How can I initialize an array without knowing its size?

Question

I have a situation, where I have to apply a criteria on an input array and reuturn another array as output which will have smaller size based upon the filtering criteria.

Now problem is I do not know the size of filtered results, so I can not initialize the array with specific value. And I do not want it to be large size will null values because I am using array.length; later on.

One way is to first loop the original input array and set a counter, and then make another loop with that counter length and initialize and fill this array[]. But is there anyway to do the job in just one loop?

Solution 1

You can't... an array's size is always fixed in Java. Typically instead of using an array, you'd use an implementation of List<T> here - usually ArrayList<T>, but with plenty of other alternatives available.

You can create an array from the list as a final step, of course - or just change the signature of the method to return a List<T> to start with.

Solution 2

Use LinkedList instead. Than, you can create an array if necessary.

Solution 3

Just return any kind of list. ArrayList will be fine, its not static.

    ArrayList<yourClass> list = new ArrayList<yourClass>();
for (yourClass item : yourArray) 
{
   list.add(item); 
}

Comments

[Noel M]

ArrayList would probably be more appropriate

[Roman]

why would you prefer ArrayList to LinkedList in this situation?

[Roman]

@Noel M: why? I think it wouldn't. We don't know the number of elements. So, with LinkedList every add (i.e. addLast) operation works in O(1) and do really a little job, while ArrayList will autoincrease its size several times and these're costly operations.

[Jon Skeet]

@Roman: I just naturally reach for an ArrayList in general. LinkedList would be fine too... it's a more expensive in terms of memory, of course, but it doesn't require copying elements on expansion.

[Jon Skeet]

On the other hand, with a LinkedList you're creating a Node object for each element. You claim that expansion is a "costly" operation - it's only a matter of creating a new array and copying the existing elements (which can be a fast array copy). I don't think it's simple to say which is uniformly "better" for this situation.

[Brian]

@Roman: See When to use LinkedList over ArrayList for discussion. That being said, your first inclination should be ArrayList.