How to quickly and conveniently create a one element arraylist

Question

Is there a Utility method somewhere that can do this in 1 line  I can t find it anywhere in Collections  or List   public List lt String gt  stringToOneElementList String s        List lt String gt  list   new ArrayList lt String gt         list add s       return list      I don t want to re-invent the wheel unless I plan on putting fancy rims on it   Well    the type can be T  and not String  but you get the point   with all the null checking  safety checks   etc

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Collections singletonList object    the list created by this method is immutable

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You can use the utility method Arrays asList and feed that result into a new ArrayList   List lt String gt  list   new ArrayList lt String gt  Arrays asList s      Other options   List lt String gt  list   new ArrayList lt String gt  Collections nCopies 1  s      and  List lt String gt  list   new ArrayList lt String gt  Collections singletonList s       ArrayList Collection  constructor  Arrays asList method  Collections nCopies method  Collections singletonList method    With Java 7   you may use the  diamond operator   replacing new ArrayList lt String gt       with new ArrayList lt  gt         Java 9  If you re using Java 9   you can use the List of method   List lt String gt  list   new ArrayList lt  gt  List of s      Regardless of the use of each option above  you may choose not to use the new ArrayList lt  gt    wrapper if you don t need your list to be mutable

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Yet another alternative is double brace initialization  e g   new ArrayList lt String gt       add s         but it is inefficient and obscure  Therefore only suitable    in code that doesn t mind memory leaks  such as most unit tests and other short-lived programs  and if none of the other solutions apply  which I think implies you ve scrolled all the way down here looking to populate a different type of container than the ArrayList in the question

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Fixed size List  The easiest way  that I know of  is to create a fixed-size single element List with Arrays asList T     like     Returns a List backed by a varargs T  return Arrays asList s     Variable size List  If it needs vary in size you can construct an ArrayList and the fixed-sizeList like  return new ArrayList lt String gt  Arrays asList s      and  in Java 7   you can use the diamond operator  lt  gt  to make it   return new ArrayList lt  gt  Arrays asList s      Single Element List  Collections can return a list with a single element with list being immutable   Collections singletonList s    The benefit here is IDEs code analysis doesn t warn about single element asList     calls

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With Java 8 Streams   Stream of object  collect Collectors toList      or if you need a set   Stream of object  collect Collectors toSet

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Seeing as Guava gets a mention  I thought I would also suggest Eclipse Collections  formerly known as GS Collections    The following examples all return a List with a single item   Lists mutable of  Just one item    Lists mutable with  Or use with    Lists immutable of  Maybe it must be immutable     Lists immutable with  And use with if you want      There are similar methods for other collections

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Very simply   Arrays asList  Hi

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The other answers all use Arrays asList    which returns an unmodifiable list  an UnsupportedOperationException is thrown if you try to add or remove an element   To get a mutable list you can wrap the returned list in a new ArrayList as a couple of answers point out  but a cleaner solution is to use Guava s Lists newArrayList    available since at least Guava 10  released in 2011     For example   Lists newArrayList  Blargle

[java] How to quickly and conveniently create a one element arraylist

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