ArrayList or List declaration in Java

Question

What is the difference between these two declarations   Declaration 1   ArrayList lt String gt  arrayList   new ArrayList lt String gt       Declaration 2   List lt String gt  arrayList   new ArrayList lt String gt

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The first declaration has to be an ArrayList  the second can be easily changed to another List type   As such  the second is preferred as it make it clear you don t require a specific implementation   Sometimes you really do need one implementation  but that is rare

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There are a few situations where you might prefer the first one to  slightly  improve performance  for example on some JVMs without a JIT compiler   Out of that kind of very specific context  you should use the first one

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The difference is that variant 1 forces you to use an ArrayList while variant 2 only guarantees you have anything that implements List lt String gt    Later on you could change that to List lt String gt  arrayList   new LinkedList lt String gt     without much hassle  Variant 1 might require you to change not only that line but other parts as well if they rely on working with an ArrayList lt String gt    Thus I d use List lt String gt  in almost any case  except when I d need to call the additional methods that ArrayList provides  which was never the case so far   ensureCapacity int  and trimToSize

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List lt String gt  arrayList   new ArrayList lt String gt       Is generic where you want to hide implementation details while returning it to client  at later point of time you may change implementation from ArrayList to LinkedList transparently   This mechanism is useful in cases where you design libraries etc   which may change their implementation details at some point of time with minimal changes on client side   ArrayList lt String gt  arrayList   new ArrayList lt String gt       This mandates you always need to return ArrayList  At some point of time if you would like to change implementation details to LinkedList  there should be changes on client side also to use LinkedList instead of ArrayList

User · Answer

List is an interface and ArrayList is an implementation of the List interface  The ArrayList class has only a few methods i e clone    trimToSize    removeRange   and ensureCapacity    in addition to the methods available in the List interface  There is not much difference in this       1  List lt String gt  l   new ArrayList lt  gt        2  ArrayList lt String gt  l   new ArrayList lt  gt       If you use the first  you will be able to call the methods available in the List interface and you cannot make calls to the new methods available in the ArrayList class  Where as  you are free to use all the methods available in the ArrayList if you use the second one    I would say the first approach is a better one because  when you are developing java applications  when you are supposed to pass the collection framework objects as arguments to the methods  then it is better to go with first approach    List lt String gt  l   new ArrayList lt  gt     doSomething l     In future due to performance constraints  if you are changing the implementation to use LinkedList or someother classes which implements List interface  instead of ArrayList  you need to  change at one point only the instantiation part    List lt String gt  l   new LinkedList lt  gt       Else you will be supposed to change at all the places  wherever  you have used the specific class implementation as method arguments

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Possibly you can refer to this link http   docs oracle com javase 6 docs api java util List html  List is an interface ArrayList LinkedList etc are classes which implement list Whenyou are using List Interface you have to itearte elements using ListIterator and can move forward and backward in the List where as in ArrayList Iterate using Iterator and its elements can be accessed unidirectional way

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It is easy to change the implementation to List to Set   Collection lt String gt  stringList   new ArrayList lt String gt       Client side stringList   new LinkedList lt String gt      stringList   new HashSet lt String gt       stringList   new HashSet lt  gt     java 1 7 and 1 8

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List is interface and ArrayList is implemented concrete class   It is always recommended to use   List lt String gt  arrayList   new ArrayList lt String gt       Because here list reference is flexible  It can also hold LinkedList or Vector object

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Whenever you have seen coding from open source community like Guava and from Google Developer  Android Library  they used this approach  List lt String gt  strings   new ArrayList lt String gt       because it s hide the implementation detail from user  You precisely   List lt String gt  strings   new ArrayList lt String gt        it s generic approach and this specialized approach   ArrayList lt String gt  strings   new ArrayList lt String gt       For Reference  Effective Java 2nd Edition  Item 52  Refer to objects by their interfaces

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Basically it allows Java to store several types of objects in one structure implementation  by generic type declaration  like class MyStructure lt T extends TT gt    which is one of Javas main features    Object-oriented approaches are based in modularity and reusability by separation of concerns - the ability to use a structure with any kind of types of object  as long as it obeys a few rules    You could just instantiate things as followed   ArrayList list   new ArrayList      instead of  ArrayList lt String gt  list   new ArrayList lt  gt       By declaring and using generic types you are informing a structure of the kind of objects it will manage and the compiler will be able to inform you if you re inserting an illegal type into that structure  for instance  Let s say      this works List list1   new ArrayList    list1 add 1   list1 add  one        does not work List lt String gt  list2   new ArrayList lt  gt     list2 add 1      compiler error here list2 add  one      If you want to see some examples check the documentation documentation          Generic version of the Box class      param  lt T gt  the type of the value being boxed     public class Box lt T gt           T stands for  Type      private T t       public void set T t    this t   t        public T get     return t        Then you could instantiate things like   class Paper          class Tissue                 Box lt Paper gt  boxOfPaper   new Box lt  gt     boxOfPaper set new Paper         Box lt Tissue gt  boxOfTissues   new Box lt  gt     boxOfTissues set new Tissue          The main thing to draw from this is you re specifying which type of object you want to box   As for using Object l   new ArrayList lt  gt      you re not accessing the List or ArrayList implementation so you won t be able to do much with the collection

[java] ArrayList or List declaration in Java

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