Android：3个活动使用相同的代码，但1次始终崩溃

Question

I am building a math app that has timed multiple choice questions for addition, subtraction, multiplication, and division. The code is pretty simple and is the same for each activity. However, My subtraction activity is very buggy and always crashes at random times. Sometimes it will crash after pressing GO, sometimes it crashes after choosing the first answer, sometimes I can get to 3 questions then it crashes. I'm confused because it does not happen with addition or multiplication. I'm wondering if it has something to do with memory?

Anyways, here is my code and error. Thanks in advance!

import android.os.CountDownTimer;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.GridLayout;
import android.widget.RelativeLayout;
import android.widget.TextView;


import java.util.*;

public class subActivity extends AppCompatActivity {

    Button button0;
    Button button1;
    Button button2;
    Button button3;
    TextView sub;
    Button startButton;
    TextView resultTextView;
    TextView pointsTextView;
    ArrayList<Integer> answers = new ArrayList<Integer>();
    int locationOfCorrectAnswer;
    int score = 0;
    int numberOfQuestions = 0;
    TextView timerTextView;
    Button playAgainButton;
    RelativeLayout gameRelativeLayout;
    GridLayout buttonLayout;
    float percent;

    public void playAgain(View view){
        score = 0;
        numberOfQuestions = 0;
        timerTextView.setText("30s");
        pointsTextView.setText("0/0");
        resultTextView.setText("");
        playAgainButton.setVisibility(View.INVISIBLE);
        generateQuestion();

        new CountDownTimer(30100, 1000) {

            @Override
            public void onTick(long millisUntilFinished) {
                buttonLayout.setVisibility(View.VISIBLE);
                sub.setVisibility(View.VISIBLE);
                timerTextView.setText(String.valueOf(millisUntilFinished/1000) + "s");
            }

            @Override
            public void onFinish() {
                playAgainButton.setVisibility(View.VISIBLE);
                timerTextView.setText("0s");
                resultTextView.setText("Your score:" + percent + "%\n Questions:" + Integer.toString(score) + "/" + numberOfQuestions);
                buttonLayout.setVisibility(View.INVISIBLE);
                sub.setVisibility(View.INVISIBLE);

            }
        }.start();
    }

    public void generateQuestion() {
        Random rand = new Random();

        int a = rand.nextInt(21);
        int b = rand.nextInt(21);
        int incorrectAnswer;

        sub.setText(a + " - " + b);
        locationOfCorrectAnswer = rand.nextInt(4);

        answers.clear();

        for (int i=0; i<4;i++){

            if(i == locationOfCorrectAnswer){
                answers.add(a-b);
            }
            else{

                incorrectAnswer = rand.nextInt((a-b));
                while (incorrectAnswer == a-b) {
                    incorrectAnswer = rand.nextInt((a-b));
                }
                answers.add(incorrectAnswer);
            }
        }

        button0.setText(Integer.toString(answers.get(0)));
        button1.setText(Integer.toString(answers.get(1)));
        button2.setText(Integer.toString(answers.get(2)));
        button3.setText(Integer.toString(answers.get(3)));

    }



    public void start(View view) {

        startButton.setVisibility(View.INVISIBLE);
        gameRelativeLayout.setVisibility(RelativeLayout.VISIBLE);
        playAgain(findViewById(R.id.playAgain));
    }

    public void chooseAnswer(View view) {

        if (view.getTag().toString().equals(Integer.toString(locationOfCorrectAnswer))) {
            score++;
            resultTextView.bringToFront();
            resultTextView.setText("Correct!");
            //resultTextView.setBackgroundColor(-16711936);

        } else {
            resultTextView.bringToFront();
            resultTextView.setText("Incorrect!");
            //resultTextView.setBackgroundColor(-65536);
        }
        numberOfQuestions++;
        pointsTextView.setText(Integer.toString(score) + "/" + numberOfQuestions);
        percent = Math.round((score/(float)numberOfQuestions)*100.0);             //get the percentage recieved
        generateQuestion();
    }

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

        startButton = (Button) findViewById(R.id.goButton);
        sub = (TextView) findViewById(R.id.subTextView);
        resultTextView = (TextView) findViewById(R.id.resultTextView);
        pointsTextView = (TextView) findViewById(R.id.pointsTextView);
        button0 = (Button) findViewById(R.id.button0);
        button1 = (Button) findViewById(R.id.button1);
        button2 = (Button) findViewById(R.id.button2);
        button3 = (Button) findViewById(R.id.button3);
        timerTextView = (TextView) findViewById(R.id.timerTextView);
        playAgainButton = (Button) findViewById(R.id.playAgain);
        gameRelativeLayout = (RelativeLayout) findViewById(R.id.gameRelativeLayout);
        buttonLayout = (GridLayout) findViewById(R.id.buttonLayout);


    }
}

This error shows up when it crashes:

 E/AndroidRuntime: FATAL EXCEPTION: main
                                                                            Process: com.example.jimmy.mathtime, PID: 14678
                                                                            java.lang.IllegalStateException: Could not execute method for android:onClick
                                                                                at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:293)
                                                                                at android.view.View.performClick(View.java:5224)
                                                                                at android.view.View$PerformClick.run(View.java:21356)
                                                                                at android.os.Handler.handleCallback(Handler.java:739)
                                                                                at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                                at android.os.Looper.loop(Looper.java:148)
                                                                                at android.app.ActivityThread.main(ActivityThread.java:5585)
                                                                                at java.lang.reflect.Method.invoke(Native Method)
                                                                                at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:730)
                                                                                at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:620)
                                                                             Caused by: java.lang.reflect.InvocationTargetException
                                                                                at java.lang.reflect.Method.invoke(Native Method)
                                                                                at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288)
                                                                                at android.view.View.performClick(View.java:5224) 
                                                                                at android.view.View$PerformClick.run(View.java:21356) 
                                                                                at android.os.Handler.handleCallback(Handler.java:739) 
                                                                                at android.os.Handler.dispatchMessage(Handler.java:95) 
                                                                                at android.os.Looper.loop(Looper.java:148) 
                                                                                at android.app.ActivityThread.main(ActivityThread.java:5585) 
                                                                                at java.lang.reflect.Method.invoke(Native Method) 
                                                                                at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:730) 
                                                                                at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:620) 
                                                                             Caused by: java.lang.IllegalArgumentException: n <= 0: -1
                                                                                at java.util.Random.nextInt(Random.java:182)
                                                                                at com.example.jimmy.mathtime.subActivity.generateQuestion(subActivity.java:84)
                                                                                at com.example.jimmy.mathtime.subActivity.chooseAnswer(subActivity.java:124)
                                                                                at java.lang.reflect.Method.invoke(Native Method) 
                                                                                at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288) 
                                                                                at android.view.View.performClick(View.java:5224) 
                                                                                at android.view.View$PerformClick.run(View.java:21356) 
                                                                                at android.os.Handler.handleCallback(Handler.java:739) 
                                                                                at android.os.Handler.dispatchMessage(Handler.java:95) 
                                                                                at android.os.Looper.loop(Looper.java:148) 
                                                                                at android.app.ActivityThread.main(ActivityThread.java:5585) 
                                                                                at java.lang.reflect.Method.invoke(Native Method) 
                                                                                at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:730) 
                                                                                at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:620) 

My XML file:

<?xml version="1.0" encoding="utf-8"?>
<android.widget.RelativeLayout 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="com.example.jimmy.mathtime.subActivity">

    <RelativeLayout
        android:id="@+id/gameRelativeLayout"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:visibility="invisible">

        <GridLayout
            android:id="@+id/buttonLayout"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:layout_alignParentStart="true"
            android:layout_below="@+id/subTextView">

            <Button
                android:id="@+id/button0"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_column="0"
                android:layout_columnWeight="1"
                android:layout_row="0"
                android:layout_rowWeight="1"
                android:background="@android:color/holo_blue_bright"
                android:onClick="chooseAnswer"
                android:tag="0"
                android:text="@string/_3"
                android:textSize="80sp" />

            <Button
                android:id="@+id/button1"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_column="1"
                android:layout_columnWeight="1"
                android:layout_row="0"
                android:layout_rowWeight="1"
                android:background="@color/colorAccent"
                android:onClick="chooseAnswer"
                android:tag="1"
                android:text="@string/_3"
                android:textSize="80sp" />

            <Button
                android:id="@+id/button2"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_column="0"
                android:layout_columnWeight="1"
                android:layout_row="1"
                android:layout_rowWeight="1"
                android:background="@android:color/holo_green_light"
                android:onClick="chooseAnswer"
                android:tag="2"
                android:text="@string/_3"
                android:textSize="80sp" />

            <Button
                android:id="@+id/button3"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_column="1"
                android:layout_columnWeight="1"
                android:layout_row="1"
                android:layout_rowWeight="1"
                android:background="@android:color/holo_purple"
                android:onClick="chooseAnswer"
                android:tag="3"
                android:text="@string/_3"
                android:textSize="80sp" />
        </GridLayout>

        <TextView
            android:id="@+id/subTextView"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignTop="@+id/timerTextView"
            android:layout_centerHorizontal="true"
            android:layout_marginTop="36dp"
            android:padding="10dp"
            android:text="@string/_3_x_3"
            android:textColor="@android:color/black"
            android:textSize="50sp"
            android:textStyle="bold" />

        <TextView
            android:id="@+id/timerTextView"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentStart="true"
            android:layout_alignParentTop="true"
            android:layout_marginStart="15dp"
            android:layout_marginTop="14dp"
            android:padding="10dp"
            android:text="@string/_30s"
            android:textSize="30sp"
            android:textStyle="bold" />

        <Button
            android:id="@+id/playAgain"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignBottom="@+id/buttonLayout"
            android:layout_centerHorizontal="true"
            android:layout_marginBottom="65dp"
            android:background="@color/colorPrimary"
            android:onClick="playAgain"
            android:text="Play_again"
            android:textSize="30sp"
            android:visibility="invisible" />

        <TextView
            android:id="@+id/pointsTextView"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentEnd="true"
            android:layout_alignTop="@+id/timerTextView"
            android:layout_marginEnd="16dp"
            android:padding="10dp"
            android:text="@string/_0_0"
            android:textSize="30sp"
            android:textStyle="bold"
            tools:ignore="RelativeOverlap" />

        <TextView
            android:id="@+id/resultTextView"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentBottom="true"
            android:layout_centerHorizontal="true"
            android:padding="20dp"
            android:text="@string/correct"
            android:textSize="30sp"
            android:textStyle="bold" />
    </RelativeLayout>

    <Button
        android:id="@+id/goButton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_centerVertical="true"
        android:backgroundTint="@android:color/holo_blue_bright"
        android:onClick="start"
        android:padding="40dp"
        android:text="@string/go"
        android:textSize="80sp"
        android:visibility="visible" />

</android.widget.RelativeLayout>

Answer 1

Your exception states:

 Caused by: java.lang.IllegalArgumentException: n <= 0: -1
                                                                            at java.util.Random.nextInt(Random.java:182)
                                                                            at com.example.jimmy.mathtime.subActivity.generateQuestion(subActivity.java:84)

Basically you pass a negative value to rand.nextInt(max) method. In your code you do the following operation:

incorrectAnswer = rand.nextInt((a-b));

If b is greater than a, you pass a negative number to nextInt function. This causes your problem. It is not allowed to do this. Check if (ab)>0!