Use of Java s Collections singletonList

Question

What is the use of Collections singletonList   in Java  I understand that it returns a list with one element  Why would I want to have a separate method to do that  How does immutability play a role here   Are there any special useful use-cases for this method rather than just being a convenient method

User · Accepted Answer

The javadoc says this       Returns an immutable list containing only the specified object  The returned list is serializable     You ask      Why would I want to have a separate method to do that    Primarily as a convenience     to save you having to write a sequence of statements to    create an empty list object add an element to it  and wrap it with an immutable wrapper    It may also be a bit faster and or save a bit of memory  but it is unlikely that these small savings will be significant    An application that creates vast numbers of singleton lists is unusual to say the least        How does immutability play a role here    It is part of the specification of the method  see above      Are there any special useful use-cases for this method  rather than just being a convenience method    Clearly  there are use-cases where it is convenient to use the singletonList method  But I don t know how you would  objectively  distinguish between an ordinary use-case and a  specially useful  one

User · Answer

From the javadoc   param  the sole object to be stored in the returned list   return an immutable list containing only the specified object    example      import java util     public class HelloWorld       public static void main String args                 create an array of string objs         String initList        One    Two    Four    One                 create one list         List list   new ArrayList Arrays asList initList             System out println  List value before    list               create singleton list         list   Collections singletonList  OnlyOneElement            list add  five      throws UnsupportedOperationException         System out println  List value after    list             Use it when  code expects a read-only list  but you only want to pass one element in it  singletonList is  thread- safe and fast

User · Answer

To answer your immutable question    Collections singletonList will create an immutable List  An immutable List  also referred to as an unmodifiable List  cannot have it s contents changed  The methods to add or remove items will throw exceptions if you try to alter the contents   A singleton List contains only that item and cannot be altered

User · Answer

Here s one view on the singleton methods      I have found these various  singleton  methods to be useful for passing a single value to an API that requires a collection of that value  Of course  this works best when the code processing the passed-in value does not need to add to the collection

User · Answer

If an Immutable Singleton collections refers to the one which having only one object and which is not further gets modified  then the same functionality can be achieved by making a collection  UnmodifiableCollection  having only one object  Since the same functionality can be achieved by Unmodifiable Collection with one object  then what special purpose the Singleton Collection serves for

User · Answer

singletonList can hold instance of any object  Object state can be modify   List lt Character gt  list   new ArrayList lt Character gt     list add  X    list add  Y    System out println  Initial list     list   List lt List lt Character gt  gt  list2   Collections singletonList list   list add  Z    System out println list   System out println list2     We can not define unmodifiableList like above

[java] Use of Java's Collections.singletonList()?

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