Why does my application stop working?

Question

So, I am trying to do an application which gets a random number between 1 and 4 ( 1 inclusive and 4 exclusive ) and after, getting that number, it would change the background color of my Main Activity to the related number:

If it gets the number 1: Change to Blue If number 2: Change to Black If 3: Change to Yellow

Here's the code:

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

    Button button = (Button) findViewById(R.id.button);
    Button button2 = (Button) findViewById(R.id.button2);

    button.setOnClickListener( new  View.OnClickListener() {

            public void onClick(View v) {

            LinearLayout layout = (LinearLayout) findViewById(R.id.layout);
            Random rnd = new Random();
            int random = rnd.nextInt(1 - 4);

                if (random == 1) {

                layout.setBackgroundColor(Color.BLUE);

                } else if (random == 2) {

                layout.setBackgroundColor(Color.BLACK);

                } else {

                layout.setBackgroundColor(Color.YELLOW);

                }


            }
        }
    );
}

The code seems fine to me and Android Studio doesn't report any errors ( only a warning because of the button2 ), but everytime I tap the button, the app just closes and says "Unfortunatly, Exercise has stopped."

My question is: Why does the application stop working after I tap the button?

( If any more information is needed, tell. I never asked and I am quite a novice in asking and Android Development )

Do you have any errors in the logcat ? if not are you sure you haven't set a filter? — Hannoun Yassir, Jun 05 '17 at 15:32
`rnd.nextInt(1 - 4)` is `rnd.nextInt(-3)`. I don't think that's what you meant. — khelwood, Jun 05 '17 at 15:35

score 1 · Answer 1 · answered Jun 05 '17 at 15:34

rnd.nextInt(1 - 4); evaluates to rnd.nextInt(-3); and that will throw an IllegalArgumentException due to a negative argument.

As you are not handling that exception yourself, bad things happen to your application. To generate random integers in a range, see How do I generate random integers within a specific range in Java?

Reference: https://docs.oracle.com/javase/7/docs/api/java/util/Random.html#nextInt(int)

score 0 · Answer 2 · answered Jun 05 '17 at 15:34

0

Check your logcat for more information.

This line seems strange to me: rnd.nextInt(1 - 4);

Is it right?

answered Jun 05 '17 at 15:34

ricardofagodoy

146
1
10

Shadov · Accepted Answer · 2017-06-05T18:22:29.463

0

The random.nextInt(int bound) method accepts the bound parameter, this is that method in JDK:

public int nextInt(int bound) {
  if (bound <= 0)
    throw new IllegalArgumentException(BadBound);

  int r = next(31);
  int m = bound - 1;
  if ((bound & m) == 0)  // i.e., bound is a power of 2
    r = (int)((bound * (long)r) >> 31);
  else {
    for (int u = r;
      u - (r = u % bound) + m < 0;
      u = next(31))
      ;
  }
  return r;
}

So, if the bound is less or equal to 0 it throws exception, your bound is -3, thus resulting in that exception. Change your bound to random.nextInt(3) + 1.

edited Jun 05 '17 at 18:22

answered Jun 05 '17 at 15:34

Shadov

5,421
2
19
38

Thank you so much. I would never figure it out on my own. EDIT: It worked btw – Kaito Hiroyuki Jun 05 '17 at 15:39

score 0 · Answer 4 · answered Jun 05 '17 at 15:38

0

Declare this: LinearLayout layout = (LinearLayout) findViewById(R.id.layout); Out side of your onClick listener.

Also Try this for your random number generator between 1 and 4

Random r = new Random();
int i1 = r.nextInt((4 - 1) + 1) + 1;

answered Jun 05 '17 at 15:38

fluffy

223
1
14

score 0 · Answer 5 · answered Jun 05 '17 at 15:44

0

int random = rnd.nextInt(1 - 4);

the problem is with above line, because it can throw an Exception.

Try to catch this Exception with try {} catch {} blocks and you will see.

Also you can change your algorithm for example use list with colors

List<Color> colors = Arrays.asList(Color.BLUE, Color.RED, Color.YELLOW);
layout.setBackgroundColor(colors.get(rand.nextInt(colors.size() - 1));

answered Jun 05 '17 at 15:44

Rivalez

83
2
11

If you will use list you won't get invalid value problems, code is shorter, more clear and gives you easy way to add many more values – Rivalez Jun 05 '17 at 15:47

score 0 · Answer 6 · edited Feb 21 '18 at 13:02

0

int random = rnd.nextInt(4);

OR

int random = rnd.nextInt(3);

edited Feb 21 '18 at 13:02

Gowthaman M

8,057
8
35
54

answered Jun 05 '17 at 15:44

Muhammad Waqas

420
4
19

Why? How is this different from the original question? What does this do and how does it solve the issue? – Zoe Jun 05 '17 at 16:35
yes ofcourse because you have used rnd.nextInt(1-4) which gives -ve value – Muhammad Waqas Jun 05 '17 at 16:36

score 0 · Answer 7 · answered Jun 05 '17 at 15:45

As stated by Bathsheba, your logic for the nextInt function is incorrect.

As detailed in the Oracle documentation for Random and nextInt(int n), the nextInt function "Returns a pseudorandom, uniformly distributed int value between 0 (inclusive) and the specified value (exclusive)".

Therefore, you are trying to generate a number between 0 and -3, which causes an "IllegalArgumentException error", since 'n' is not positive.

One correct way to generate a random number between 1 (inclusive) and 4 (exclusive) is the following...

int random = rnd.nextInt(3) + 1

'rnd.nextInt(2)' will generate either 0, 1, or 2
adding 1 to these options gives you the desired outputs of 1, 2, or 3

Why does my application stop working?

7 Answers7