ArrayList initialization equivalent to array initialization

Question

I am aware that you can initialize an array during instantiation as follows   String   names   new String     Ryan    Julie    Bob      Is there a way to do the same thing with an ArrayList  Or must I add the contents individually with array add

User · Answer

Well  in Java there s no literal syntax for lists  so you have to do  add     If you have a lot of elements  it s a bit verbose  but you could either    use Groovy or something like that use Arrays asList array    2 would look something like   String   elements   new String     Ryan    Julie    Bob    List list   new ArrayList Arrays asList elements      This results in some unnecessary object creation though

User · Answer

Arrays asList  Ryan    Julie    Bob

User · Answer

The selected answer is  ArrayList lt Integer gt  Arrays asList 1 2 3 5 8 13 21     However  its important to understand the selected answer internally copies the elements several times before creating the final array  and that there is a way to reduce some of that redundancy   Lets start by understanding what is going on    First  the elements are copied into the Arrays ArrayList lt T gt  created by the static factory Arrays asList T       This does not the produce the same class as java lang ArrayListdespite having the same simple class name   It does not implement methods like remove int  despite having a List interface  If you call those methods it will throw an UnspportedMethodException  But if all you need is a fixed-sized list  you can stop here   Next the Arrays ArrayList lt T gt  constructed in  1  gets passed to the constructor ArrayList lt  gt  Collection lt T gt   where the collection toArray   method is called to clone it        public ArrayList Collection lt   extends E gt  collection           Object   a   collection toArray       Next the constructor decides whether to adopt the cloned array  or copy it again to remove the subclass type   Since Arrays asList T     internally uses an array of type T  the very same one we passed as the parameter  the constructor always rejects using the clone unless T is a pure Object   E g  String  Integer  etc all get copied again  because they extend Object    if  a getClass      Object   class                Arrays asList T     is always true here        when T subclasses object     Object   newArray   new Object a length       System arraycopy a  0  newArray  0  a length       a   newArray    array   a  size   a length     Thus  our data was copied 3x just to explicitly initialize the ArrayList   We could get it down to 2x if we force Arrays AsList T     to construct an Object   array  so that ArrayList can later adopt it  which can be done as follows    List lt Integer gt   List lt   gt   new ArrayList lt  gt  Arrays asList  Object  1  2  3  4  5      Or maybe just adding the elements after creation might still be the most efficient

User · Answer

Yes   new ArrayList lt String gt         add  A       add  B         What this is actually doing is creating a class derived from ArrayList lt String gt   the outer set of braces do this  and then declare a static initialiser  the inner set of braces   This is actually an inner class of the containing class  and so it ll have an implicit this pointer  Not a problem unless you want to serialise it  or you re expecting the outer class to be garbage collected   I understand that Java 7 will provide additional language constructs to do precisely what you want   EDIT  recent Java versions provide more usable functions for creating such collections  and are worth investigating over the above  provided at a time prior to these versions

User · Answer

How about this one   ArrayList lt String gt  names   new ArrayList lt String gt     Collections addAll names   Ryan    Julie    Bob

User · Answer

Arrays asList can help here   new ArrayList lt Integer gt  Arrays asList 1 2 3 5 8 13 21

User · Answer

This is how it is done using the fluent interface of the op4j Java library  1 1  was released Dec  10   -  List lt String gt  names   Op onListFor  Ryan    Julie    Bob   get      It s a very cool library that saves you a tonne of time

User · Answer

Here is the closest you can get   ArrayList lt String gt  list   new ArrayList Arrays asList  Ryan    Julie    Bob       You can go even simpler with   List lt String gt  list   Arrays asList  Ryan    Julie    Bob     Looking at the source for Arrays asList  it constructs an ArrayList  but by default is cast to List   So you could do this  but not reliably for new JDKs    ArrayList lt String gt  list    ArrayList lt String gt  Arrays asList  Ryan    Julie    Bob

[java] ArrayList initialization equivalent to array initialization

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