.get(0));
            }
        });


    }

    class getUsers extends AsyncTask<Void, Void, Void> {

        String error = "";

        @Override
        protected Void doInBackground(Void... voids) {
            try {
                Class.forName("com.mysql.jdbc.Driver");
                Connection connection = DriverManager.getConnection("jdbc:mysql://192.168.100.5:3306/cardetailingdb", "lau", "lau");
                Statement statement = connection.createStatement();
                ResultSet resultSet = statement.executeQuery("SELECT * FROM users");

                while (resultSet.next()) {
                    userList.add(resultSet.getString(1));
                }
            } catch (Exception e) {
                error = e.toString();
            }
            return null;
        }

    }
}

Answer 1

My guess is that the arraylist is empty or cannot be accessed.

The list is empty. You can see the stacktrace in the logcat for crash details.

But I use the same code on a button with a setOnClickListener, the text is updated by the string found in the arrayList.

That's because it's is not executed syncronously in onCreate() but later on clicking the button.

I want to make the app automatically update the Text onloading the activity.

You can add an onPostExecute() to your async task that gets run on the main thread once the background call completes. You should also check that the async call actually succeeded and not blindly trust that userList has elements in it.

Answer 2

In type of async task you should ensured first your all data is populated. Soo make sure background is completed and List is not empty. Hope will be work fine. For crashing issues you can add a check point like:

if(userList.size()>0){ text.setText(userList.get(0)); }