java ArrayList - how can I check if an index exists

Question

I m using ArrayList lt String gt  and I add data at specific indices  how can I check if a specific index exists  Should I simply get   and check the value  Or should I wait for an exception  Is there another way  Update Thank you for your answers  but because I m only adding stuff at specific indices  the length of the list will not show me which are available

User · Accepted Answer

The method arrayList size   returns the number of items in the list - so if the index is greater than or equal to the size    it doesn t exist   if index  gt   myList size         index not exists  else      index exists

User · Answer

Quick and dirty test for whether an index exists or not  in your implementation replace list With your list you are testing   public boolean hasIndex int index       if index  lt  list size            return true      return false      or for 2Dimensional ArrayLists     public boolean hasRow int row       if row  lt   matrix size            return true      return false

User · Answer

a simple way to do this   try     list get  index        catch   IndexOutOfBoundsException e       if list isEmpty      index  gt   list size            Adding new item to list

User · Answer

While you got a dozen suggestions about using the size of your list  which work for lists with linear entries  no one seemed to read your question   If you add entries manually at different indexes none of these suggestions will work  as you need to check for a specific index   Using if   list get index     null   will not work either  as get   throws an exception instead of returning null   Try this   try       list get  index      catch   IndexOutOfBoundsException e         list add  index  new Object          Here a new entry is added if the index does not exist  You can alter it to do something different

User · Answer

Since java-9 there is a standard way of checking if an index belongs to the array - Objects checkIndex      List lt Integer gt  ints   List of 1 2 3   System out println Objects checkIndex 1 ints size         1 System out println Objects checkIndex 10 ints size        IndexOutOfBoundsException

User · Answer

Regarding your update  which probably should be another question   You should use an array of these objects instead an ArrayList  so you can simply check the value for null   Object   array   new Object MAX ENTRIES      if   array  8      null           not available   else         do something     Best-Practice  If you don t have hundred of entries in your array you should consider organizing it as a class to get rid of the magic numbers 3 8 etc   Control flow using exception is bad practice

User · Answer

You could check for the size of the array   package sojava  import java util ArrayList   public class Main       public static Object get ArrayList list  int index            if  list size    gt  index    return list get index             return null             public static void main String   args            ArrayList list   new ArrayList            list add      list add      list add                      System out println get list  4               prints  null

User · Answer

If your index is less than the size of your list then it does exist  possibly with null value  If index is bigger then you may call ensureCapacity    to be able to use that index   If you want to check if a value at your index is null or not  call get

User · Answer

You can check the size of an ArrayList using the size   method  This will return the maximum index  1

User · Answer

This is what you need      public boolean indexExists final List list  final int index        return index  gt   0  amp  amp  index  lt  list size        Why not use an plain old array  Indexed access to a List is a code smell I think

User · Answer

Usually I just check if the index is less than the array size  if  index  lt  list size                  If you are also concerned of index being a negative value  use following  if  index  gt   0  amp  amp  index  lt  list size

[java] java: ArrayList - how can I check if an index exists?

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