Dynamic ArrayList in Java

Question

I have one doubt in java, can we create dynamic ArrayList or String[] inside a for-loop. For example in my app there are categories field is there (not static, obtaining from server), I want to create ArrayList or String[] object based on size of categories. How can I create a dynamic ArrayList in side a loop?

My code is as follows:

for(int i =0; i<20; i++)
{
    ArrayList <String> list(i) = new ArrayList<String>();
}

can we create an object like this? based on category position, I can call the list items like list(positon), can we do like this java?

You can create both. In case of list : ArrayList <String> l = new ArrayList<String>(); Array : String[] l = new String[size] //In case you know the size. — TJ-
– TJ-, Commented Oct 8, 2012 at 7:41
You can just keep adding items to an ArrayList... or what are you trying todo? — dngfng
– dngfng, Commented Oct 8, 2012 at 7:42
I know how to add elements to arraylist but i dont know how to create dynamic ArrayList object based on Size — Ravikumar11
– Ravikumar11, Commented Oct 8, 2012 at 7:59

Samir Mangroliya · Accepted Answer · 2012-10-08 07:43:50Z

8

ArrayList is a dynamic array, i.e. you can add new elements like,

ArrayList <String> list = new ArrayList<String>();
for(int i =0; i<20; i++)
{
 list.add("string"+i);
}

i can call the list items like list(positon),can we do like this java?

ArrayList have get(position) method to get Object from position.

String str1 = list.get(0);//0th position

edited Oct 8, 2012 at 7:43

answered Oct 8, 2012 at 7:42

Samir Mangroliya

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Comments

Tudor · Accepted Answer · 2012-10-08 07:41:50Z

The whole point of ArrayList is that you don't have to worry about size. It will resize itself dynamically as more items get added. You can see the source code of add:

410       public boolean add(E e) {
411           ensureCapacityInternal(size + 1);  // Increments modCount!!
412           elementData[size++] = e;
413           return true;
414       }

ensureCapacityInternal:

183       private void ensureCapacityInternal(int minCapacity) {
184           modCount++;
185           // overflow-conscious code
186           if (minCapacity - elementData.length > 0)
187               grow(minCapacity);
188       }

and grow:

204       private void grow(int minCapacity) {
205           // overflow-conscious code
206           int oldCapacity = elementData.length;
207           int newCapacity = oldCapacity + (oldCapacity >> 1);
208           if (newCapacity - minCapacity < 0)
209               newCapacity = minCapacity;
210           if (newCapacity - MAX_ARRAY_SIZE > 0)
211               newCapacity = hugeCapacity(minCapacity);
212           // minCapacity is usually close to size, so this is a win:
213           elementData = Arrays.copyOf(elementData, newCapacity);
214       }

grow array is serious overhead for arrayList. As it is coping array will create a hole new instance will.
@Quoi: Yes but as you can see the capacity grows linearly with the number items already in the list, so the resizing will be less and less frequent.
If I interpret the question correctly, the point is that he wants to create one arraylist per category. For example, if he on the server has a list of books sorted into the categories: thriller, drama, comedy. He wants to create 3 (the size of categories) arraylists, each representing one category. This is my interpretation of the question, at least. Then he'll add all thriller books to the thriller category arraylist, and the same for the other two categories.
@Alderath: Then it sound like the needs a Map with ArrayLists as values.

Alderath · Accepted Answer · 2012-10-08 09:09:34Z

If I understood what you want to do, I'd do as follows. (And btw, if this is correct, the terminology for describing what you want to do is "nested arraylists", not "dynamic arraylists". Arraylists are (like others have pointed out already) dynamic by default).

List<List<String>> lists = new Arraylist<List<String>>();

for(int i = 0; i < getNumCategoriesFromServer(); i++){
    lists.add(new ArrayList<String>());
}

Now you can do, e.g.:

lists.get(0).add(someItemFromCategoryZero);
lists.get(j).add(someItemFromCategoryJ);

list.get(i) will return the list for category i.

You can also make an array of ArrayLists, but that will not ensure type safety at compile time.

Roy · Accepted Answer · 2012-10-08 08:11:22Z

-1

Are you trying to create an array with each element being an ArrayList? If so,

ArrayList[] list = new ArrayList[20];
for(int i=0; i<20; i++)
{
    list[i] = new ArrayList<String>();
}

answered Oct 8, 2012 at 8:11

Roy

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