Best way to get value from Collection by index

Question

What is the best way to get value from java util Collection by index

User · Answer

In general  there is no good way  as Collections are not guaranteed to have fixed indices   Yes  you can iterate through them  which is how toArray  and other functions  work   But the iteration order isn t necessarily fixed  and if you re trying to index into a general Collection  you re probably doing something wrong   It would make more sense to index into a List

User · Answer

Convert the collection into an array by using function   Object   toArray Object   a

User · Answer

use for each loop     ArrayList lt Character gt  al   new ArrayList lt  gt         String input  hello    for  int i   0  i  lt  input length    i         al add input charAt i       for  Character ch   al                       System Out println ch

User · Answer

You shouldn t  a Collection avoids talking about indexes specifically because it might not make sense for the specific collection  For example  a List implies some form of ordering  but a Set does not   Collection lt String gt  myCollection   new HashSet lt String gt     myCollection add  Hello    myCollection add  World     for  String elem   myCollection        System out println  elem       elem      System out println  myCollection toArray   0        myCollection toArray   0      gives me   elem   World elem   Hello myCollection toArray   0    World   whilst   myCollection   new ArrayList lt String gt     myCollection add  Hello    myCollection add  World     for  String elem   myCollection        System out println  elem       elem      System out println  myCollection toArray   0        myCollection toArray   0      gives me   elem   Hello elem   World myCollection toArray   0    Hello   Why do you want to do this  Could you not just iterate over the collection

User · Answer

you definitively want a List       The List interface provides four methods for positional  indexed  access to list elements     Lists  like Java arrays  are zero based    Also     Note that these operations may execute in time proportional to the index value for some    implementations  the LinkedList class  for example   Thus  iterating over the elements in a   list is typically preferable to indexing through it if the caller does not know the    implementation    If you need the index in order to modify your collection you should note that List provides a special ListIterator that allow you to get the index   List lt String gt  names   Arrays asList  Davide    Francesco    Angelocola    ListIterator lt String gt  i   names listIterator     while  i hasNext          System out format    d   s n   i nextIndex    i next

User · Answer

I agree with Matthew Flaschen s answer and just wanted to show examples of the options for the case you cannot switch to List  because a library returns you a Collection    List list   new ArrayList theCollection   list get 5     Or  Object   list2   theCollection toArray    doSomethingWith list 2      If you know what generics is I can provide samples for that too   Edit  It s another question what the intent and semantics of the original collection is

User · Answer

You can get the value from collection using for-each loop or using iterator interface  For a Collection c  for   lt ElementType gt  elem  c         System out println elem   or Using Iterator Interface   Iterator it   c iterator             while  it hasNext             System out println it next

User · Answer

You must either wrap your collection in a list  new ArrayList c   or use c toArray   since Collections have no notion of  index  or  order

User · Answer

It would be just as convenient to simply convert your collection into a list whenever it updates  But if you are initializing  this will suffice   for String i   collectionlist       arraylist add i       whateverIntID   arraylist indexOf i       Be open-minded

User · Answer

I agree that this is generally a bad idea  However  Commons Collections had a nice routine for getting the value by index if you really need to   CollectionUtils get collection  index

[java] Best way to get value from Collection by index

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