Question

App is crashing at startup? Any idea? Thanks!
import androidx.appcompat.app.AppCompatActivity;

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

import java.util.ArrayList;
import java.util.Random;

public class MainActivity extends AppCompatActivity {

    private EditText teamOneText;
    private EditText teamTwoText;
    private Button selectButton;
    private TextView resultView;

    Random r = new Random();

    ArrayList<EditText> editTextList = new ArrayList<>();
    ArrayList<String> editTextValues = new ArrayList<>();

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

        editTextList.add(teamOneText);
        editTextList.add(teamTwoText);

        for(int i=0;i<editTextList.size();i++){
            editTextValues.add(editTextList.get(i).getText().toString());
        }

        teamOneText = findViewById(R.id.team_one);
        teamTwoText = findViewById(R.id.team_two);
        resultView = findViewById(R.id.result_view);
        selectButton = findViewById(R.id.select_button);

        selectButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                resultView.setText(editTextValues.get(r.nextInt(editTextValues.size())));
            }
        });
    }
}

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/result_view"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Result shown here"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.70" />

    <Button
        android:id="@+id/select_button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Select Random Team"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.5"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.40" />

    <EditText
        android:id="@+id/team_one"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:ems="10"
        android:inputType="textPersonName"
        android:text="Team's name"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.70"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.15" />

    <EditText
        android:id="@+id/team_two"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:ems="10"
        android:inputType="textPersonName"
        android:text="Team's name"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.70"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.25" />

    <TextView
        android:id="@+id/team1_label"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Team 1"
        android:textSize="18sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.15"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.16" />

    <TextView
        android:id="@+id/team2_label"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Team 2"
        android:textSize="18sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.15"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.26" />

</androidx.constraintlayout.widget.ConstraintLayout>

strings.xml

<resources>
    <string name="app_name">TeamSelector</string>
</resources>

Answer 1

The app is crashing at startup because there is a problem in MainActivity,java file's OnCreate() function

You are adding the EditText teamOneText and EditText teamTwoText before initialising it.

i.e.

you have written:

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

        editTextList.add(teamOneText);  //adding teamOneText without initialising
        editTextList.add(teamTwoText);  //adding teamOneText without initialising

        for(int i=0;i<editTextList.size();i++){
            editTextValues.add(editTextList.get(i).getText().toString());
        }

        teamOneText = findViewById(R.id.team_one); //initailising after using it
        teamTwoText = findViewById(R.id.team_two); //initailising after using it
        resultView = findViewById(R.id.result_view);
        selectButton = findViewById(R.id.select_button);

        selectButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                resultView.setText(editTextValues.get(r.nextInt(editTextValues.size())));
            }
        });
    }

In this onCreate function you have to initialise first then use it.

So the correct code for the MainActivity,java will be as follows:

**********************************************************************************************************************************************

import androidx.appcompat.app.AppCompatActivity;

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

import java.util.ArrayList;
import java.util.Random;

public class MainActivity extends AppCompatActivity {

    private EditText teamOneText;
    private EditText teamTwoText;
    private Button selectButton;
    private TextView resultView;

    Random r = new Random();

    ArrayList<EditText> editTextList = new ArrayList<>();
    ArrayList<String> editTextValues = new ArrayList<>();

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


        teamOneText = findViewById(R.id.team_one); //INITIALISING THE EditTExt teamOneText
        teamTwoText = findViewById(R.id.team_two); //INITIALISING THE EditTExt teamTwoText
        
        editTextList.add(teamOneText);  //Then using EditText teamOneText  and adding to the list 
        editTextList.add(teamTwoText); //Then using EditText teamTwoText and adding to the list

        for(int i=0;i<editTextList.size();i++){
            editTextValues.add(editTextList.get(i).getText().toString());
        }

        
        resultView = findViewById(R.id.result_view);
        selectButton = findViewById(R.id.select_button);

        selectButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                resultView.setText(editTextValues.get(r.nextInt(editTextValues.size())));
            }
        });
    }
}

***********************************************************************************************************************************************

**Correct this thing the app will not crash

After running this corrected code the app will start like this