java.lang.NullPointerException with arrays

Question

Hey the names in the program are in portuguese but I think its understandable, if you have any doubt just ask and i'll translate.

So I'm getting a NullPointerException with these. The array Vector_Canais is initialized in the constructor:

public Box(int capacidade) {
        Time a = new Time();
        Vector_Canais = new Canal[DEFAULT_SIZE];
    }

public static void novoCanal() {
        Scanner in = new Scanner(System.in);

        Cnl = in.nextLine();
        Vector_Canais[i] = new Canal(Cnl);      
        i++;
    }

    public static String listaCanais(int i) {
        return (Vector_Canais[i].getCanal());
    }

public static void listaCanais() {
        for (int a = 0; a < 100; a++) {
            if (Box.listaCanais(a) != null) {
                System.out.println(Box.listaCanais(a));
            }
        }

i is initialized at 0. Any ideas?

Please copy/paste the stack trace from your exception, and show us the line in your code that caused it. Also show the code that initializes Vector_Canais. — nos
– nos, Commented Dec 13, 2012 at 12:22
Better post all your code. What you have posted doesn't make any sense. For example, Time a = new Time(); does nothing - the scope of a is the method and you don't use it, so why define it? — Bohemian
– Bohemian ♦, Commented Dec 13, 2012 at 12:33

Theodoros Chatzigiannakis · Accepted Answer · 2012-12-13 12:26:36Z

3

Most probably you haven't initialized the array Vector_Canais or you haven't initialized all of its members (for instance, novoCanal hasn't been called 100 times before). Remember, making an array of size 100 (of a class) means making an array of 100 null slots, not 100 objects. Until you call a constructor for each of them, they may throw this.

answered Dec 13, 2012 at 12:26

Theodoros Chatzigiannakis

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5 Comments

deftz Over a year ago

I think thats my problem. Thanks!

Theodoros Chatzigiannakis Over a year ago

Glad to be of help. If this really is the solution for your problem, please consider marking the answer as correct.

deftz Over a year ago

Followup question: how do I make the rest 99 channels if I don't want to/can't make them all at the same time?

Theodoros Chatzigiannakis Over a year ago

In that case, you can modify your null check to look like this: if (Vector_Canais[a] != null && Box.listaCanais(a) != null) This way, you don't have to initialize them all before looping, but the loop ignores those that haven't been initialized.

deftz Over a year ago

Thanks i've made a counter though and compared the cycle with the counter so it would only scan the channels created.

Bohemian · Accepted Answer · 2012-12-13 12:23:00Z

1

You haven't shown where Vector_Canais is initialized - my guess is you are not initializing it.

Try this:

static private Vector_Canais Canal[] = new Canal[100]; // for example

answered Dec 13, 2012 at 12:23

Bohemian♦

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4 Comments

m0skit0 Over a year ago

Java conventions! Variable names start with lowercase ;)

Bohemian Over a year ago

@m0skit0 Come on... this guy is waaay off coping with naming conventions. I'm just trying to get him up and running with as little info as possible in case it fries his brain

deftz Over a year ago

Sorry I am here: public Box(int capacidade) { Time a = new Time(); Vector_Canais = new Canal[DEFAULT_SIZE]; }

Bohemian Over a year ago

"edit" your question and add what code you need to there, not here in a comment

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