Fixing Array out of index Error

Question

Im working myself through the Android Programming 3rd edition book by The big nerd ranch. In Chapter 2 theres a challenge to add previous button to the app. I have added the previous button and it works fine as long as i dont try to previous the first question. I dont know exactly how to loop the button so whenever i previous the first question, it goes by default to the last question. The next button does loop through all the question, however the previous button does not. Im just learning the android environment so im sure this is a simple question. How can i fix my current error?

    mNextButton = (Button) findViewById(R.id.next_button);
    mNextButton.setOnClickListener(new View.OnClickListener(){
        @Override
        public void onClick(View v) {
            mCurrentIndex = (mCurrentIndex + 1) % mQuestionBank.length;
            updateQuestion();

            }
        });


    mPreviousButton = (Button) findViewById(R.id.previous_button);
    mPreviousButton.setOnClickListener(new View.OnClickListener(){
        @Override
        public void onClick(View v) {
            mCurrentIndex = (mCurrentIndex - 1) % mQuestionBank.length;
            updatePrevQuestion();

        }
    });

    updateQuestion();
    updatePrevQuestion();

}

private void updatePrevQuestion(){
    int question = mQuestionBank[mCurrentIndex].getTextResId();
    mQuestionTexView.setText(question);
}

private void updateQuestion(){
    int question = mQuestionBank[mCurrentIndex].getTextResId();
    mQuestionTexView.setText(question);
}

Here is the error Im getting

6-06 19:18:09.158 14727-14727/com.example.dany_user.geo_quiz E/AndroidRuntime: FATAL EXCEPTION: main

Process: com.example.dany_user.geo_quiz, PID: 14727

java.lang.ArrayIndexOutOfBoundsException: length=6; index=-1

at com.example.dany_user.geo_quiz.QuizActivity.updatePrevQuestion(QuizActivity.java:98)
                                                                                at com.example.dany_user.geo_quiz.QuizActivity.access$400(QuizActivity.java:11)
                                                                                at com.example.dany_user.geo_quiz.QuizActivity$5.onClick(QuizActivity.java:87)
                                                                                at android.view.View.performClick(View.java:6205)
                                                                                at android.widget.TextView.performClick(TextView.java:11103)
                                                                                at android.view.View$PerformClick.run(View.java:23653)
                                                                                at android.os.Handler.handleCallback(Handler.java:751)
                                                                                at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                                at android.os.Looper.loop(Looper.java:154)
                                                                                at android.app.ActivityThread.main(ActivityThread.java:6682)
                                                                                at java.lang.reflect.Method.invoke(Native Method)
                                                                                at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1520)
                                                                                at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1410)

Ted Hopp · Accepted Answer · 2017-06-07 01:36:41Z

2

The problem is that (-1 % n) is -1 for any n in Java. To ensure that you wrap around to the end of the array, you can use this:

mCurrentIndex = (mCurrentIndex + mQuestionBank.length - 1) % mQuestionBank.length;

That will result in mCurrentIndex always being in range after evaluation if it was already in range.

If you don't want to wrap around (and I'd suggest treating the "previous" and "next" buttons symmetrically), then you can do one or both of the following:

Do nothing in the event handler if mCurrentIndex is at an extreme value
Disable the relevant button if mCurrentIndex is at an extreme value, so the event handler cannot be triggered.

answered Jun 7, 2017 at 1:36

Ted Hopp

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