List Map String String vs List extends Map String String

Question

Is there any difference between  List lt Map lt String  String gt  gt    and  List lt   extends Map lt String  String gt  gt       If there is no difference  what is the benefit of using   extends

User · Answer

Today  I have used this feature  so here s my very fresh real-life example   I have changed class and method names to generic ones so they won t distract from the actual point    I have a method that s meant to accept a Set of A objects that I originally wrote with this signature   void myMethod Set lt A gt  set    But it want to actually call it with Sets of subclasses of A  But this is not allowed   The reason for that is  myMethod could add objects to set that are of type A  but not of the subtype that set s objects are declared to be at the caller s site  So this could break the type system if it were possible    Now here come generics to the rescue  because it works as intended if I use this method signature instead    lt T extends A gt  void myMethod Set lt T gt  set    or shorter  if you don t need to use the actual type in the method body   void myMethod Set lt   extends A gt  set    This way  set s type becomes a collection of objects of the actual subtype of A  so it becomes possible to use this with subclasses without endangering the type system

User · Answer

As you mentioned  there could be two below versions of defining a List    List lt   extends Map lt String  String gt  gt  List lt   gt    2 is very open  It can hold any object type  This may not be useful in case you want to have a map of a given type  In case someone accidentally puts a different type of map  for example  Map lt String  int gt   Your consumer method might break   In order to ensure that List can hold objects of a given type  Java generics introduced   extends  So in  1  the List can hold any object which is derived from Map lt String  String gt  type  Adding any other type of data would throw an exception

User · Answer

You cannot assign expressions with types such as List lt NavigableMap lt String String gt  gt  to the first    If you want to know why you can t assign List lt String gt  to List lt Object gt  see a zillion other questions on SO

User · Answer

The difference is that  for example  a  List lt HashMap lt String String gt  gt    is a  List lt   extends Map lt String String gt  gt    but not a  List lt Map lt String String gt  gt    So   void withWilds  List lt   extends Map lt String String gt  gt  foo     void noWilds  List lt Map lt String String gt  gt  foo      void main  String   args        List lt HashMap lt String String gt  gt  myMap       withWilds  myMap       Works     noWilds  myMap       Compiler error     You would think a List of HashMaps should be a List of Maps  but there s a good reason why it isn t   Suppose you could do   List lt HashMap lt String String gt  gt  hashMaps   new ArrayList lt HashMap lt String String gt  gt      List lt Map lt String String gt  gt  maps   hashMaps     Won t compile                                               but imagine that it could  Map lt String String gt  aMap   Collections singletonMap  foo   bar       Not a HashMap  maps add  aMap       Perfectly legal  adding a Map to a List of Maps      But maps and hashMaps are the same object  so this should be the same as  hashMaps add  aMap       Should be illegal  aMap is not a HashMap    So this is why a List of HashMaps shouldn t be a List of Maps

User · Answer

What I m missing in the other answers is a reference to how this relates to co- and contravariance and sub- and supertypes  that is  polymorphism  in general and to Java in particular  This may be well understood by the OP  but just in case  here it goes   Covariance  If you have a class Automobile  then Car and Truck are their subtypes  Any Car can be assigned to a variable of type Automobile  this is well-known in OO and is called polymorphism  Covariance refers to using this same principle in scenarios with generics or delegates  Java doesn t have delegates  yet   so the term applies only to generics   I tend to think of covariance as standard polymorphism what you would expect to work without thinking  because   List lt Car gt  cars  List lt Automobile gt  automobiles   cars     You d expect this to work because Car is-a Automobile  but    throws inconvertible types compile error    The reason of the error is  however  correct  List lt Car gt  does not inherit from List lt Automobile gt  and thus cannot be assigned to each other  Only the generic type parameters have an inherit relationship  One might think that the Java compiler simply isn t smart enough to properly understand your scenario there  However  you can help the compiler by giving him a hint   List lt Car gt  cars  List lt   extends Automobile gt  automobiles   cars       no error   Contravariance  The reverse of co-variance is contravariance  Where in covariance the parameter types must have a subtype relationship  in contravariance they must have a supertype relationship  This can be considered as an inheritance upper-bound  any supertype is allowed up and including the specified type   class AutoColorComparer implements Comparator lt Automobile gt      public int compare Automobile a  Automobile b               Return comparison of colors         This can be used with Collections sort   public static  lt T gt  void sort List lt T gt  list  Comparator lt   super T gt  c      Which you can call like this  without errors  List lt Car gt  cars   getListFromSomewhere    Collections sort cars  new AutoColorComparer       You could even call it with a comparer that compares objects and use it with any type   When to use contra or co-variance   A bit OT perhaps  you didn t ask  but it helps understanding answering your question  In general  when you get something  use covariance and when you put something  use contravariance  This is best explained in an answer to Stack Overflow question How would contravariance be used in Java generics    So what is it then with List lt   extends Map lt String  String gt  gt   You use extends  so the rules for covariance applies  Here you have a list of maps and each item you store in the list must be a Map lt string  string gt  or derive from it  The statement List lt Map lt String  String gt  gt  cannot derive from Map  but must be a Map   Hence  the following will work  because TreeMap inherits from Map   List lt Map lt String  String gt  gt  mapList   new ArrayList lt Map lt String  String gt  gt     mapList add new TreeMap lt String  String gt        but this will not   List lt   extends Map lt String  String gt  gt  mapList   new ArrayList lt   extends Map lt String  String gt  gt     mapList add new TreeMap lt String  String gt        and this will not work either  because it does not satisfy the covariance constraint   List lt   extends Map lt String  String gt  gt  mapList   new ArrayList lt   extends Map lt String  String gt  gt     mapList add new ArrayList lt String gt           This is NOT allowed  List does not implement Map   What else   This is probably obvious  but you may have already noted that using the extends keyword only applies to that parameter and not to the rest  I e   the following will not compile   List lt   extends Map lt String  String gt  gt  mapList   new List lt   extends Map lt String  String gt  gt     mapList add new TreeMap lt String  Element gt         This is NOT allowed   Suppose you want to allow any type in the map  with a key as string  you can use extend on each type parameter  I e   suppose you process XML and you want to store AttrNode  Element etc in a map  you can do something like   List lt   extends Map lt String    extends Node gt  gt  listOfMapsOfNodes   new         Now you can do  listOfMapsOfNodes add new TreeMap lt Sting  Element gt      listOfMapsOfNodes add new TreeMap lt Sting  CDATASection gt

[java] List<Map<String, String>> vs List<? extends Map<String, String>>

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