How to get the first element of the List or Set

Question

I d like to know if I can get the first element of a list or set   Which method to use

User · Answer

In Java   8 you could also use the Streaming API   Optional lt String gt  first   set stream   findFirst       Useful if the Set List may be empty

User · Answer

See the javadoc   of List  list get 0     or Set  set iterator   next      and check the size before using the above methods by invoking isEmpty     list or set isEmpty

User · Answer

Collection c   Iterator iter   c iterator     Object first   iter next       This is the closest you ll get to having the  first  element of a Set   You should realize that it has absolutely no meaning for most implementations of Set   This may have meaning for LinkedHashSet and TreeSet  but not for HashSet

User · Answer

I m surprised that nobody suggested guava solution yet   com google common collect Iterables get collection  0     or com google common collect Iterables get collection  0  defaultValue     or com google common collect Iterables getFirst collection  defaultValue    or if you expect single element   com google common collect Iterables getOnlyElement collection  defaultValue     or com google common collect Iterables getOnlyElement collection

User · Answer

Let s assume that you have a List lt String gt  strings that you want the first item from   There are several ways to do that   Java  pre-8    String firstElement   null  if   strings isEmpty    amp  amp  strings size    gt  0        firstElement   strings get 0       Java 8   Optional lt String gt  firstElement   strings stream   findFirst      Guava  String firstElement   Iterables getFirst strings  null     Apache commons  4    String firstElement    String  IteratorUtils get strings  0     Apache commons  before 4   String firstElement    String  CollectionUtils get strings  0     Followed by or encapsulated within the appropriate checks or try-catch blocks   Kotlin   In Kotlin both Arrays and most of the Collections  eg  List  have a first method call  So your code would look something like this  for a List   val stringsList  List lt String  gt    listOf  a    b   null  val first  String    stringsList first     for an Array   val stringArray  Array lt String  gt    arrayOf  a    b   null  val first  String    stringArray first     Followed by or encapsulated within the appropriate checks or try-catch blocks   Kotlin also includes safer ways to do that for kotlin collections  for example firstOrNull or getOrElse  or getOrDefault when using JRE8

User · Answer

Set  set toArray   0     List  list get 0

User · Answer

java8 and further  Set lt String gt  set   new TreeSet lt  gt         set add  2        set add  1        set add  3        String first   set stream   findFirst   get      This will help you retrieve the first element of the list or set  Given that the set or list is not empty  get   on empty optional will throw java util NoSuchElementException   orElse   can be used as   this is just a work around - not recommended    String first   set stream   findFirst   orElse      set removeIf String  isEmpty     Below is the appropriate approach    Optional lt String gt  firstString   set stream   findFirst    if firstString isPresent         String first   firstString get        Similarly first element of the list can be retrieved   Hope this helps

User · Answer

This is not an exact answer to this question  but in case the objects should be sorted SortedSet has a first   method   SortedSet lt String gt  sortedSet   new TreeSet lt String gt     sortedSet add  2    sortedSet add  1    sortedSet add  3    String first   sortedSet first      first  1    The sorted objects must implement the Comparable interface  like String does

User · Answer

You can use the get index  method to access an element from a List    Sets  by definition  simply contain elements and have no particular order  Therefore  there is no  first  element you can get  but it is possible to iterate through it using iterator  using the for each loop  or convert it to an array using the toArray   method

[java] How to get the first element of the List or Set?

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