Initial size for the ArrayList

Question

You can set the initial size for an ArrayList by doing  ArrayList lt Integer gt  arr new ArrayList lt Integer gt  10     However  you can t do  arr add 5  10     because it causes an out of bounds exception   What is the use of setting an initial size if you can t access the space you allocated   The add function is defined as add int index  Object element  so I am not adding to index 10

User · Answer

If you want to add 10 items to your ArrayList you may try that   for  int i   0  i  lt  10  i        arr add i     If you have already declare an array size variable you would use the variable size instead of number  10

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If you want to add the elements with index  you could instead use an array       String    test   new String length       test 0     add

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ArrayList myList   new ArrayList 10        myList add 3   DDD        myList add 9   III        myList add 0   AAA        myList add 1   BBB         for String item myList           System out println  inside list     item             Declare the initial capasity of arraylist is nothing but saving shifting time in internally  when we add the element internally it check the capasity to increase the capasity  you could add the element at 0 index initially then 1 and so on

User · Answer

10 is the initial capacity of the AL  not the size  which is 0   You should mention the initial capacity to some high value when you are going to have a lots of elements  because it avoids the overhead of expanding the capacity as you keep adding elements

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I guess an exact answer to your question would be   Setting an intial size on an ArrayList reduces the nr  of times internal memory re-allocation has to occur  The list is backed by an array  If you specify i e  initial capacity 0  already at the first insertion of an element the internal array would have to be resized  If you have an approximate idea of how many elements your list would hold  setting the initial capacity would reduce the nr  of memory re-allocations happening while you use the list

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Capacity of an ArrayList isn t the same as its size  Size is equal to the number of elements contained in the ArrayList  and any other List implementation    The capacity is just the length of the underlying array which is used to internaly store the elements of the ArrayList  and is always greater or equal to the size of the list   When calling set index  element  on the list  the index relates to the actual number of the list elements   size   which is zero in your code  therefore the AIOOBE is thrown   not to the array length   capacity   which is an implementation detail specific to the ArrayList    The set method is common to all List implementations  such as LinkedList  which isn t actually implemented by an array  but as a linked chain of entries   Edit  You actually use the add index  element  method  not set index  element   but the principle is the same here

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I faced with the similar issue  and just knowing the arrayList is a resizable-array implementation of the List interface  I also expect you can add element to any point  but at least have the option to define the initial size   Anyway  you can create an array first and convert that to a list like     int index   5    int size   10     Integer   array   new Integer size     array index    value          List lt Integer gt  list   Arrays asList array     or    List lt Integer gt  list   Arrays asList new Integer size      list set index  value

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if you want to use Collections fill list  obj   in order to fill the list with a repeated object alternatively you can use  ArrayList lt Integer gt  arr new ArrayList lt Integer gt  Collections nCopies 10  0      the line copies 10 times 0 in to your ArrayList

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Although your arraylist has a capacity of 10  the real list has no elements here  The add method is used to insert a element to the real list  Since it has no elements  you can t insert an element to the index of 5

User · Answer

Being late to this  but after Java 8  I personally find this following approach with the Stream API more concise and can be an alternative to the accepted answer   For example   Arrays stream new int size   boxed   collect Collectors toList     where size is the desired List size and without the disadvantage mentioned here  all elements in the List are initialized as 0    I did a quick search and did not see stream in any answers posted - feel free to let me know if this answer is redundant and I can remove it

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contrib   List  lt Destination  gt  destinations   Collections nCopies source size    Destination class newInstance

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Right now there are no elements in your list so you cannot add to index 5 of the list when it does not exist  You are confusing the capacity of the list with its current size    Just call   arr add 10    to add the Integer to your ArrayList

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If you want a list with a predefined size you can also use   List lt Integer gt  arr   Arrays asList new Integer 10

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This might help someone -   ArrayList lt Integer gt  integerArrayList   new ArrayList lt  gt  Arrays asList new Integer 10

User · Answer

You re confusing the size of the array list with its capacity    the size is the number of elements in the list  the capacity is how many elements the list can potentially accommodate without reallocating its internal structures    When you call new ArrayList lt Integer gt  10   you are setting the list s initial capacity  not its size  In other words  when constructed in this manner  the array list starts its life empty   One way to add ten elements to the array list is by using a loop   for  int i   0  i  lt  10  i        arr add 0       Having done this  you can now modify elements at indices 0  9

User · Answer

My two cents on Stream  I think it s better to use  IntStream generate i - gt  MyClass contruct              limit INT SIZE            collect Collectors toList       with the flexibility to put any initial values

[java] Initial size for the ArrayList

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