Why the arraylist empties itself when it is called optionPressed method?

Question

Making a simple BrainTrain app, the updateAnswer method gives out random answers to the question but the issue comes up whenever i call the optionsPressed method and try getting any of the object, the arrayList gives out IndexOutOfBoundsException .

package com.example.nishantsaini.braintrain;

import android.graphics.Color;
import android.graphics.drawable.ColorDrawable;
import android.graphics.drawable.Drawable;
import android.os.CountDownTimer;
import android.support.v4.content.ContextCompat;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.LinearLayout;
import android.widget.TextView;

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

import static android.R.color.black;
import static android.R.color.holo_blue_bright;
import static android.R.color.holo_blue_dark;
import static android.R.color.holo_blue_light;
import static android.R.color.holo_green_light;



public class MainActivity extends AppCompatActivity {

Random rnd = new Random();
boolean gameisActive = false;
int count = 0;
CountDownTimer cd;
int var1,var2;
ArrayList<Button> options;




@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    cd = new CountDownTimer(30000, 1000) {
        @Override
        public void onTick(long l) {
            Log.i("Time Left:", Long.toString(l / 1000));
            TextView timer = (TextView) findViewById(R.id.timer);
            timer.setText("0:" + String.format("%02d", (l / 1000)));

        }

        @Override
        public void onFinish() {
            TextView score = (TextView) findViewById(R.id.Score);
            LinearLayout onFinishLayout = (LinearLayout) findViewById(R.id.onFinishLayout);
            /*score.setText( "You got " + getScore() + "right!");
            */

        }
    };
    options = new ArrayList<>();
    options.add((Button) findViewById(R.id.option2));
    options.add((Button) findViewById(R.id.option1));
    options.add((Button) findViewById(R.id.option3));
    options.add((Button) findViewById(R.id.option4));
}

public void start(View view) {



    cd.start();

    Button start = (Button) findViewById(R.id.start);
    updateQuestion();
    updateAnswers(options);

    start.setText("Replay");

}

public void optionPressed(View view){

    ColorDrawable blue_d = new ColorDrawable(getResources().getColor(R.color.blue_b));
    ColorDrawable blue_l = new ColorDrawable(getResources().getColor(R.color.blue_l));
    ColorDrawable blue_b = new ColorDrawable(getResources().getColor(R.color.blue_b));
    ColorDrawable green = new ColorDrawable(getResources().getColor(R.color.green));

    Drawable color = blue_b;
    while(options.size() > 0) {
        int index = rnd.nextInt(options.size());
        Button b = options.get(index);

        if (color == blue_b){
            color = blue_d;
            b.setBackground(color);
        }
        else if (color == blue_d){
            color = green;
            b.setBackground(color);

        }
        else if (color == green){
            color = blue_l;
            b.setBackground(color);

        }
        else if (color == blue_l)
        {
            color = blue_b;
            b.setBackground(color);
        }
        options.remove(index);
    }

    updateQuestion();
    updateScore();
    updateAnswers(options);

}

public void updateQuestion(){
    var1 = 5 + (int)(Math.random()*20);
    var2 = 5 + (int)(Math.random()*20);
    TextView question = (TextView) findViewById(R.id.question);
    question.setText(Integer.toString(var1) +" + " + Integer.toString(var2) + " = ");
    question.setPadding(0,50,0,0);
}

public void updateScore(){

    count++;
    int correct = 0;
    TextView score = (TextView) findViewById(R.id.points);
    score.setText(Integer.toString(correct) + "/" + Integer.toString(count));
}

public void updateAnswers(ArrayList<Button> arrayList ){

    Button b;
    int answer = var1 + var2;
    int indexAtWhichRealAnswerGoes = 1+ (int) (Math.random()*3);
    int id ;
    Log.i("arraylist size",Integer.toString(options.size()));

    b = arrayList.get(indexAtWhichRealAnswerGoes);
    b.setText(Integer.toString(answer));
    id = b.getId();
    arrayList.remove(indexAtWhichRealAnswerGoes);
    for (int i = 0; i < arrayList.size(); i++) {
        int randomanswer = (answer-7) + (int)(Math.random()*(answer+7));
        b = arrayList.get(i);
        b.setText(Integer.toString(randomanswer));

    }
    arrayList.add((Button) findViewById(id));
    Log.i("arraylist size",Integer.toString(arrayList.size()));

}
}

Answer 1

So, you call updateAnswers(options); using the list that you previously emptied

while(options.size() > 0) {
    int index = rnd.nextInt(options.size());
    ...
    options.remove(index);
}
...
updateAnswers(options);

At that point, the list is empty.

In the method, you safely used for (int i = 0; i < arrayList.size(); i++) { to prevent any error but before that we see

int indexAtWhichRealAnswerGoes = 1+ (int) (Math.random()*3);
...
b = arrayList.get(indexAtWhichRealAnswerGoes);

You get that random index without checking the size of the list. ( List that is empty at that point).

---

If you want to keep the list with its value, but the logic can't be change so the remove is necessary, you need to do a copy of the list to work on that one, and pass the original to the method updateAnswers .

List<Button> copyOptions = new ArrayList<>(options);

It will share the same instance, but you can update the list copyOptions without having any impact on options