Java generics - ArrayList initialization

Question

It is known that arraylist init  should be like this  ArrayList lt A gt  a   new ArrayList lt A gt     ArrayList lt Integer gt  a   new ArrayList lt Number gt        compile-time error   so  why does java allow these         1  ArrayList lt   extends Object gt  a1   new ArrayList lt Object gt     2  ArrayList lt   gt  a2   new ArrayList lt Integer gt       then  if they are correct why doesn t allow these    1  a1 add 3   2  a2 add 3     the compiler message is   The method add int  capture 1-of   extends Object  in the type ArrayList is not applicable for the arguments  int   more general     1  a1 add null e     2  a2 add   e     I read about this but it is good to hear from you  thanks  the other amusing point is     ArrayList lt ArrayList lt   gt  gt  a   new ArrayList lt ArrayList lt   gt  gt        correct  ArrayList lt   gt  a   new ArrayList lt   gt        wrong  I know it s reason but I have some  question in my mind that mentioned above

User · Answer

ArrayList lt Integer gt  a   new ArrayList lt Number gt        Does not work because the fact that Number is a super class of Integer does not mean that List lt Number gt  is a super class of List lt Integer gt   Generics are removed during compilation and do not exist on runtime  so parent-child relationship of collections cannot be be implemented  the information about element type is simply removed    ArrayList lt   extends Object gt  a1   new ArrayList lt Object gt     a1 add 3     I cannot explain why it does not work  It is really strange but it is a fact  Really syntax  lt   extends Object gt  is mostly used for return values of methods  Even in this example Object o   a1 get 0  is valid   ArrayList lt   gt  a   new ArrayList lt   gt      This does not work because you cannot instantiate list of unknown type

User · Answer

You can t assign a List lt Number gt  to a reference of type List lt Integer gt  because List lt Number gt  allows types of numbers other than Integer  If you were allowed to do that  the following would be allowed   List lt Number gt  numbers   new ArrayList lt Number gt     numbers add 1 1      add a double List lt Integer gt  ints   numbers  Integer fail   ints get 0      ClassCastException    The type List lt Integer gt  is making a guarantee that anything it contains will be an Integer  That s why you re allowed to get an Integer out of it without casting  As you can see  if the compiler allowed a List of another type such as Number to be assigned to a List lt Integer gt  that guarantee would be broken   Assigning a List lt Integer gt  to a reference of a type such as List lt   gt  or List lt   extends Number gt  is legal because the   means  some unknown subtype of the given type   where the type is Object in the case of just   and Number in the case of   extends Number    Since   indicates that you do not know what specific type of object the List will accept  it isn t legal to add anything but null to it  You are  however  allowed to retrieve any object from it  which is the purpose of using a   extends X bounded wildcard type  Note that the opposite is true for a   super X bounded wildcard type    a List lt   super Integer gt  is  a list of some unknown type that is at least a supertype of Integer   While you don t know exactly what type of List it is  could be List lt Integer gt   List lt Number gt   List lt Object gt   you do know for sure that whatever it is  an Integer can be added to it   Finally  new ArrayList lt   gt    isn t legal because when you re creating an instance of a paramterized class like ArrayList  you have to give a specific type parameter  You could really use whatever in your example  Object  Foo  it doesn t matter  since you ll never be able to add anything but null to it since you re assigning it directly to an ArrayList lt   gt  reference

User · Answer

The key lies in the differences between references and instances and what the reference can promise and what the instance can really do   ArrayList lt A gt  a   new ArrayList lt A gt       Here a is a reference to an instance of a specific type - exactly an array list of As   More explicitly  a is a reference to an array list that will accept As and will produce As   new ArrayList lt A gt    is an instance of an array list of As  that is  an array list that will accept As and will produce As   ArrayList lt Integer gt  a   new ArrayList lt Number gt        Here  a is a reference to exactly an array list of Integers  i e  exactly an array list that can accept Integers and will produce Integers   It cannot point to an array list of Numbers   That array list of Numbers can not meet all the promises of ArrayList lt Integer gt  a  i e  an array list of Numbers may produce objects that are not Integers  even though its empty right then    ArrayList lt Number gt  a   new ArrayList lt Integer gt        Here  declaration of a says that a will refer to exactly an array list of Numbers  that is  exactly an array list that will accept Numbers and will produce Numbers   It cannot point to an array list of Integers  because the type declaration of a says that a can accept any Number  but that array list of Integers cannot accept just any Number  it can only accept Integers   ArrayList lt   extends Object gt  a  new ArrayList lt Object gt       Here a is a  generic  reference to a family of types rather than a reference to a specific type  It can point to any list that is member of that family   However  the trade-off for this nice flexible reference is that they cannot promise all of the functionality that it could if it were a type-specific reference  e g  non-generic    In this case  a is a reference to an array list that will produce Objects  But  unlike a type-specific list reference  this a reference cannot accept any Object    i e  not every member of the family of types that a can point to can accept any Object  e g  an array list of Integers can only accept Integers    ArrayList lt   super Integer gt  a   new ArrayList lt Number gt       Again  a is a reference to a family of types  rather than a single specific type    Since the wildcard uses super  this list reference can accept Integers  but it cannot produce Integers  Said another way  we know that any and every member of the family of types that a can point to can accept an Integer   However  not every member of that family can produce Integers   PECS - Producer extends  Consumer super - This mnemonic helps you remember that using  extends means the generic type can produce the specific type  but cannot accept it   Using super means the generic type can consume  accept  the specific type  but cannot produce it    ArrayList lt ArrayList lt   gt  gt  a   An array list that holds references to any list that is a member of a family of array lists types     new ArrayList lt ArrayList lt   gt  gt        correct   An instance of an array list that holds references to any list that is a member of a family of array lists types   ArrayList lt   gt  a   An reference to any array list  a member of the family of array list types      new ArrayList lt   gt      ArrayList lt   gt  refers to any type from a family of array list types  but you can only instantiate a specific type     See also How can I add to List lt   extends Number gt  data structures

User · Answer

A lot of this has to do with polymorphism   When you assign  X   new Y      X can be much less  specific  than Y  but not the other way around   X is just the handle you are accessing Y with  Y is the real instantiated thing     You get an error here because Integer is a Number  but Number is not an Integer   ArrayList lt Integer gt  a   new ArrayList lt Number gt        compile-time error   As such  any method of X that you call must be valid for Y   Since X is more generally it probably shares some  but not all of Y s methods   Still  any arguments given must be valid for Y   In your examples with add  an int  small i  is not a valid Object or Integer   ArrayList lt   gt  a   new ArrayList lt   gt       This is no good because you can t actually instantiate an array list containing   s    You can declare one as such  and then damn near anything can follow in new ArrayList lt Whatever gt

User · Answer

You have strange expectations  If you gave the chain of arguments that led you to them  we might spot the flaw in them  As it is  I can only give a short primer on generics  hoping to touch on the points you might have misunderstood   ArrayList lt   extends Object gt  is an ArrayList whose type parameter is known to be Object or a subtype thereof   Yes  extends in type bounds has a meaning other than direct subclass   Since only reference types can be type parameters  this is actually equivalent to ArrayList lt   gt    That is  you can put an ArrayList lt String gt  into a variable declared with ArrayList lt   gt   That s why a1 add 3  is a compile time error  a1 s declared type permits a1 to be an ArrayList lt String gt   to which no Integer can be added   Clearly  an ArrayList lt   gt  is not very useful  as you can only insert null into it  That might be why the Java Spec forbids it      It is a compile-time error if any of   the type arguments used in a class   instance creation expression are   wildcard type arguments   ArrayList lt ArrayList lt   gt  gt  in contrast is a functional data type  You can add all kinds of ArrayLists into it  and retrieve them  And since ArrayList lt   gt  only contains but is not a wildcard type  the above rule does not apply

User · Answer

Think of the   as to mean  unknown   Thus   ArrayList lt   extends Object gt   is to say  an unknown type that  or as long as it extends Object   Therefore  needful to say  arrayList add 3  would be putting something you know  into an unknown  I e  Forgetting

[java] Java generics - ArrayList initialization

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