java array declaration

Question

I get the error shown in the comment when I call my class's constructor (when I remove the array parts everything goes fine). Is this because of a wrong declaration of the array seq?

public class FibIt implements SeqIt{
    public int counter;
    public int ptr;
    public int [] seq;

    public FibIt(Fib x)
    {   counter=0;
        ptr=0;
        seq[0]=x.first1;         //gives me an error here saying Exception in 
                                     //thread "main" java.lang.NullPointerException
                                 //at FibIt.<init>(FibIt.java:9)
                                     //at Main.main(Main.java:6)
        seq[1]=x.first2;
        for (int i=2; seq[i-1]<=x.last; i++)
            {seq[i]=seq[i-1]+seq[i-2];}
    }
    @Override
    public int func2() {
        // TODO Auto-generated method stub
        ptr++;
        return seq[ptr-1];
    }

}

Austin · Accepted Answer · 2012-11-19 07:37:40Z

6

You have to initialize your array, so something like public int[] seq = new int[10];

Then replace 10 with whatever size you need.

And I was just about to answer your question when @Jack posted a good solution. ArrayList<Integer> is pretty useful if you don't know the size of the array.

edited Nov 19, 2012 at 7:37

answered Nov 14, 2012 at 4:35

Austin

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6 Comments

CSawy Over a year ago

what if i dont know the size [depends on the input]?

Jack Over a year ago

Then you have to wait to initialize it until you know the size or use a different data structure, which for java is the ArrayList. Arrays can't be resized dynamically.

Paul Bellora Over a year ago

@user1763553 You could initialize it in the constructor instead, or yeah, just use a collection.

Jack Over a year ago

@user1763553 in your example, since you are calculating the fibonacci sequence up to parameter x.last, you could initialize it inside: seq = new int[x.last].

CSawy Over a year ago

the problem is that this is a sequence so i will never know its size even after putting some elements. i dont know when it will end so how will the ArrayList method be useful? and how to implement it? thanks!

|

Jack · Accepted Answer · 2012-11-14 04:37:34Z

2

You need to initialize the array. One thing is declaration, other thing is inizialization.

int[] seq declares a variable of name seq which is an array of int. Then you need to effectively inizialize it by assigning to it a constructor for an array: new int[dimension]

answered Nov 14, 2012 at 4:37

Jack

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Comments

vels4j · Accepted Answer · 2012-11-14 04:37:26Z

1

Yes, you have only declared the array but not initilized.

public int [] seq = new int[anySize];

answered Nov 14, 2012 at 4:37

vels4j

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