for循环中带有ArrayList的奇怪错误

Question

I have a very weird problem, the problem only presents itself in case 3 & 4.

                for(Sector sector : sectoren)
    {
        switch(sector.getCode())
        {
            case 1:
                for(int i = 0; i<3; i++)
                {
                    for(int j=0;j<3; j++)
                    {
                        Gebiedskaart gbk = gebiedskaarten.get(r.nextInt(gebiedskaarten.size()));
                        vakken[i][j].setGbk(gbk);
                        gebiedskaarten.remove(gbk);
                    }
                vakken[2][2].setGbk(null);
                }
            case 2:
                for(int i = 0; i<3; i++)
                {
                    for(int j=4;j<7; j++)
                    {
                        Gebiedskaart gbk = gebiedskaarten.get(r.nextInt(gebiedskaarten.size()));
                        vakken[i][j].setGbk(gbk);
                        gebiedskaarten.remove(gbk);
                    }
                }
                vakken[2][4].setGbk(null);
            case 3:
                for(int i=4; i<7; i++)
                {
                    for(int j=0;j<3; j++)
                    {
                        System.out.println(gebiedskaarten.size());
                        Gebiedskaart gbk = gebiedskaarten.get(r.nextInt(gebiedskaarten.size()));
                        vakken[i][j].setGbk(gbk);
                        gebiedskaarten.remove(gbk);
                    }
                }
                vakken[4][2].setGbk(null);
           case 4:
                for(int i = 4; i<7; i++)
                {
                    for(int j=4;j<7; j++)
                    {
                        Gebiedskaart gbk = gebiedskaarten.get(r.nextInt(gebiedskaarten.size()));
                        vakken[i][j].setGbk(gbk);
                        //gebiedskaarten.remove(gbk); doet iets raar? moeten we nog naar kijken, hij blijft gewoon verwijderen tot de lijst leeg is
                    }
                }
                vakken[4][4].setGbk(null);
        }
    }

in the code you can see that I assign a value to an array, and then i remove that value from the ArrayList. You can see I have put the System.out.println there, to see what it does, and this is the output:

34
33
32
31
30
29
28
27
26
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0

And then the error obviously:

Exception in thread "main" java.lang.IllegalArgumentException: n must be positive
    at java.util.Random.nextInt(Random.java:300)
    at domein.Spelbord.wijsGebiedskaartenToe(Spelbord.java:123)
    at domein.Spelbord.<init>(Spelbord.java:19)
    at domein.Spel.setWereldkaart(Spel.java:18)
    at domein.DomeinController.maakWereldkaart(DomeinController.java:39)
    at ui.Uc1.maakWereldkaart(Uc1.java:42)
    at ui.ConsoleApplicatie.behandelUc1(ConsoleApplicatie.java:67)
    at ui.ConsoleApplicatie.startSpel(ConsoleApplicatie.java:29)
    at StartUp.main(StartUp.java:12)
Java Result: 1

I don't have a clue of whats going on, maybe you guys have?

Answer 1

It is because you do:

 r.nextInt(gebiedskaarten.size());

when gebiedskaarten.size() == 0

==> you try get a random number from 0 to 0 ==> Exception

Update: it also looks like you forgot the break; statements in your switch-case! I guess you want to be your code logic to be like that:

 switch(sector.getCode()) {
 case 1: /*do something*/ break;
 case 2: /*do something else */ break;
 //...
 default: //do some default action
 }

see also related question: Why do we need break after case statements?

Answer 2

You are calling Random.nextInt() with a value of 0, which is not supported.

As you see from the output of System.out , the array finally has a size of 0, and you are passing the array's size to nextInt() .

See also http://docs.oracle.com/javase/7/docs/api/java/util/Random.html#nextInt(int)

Answer 3

I do not know why your loop runs so many times, while only 9 (3 * 3) times is expected, but assuming that the code you posted starts when gebiedskaarten.size() == 9 , it does the follow for 9 iterations:

• System.out.println(gebiedskaarten.size())
  • Prints the current size.
• Gebiedskaart gbk = gebiedskaarten.get(r.nextInt(gebiedskaarten.size()))
  • First executes r.nextInt(...) with gebiedskaarten.size() , obtaining a new random value.
  • Then calls gebiedskaarten.get(...) with that random number and assigns it to gbk .
• vakken[i][j].setGbk(gbk);
  • Calls setGbk(gbk) on the i, j entry of vakken .
• gebiedskaarten.remove(gbk);
  • Removes gbk from the gebiedskaarten collection, reducing the size by one.

Where it goes wrong, is that a certain point, gebiedskaarten.size() becomes 0 , and you try to obtain a new random via r.nextInt(...) on 0 , which is not possible, as it needs to be strictly positive.
Note however that calling gebiedskaarten.get(0) is valid, so you have an one-off error.

I also strongly suggest you to move from Dutch naming conventions to English naming conventions, as it will help you a lot in either your career future and in posting questions online.

Also, for completeness' sake, as @donfuxx has pointed out, your switch statements are not conform standards, which lead to errors in 99% of the time, it should be such:

switch (variable) {
    case 1:
        //do interesting things
        break;
    case 2:
        //do interesting things
        break;
    case 3:
        //do interesting things
        break;
    default:
        //either allowable base case, or may-not-happen error case
        break;

As you see I have added a break at the end of every case , this is neccessary to prevent it from falling through the statements. If there is no break at the end of case 1 , then case 2 will also get executed.