CODEWITHSUNDEEP
javaarraylist
Ravikumar11
Ravikumar11

Reputation: 449

Dynamic ArrayList in Java

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?

Upvotes: 6

Views: 61778

Answers (4)

Alderath
Alderath

Reputation: 3859

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.

Upvotes: 2

Roy
Roy

Reputation: 984

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>();
}

Upvotes: -1

Samir Mangroliya
Samir Mangroliya

Reputation: 40416

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

Upvotes: 8

Tudor
Tudor

Reputation: 62439

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       }

Upvotes: 5

Related Questions