Difference between List List List T List E and List Object

Question

What are the differences between List  List lt   gt   List lt T gt   List lt E gt   and List lt Object gt    1  List  List  is a raw type  therefore not typesafe  It will only generate a runtime error when the casting is bad  We want a compile time error when the cast is bad  Not recommended to use   2  List lt   gt   List lt   gt  is an unbounded wildcard  But I m not sure what it s for  I can print a List lt   gt  without issue   public static void test List lt   gt  list       System out println list        Works     Why can t I add items to a List lt   gt    public static void test List lt   gt  list       list add new Long 2           Error     list add  2                   Error     System out println list       3  List lt T gt   public static void test List lt T gt  list        T cannot be resolved     System out println list       I don t understand this syntax  I saw something like this  and it works   public  lt T gt  T   toArray T   a       return a         Sometimes  I see  lt T gt   or  lt E gt   or  lt U gt    lt T E gt   Are they all the same or do they represent something different   4  List lt Object gt   This gives the error  The method test List lt Object gt   is not applicable for the argument List lt String gt     public static void test List lt Object gt  list       System out println list       If I try this then I got  Cannot cast from List lt String gt  to List lt Object gt     test  List lt Object gt   names     I am confused  String is a subclass of Object  so why isn t List lt String gt  a subclass of List lt Object gt

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The reason you cannot cast List lt String gt  to List lt Object gt  is that it would allow you to violate the constraints of the List lt String gt    Think about the following scenario  If I have a List lt String gt   it is supposed to only contain objects of type String   Which is a final class   If I can cast that to a List lt Object gt   then that allows me to add Object to that list  thus violating the original contract of List lt String gt    Thus  in general  if class C inherits from class P  you cannot say that GenericType lt C gt  also inherits from GenericType lt P gt    N B  I already commented on this in a previous answer but wanted to expand on it

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You re right  String is a subset of Object  Since String is more  precise  than Object  you should cast it to use it as an argument for System out println

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Theory  String   can be cast to Object    but  List lt String gt  cannot be cast to List lt Object gt    Practice  For lists it is more subtle than that  because at compile time the type of a List parameter passed to a method is not checked   The method definition might as well say List lt   gt  - from the compiler s point of view it is equivalent   This is why the OP s example  2 gives runtime errors not compile errors   If you handle a List lt Object gt  parameter passed to a method carefully so you don t force a type check on any element of the list  then you can have your method defined using List lt Object gt  but in fact accept a List lt String gt  parameter from the calling code   A   So this code will not give compile or runtime errors and will actually  and maybe surprisingly   work   public static void main String   args        List argsList   new ArrayList lt String gt         argsList addAll Arrays asList args        test argsList       The object passed here is a List lt String gt     public static void test List lt Object gt  set        List lt Object gt  params   new ArrayList lt  gt         This is a List lt Object gt      params addAll set            Each String in set can be added to List lt Object gt      params add new Long 2        A Long can be added to List lt Object gt      System out println params       B   This code will give a runtime error   public static void main String   args        List argsList   new ArrayList lt String gt         argsList addAll Arrays asList args        test1 argsList       test2 argsList      public static void test1 List lt Object gt  set        List lt Object gt  params   set      Surprise   Runtime error    public static void test2 List lt Object gt  set        set add new Long 2             Also a runtime error     C   This code will give a runtime error  java lang ArrayStoreException  java util Collections UnmodifiableRandomAccessList Object      public static void main String   args        test args      public static void test Object   set        Object   params   set        This is OK even at runtime     params 0    new Long 2       Surprise   Runtime error     In B  the parameter set is not a typed List at compile time  the compiler sees it as List lt   gt    There is a runtime error because at runtime  set becomes the actual object passed from main    and that is a List lt String gt    A List lt String gt  cannot be cast to List lt Object gt    In C  the parameter set requires an Object     There is no compile error and no runtime error when it is called with a String   object as the parameter   That s because String   casts to Object     But the actual object received by test   remains a String    it didn t change   So the params object also becomes a String     And element 0 of a String   cannot be assigned to a Long    Hopefully I have everything right here  if my reasoning is wrong I m sure the community will tell me   UPDATED  I have updated the code in example A so that it actually compiles  while still showing the point made

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The notation List lt   gt  means  a list of something  but I m not saying what    Since the code in test works for any kind of object in the list  this works as a formal method parameter   Using a type parameter  like in your point 3   requires that the type parameter be declared  The Java syntax for that is to put  lt T gt  in front of the function  This is exactly analogous to declaring formal parameter names to a method before using the names in the method body   Regarding List lt Object gt  not accepting a List lt String gt   that makes sense because a String is not Object  it is a subclass of Object  The fix is to declare public static void test List lt   extends Object gt  set       But then the extends Object is redundant  because every class directly or indirectly extends Object

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For the last part  Although String is a subset of Object  but List lt String  is not inherited from List lt Object

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In your third point   T  cannot be resolved because its not declared  usually when you declare a generic class you can use  T  as the name of the bound type parameter  many online examples including oracle s tutorials use  T  as the name of the type parameter  say for example  you declare a class like   public class FooHandler lt T gt       public void operateOnFoo T foo      some foo handling code here         you are saying that FooHandler s operateOnFoo method expects a variable of type  T  which is declared on the class declaration itself  with this in mind  you can later add another method like  public void operateOnFoos List lt T gt  foos    in all the cases either T  E or U there all identifiers of the type parameter  you can even have more than one type parameter which uses the syntax  public class MyClass lt Atype AnotherType gt       in your forth ponint although efectively Sting is a sub type of Object  in generics classes there is no such relation  List lt String gt  is not a sub type of List lt Object gt  they are two diferent types from the compiler point of view  this is best explained in this blog entry

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1  Correct  2  You can think of that one as  read only  list  where you don t care about the type of the items Could e g  be used by a method that is returning the length of the list    3  T  E and U are the same  but people tend to use e g  T for type  E for Element  V for value and K for key  The method that compiles says that it took an array of a certain type  and returns an array of the same type   4  You can t mix oranges and apples  You would be able to add an Object to your String list if you could pass a string list to a method that expects object lists   And not all objects are strings

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I would advise reading Java puzzlers  It explains inheritance  generics  abstractions  and wildcards in declarations quite well   http   www javapuzzlers com

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Let us talk about them in the context of Java history     List     List  means it can include any Object  List was in the release before Java 5 0  Java 5 0 introduced List  for backward compatibility    List list new  ArrayList    list add anyObject      List lt   gt         means unknown Object not any Object  the wildcard   introduction is for solving the problem built by Generic Type  see wildcards  but this also causes another problem   Collection lt   gt  c   new ArrayList lt String gt     c add new Object        Compile time error    List lt  T gt  List lt  E gt     Means generic Declaration at the premise of none T or E type in your project Lib    List lt  Object gt  means  generic parameterization

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Problem 2 is OK  because   System out println set    means  System out println set toString      set is an instance of List  so complier will call List toString     public static void test List lt   gt  set   set add new Long 2      -- gt  Error   set add  2         -- gt  Error System out println set      Element   will not promise Long and String  so complier will  not accept Long and String Object  public static void test List lt String gt  set   set add new Long 2      -- gt  Error set add  2         -- gt  Work System out println set     Element String promise it a String  so complier will accept String Object   Problem 3  these symbols are same  but you can give them differet specification  For example   public  lt T extends Integer E extends String gt  void p T t  E e       Problem 4  Collection does not allow type parameter covariance  But array does allow covariance

[java] Difference between List, List<?>, List<T>, List<E>, and List<Object>

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