Java Get first item from a collection

Question

If I have a collection  such as Collection lt String gt  strs  how can I get the first item out  I could just call an Iterator  take its first next    then throw the Iterator away  Is there a less wasteful way to do it

User · Answer

You can do a casting  For example  if exists one method with this definition  and you know that this method is returning a List   Collection lt String gt  getStrings      And after invoke it  you need the first element  you can do it like this   List lt String gt  listString    List  getStrings    String firstElement    listString isEmpty     null   listString get 0

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It sounds like your Collection wants to be List-like  so I d suggest   List lt String gt  myList   new ArrayList lt String gt         String first   myList get 0

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Iterables get yourC  indexYouWant   Because really  if you re using Collections  you should be using Google Collections

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It totally depends upon which implementation you have used  whether arraylist linkedlist  or other implementations of set   if it is set then you can directly get the first element   their can be trick loop over the collection   create a variable of value 1 and get value when flag value is 1 after that break that loop   if it is list s implementation then it is easy by defining index number

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Guava provides an onlyElement Collector  but only use it if you expect the collection to have exactly one element   Collection lt String gt  stringCollection        String string   collection stream   collect MoreCollectors onlyElement      If you are unsure of how many elements there are  use findFirst   Optional lt String gt  optionalString   collection stream   findFirst

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Functional way    public static  lt T gt  Optional lt T gt  findFirst List lt T gt  result        return Optional ofNullable result               map List  stream               flatMap Stream  findFirst       above code snippet preserve  from NullPointerException and IndexOutOfBoundsException

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In Java 8 you have some many operators to use  for instance limit              Operator that limit the total number of items emitted through the pipeline    Shall print     1      throws InterruptedException      Test public void limitStream   throws InterruptedException       List lt Integer gt  list   Arrays asList 1  2  3  1  4  2  3                                  stream                                   limit 1                                  collect toList         System out println list

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Looks like that is the best way to do it   String first   strs iterator   next      Great question     At first  it seems like an oversight for the Collection interface   Note that  first  won t always return the first thing you put in the collection  and may only make sense for ordered collections   Maybe that is why there isn t a get item  call  since the order isn t necessarily preserved   While it might seem a bit wasteful  it might not be as bad as you think   The Iterator really just contains indexing information into the collection  not a usually a copy of the entire collection   Invoking this method does instantiate the Iterator object  but that is really the only overhead  not like copying all the elements    For example  looking at the type returned by the ArrayList lt String gt  iterator   method  we see that it is ArrayList  Itr   This is an internal class that just accesses the elements of the list directly  rather than copying them   Just be sure you check the return of iterator   since it may be empty or null depending on the implementation

User · Answer

There is no such a thing as  first  item in a Collection because it is    well simply a collection    From the Java doc s Collection iterator   method      There are no guarantees concerning the order in which the elements are returned      So you can t   If you use another interface such as List  you can do the following   String first   strs get 0     But directly from a Collection this is not possible

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You could do this   String strz     strs toArray String strs size      String theFirstOne   strz 0     The javadoc for Collection gives the following caveat wrt ordering of the elements of the array      If this collection makes any guarantees as to what order its elements are returned by its iterator  this method must return the elements in the same order

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In java 8   Optional lt String gt  firstElement   collection stream   findFirst      For older versions of java  there is a getFirst method in Guava Iterables   Iterables getFirst iterable  defaultValue

User · Answer

If you know that the collection is a queue then you can cast the collection to a queue and get it easily   There are several structures you can use to get the order  but you will need to cast to it

[java] Java: Get first item from a collection

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