What is the difference between E T and for Java generics

Question

I come across Java code like this   public interface Foo lt E gt      public interface Bar lt T gt      public interface Zar lt   gt       What is the difference among all three of the above and what do they call this type of class or interface declarations in Java

User · Accepted Answer

Well there s no difference between the first two - they re just using different names for the type parameter  E or T    The third isn t a valid declaration -   is used as a wildcard which is used when providing a type argument  e g  List lt   gt  foo       means that foo refers to a list of some type  but we don t know what   All of this is generics  which is a pretty huge topic  You may wish to learn about it through the following resources  although there are more available of course    Java Tutorial on Generics Language guide to generics Generics in the Java programming language Angelika Langer s Java Generics FAQ  massive and comprehensive  more for reference though

User · Answer

The most commonly used type parameter names are   E - Element  used extensively by the Java Collections Framework  K - Key N - Number T - Type V - Value S U V etc  - 2nd  3rd  4th types   You ll see these names used throughout the Java SE API

User · Answer

compiler will make a capture for each wildcard  e g   question mark in List  when it makes up a function like   foo List lt   gt  list        list put list get       ERROR  capture and Object are not identical type      However a generic type like V would be ok and making it a generic method    lt V gt void foo List lt V gt  list        list put list get

User · Answer

The previous answers explain type parameters  T  E  etc    but don t explain the wildcard       or the differences between them  so I ll address that   First  just to be clear  the wildcard and type parameters are not the same  Where type parameters define a sort of variable  e g   T  that represents the type for a scope  the wildcard does not  the wildcard just defines a set of allowable types that you can use for a generic type  Without any bounding  extends or super   the wildcard means  use any type here    The wildcard always come between angle brackets  and it only has meaning in the context of a generic type   public void foo List lt   gt  listOfAnyType            pass a List of any type   never  public  lt   gt    bar   someType            error  Must use type params here   or  public class MyGeneric             error     public   getFoo                error             It gets more confusing where they overlap   For example   List lt T gt  fooList      A list which will be of type T  when T is chosen                       Requires T was defined above in this scope List lt   gt  barList      A list of some type  decided elsewhere  You can do                      this anywhere  no T required    There s a lot of overlap in what s possible with method definitions   The following are  functionally  identical   public  lt T gt  void foo List lt T gt  listOfT        public void bar List lt   gt  listOfSomething           So  if there s overlap  why use one or the other   Sometimes  it s honestly just style  some people say that if you don t need a type param  you should use a wildcard just to make the code simpler more readable   One main difference I explained above  type params define a type variable  e g   T  which you can use elsewhere in the scope  the wildcard doesn t   Otherwise  there are two big differences between type params and the wildcard   Type params can have multiple bounding classes  the wildcard cannot   public class Foo  lt T extends Comparable lt T gt   amp  Cloneable gt          The wildcard can have lower bounds  type params cannot   public void bar List lt   super Integer gt  list          In the above the List lt   super Integer gt  defines Integer as a lower bound on the wildcard  meaning that the List type must be Integer or a super-type of Integer   Generic type bounding is beyond what I want to cover in detail   In short  it allows you to define which types a generic type can be   This makes it possible to treat generics polymorphically   E g  with   public void foo List lt   extends Number gt  numbers          You can pass a List lt Integer gt   List lt Float gt   List lt Byte gt   etc  for numbers   Without type bounding  this won t work -- that s just how generics are   Finally  here s a method definition which uses the wildcard to do something that I don t think you can do any other way   public static  lt T extends Number gt  void adder T elem  List lt   super Number gt  numberSuper        numberSuper add elem       numberSuper can be a List of Number or any supertype of Number  e g   List lt Object gt    and elem must be Number or any subtype   With all the bounding  the compiler can be certain that the  add   is typesafe

User · Answer

It s more convention than anything else    T is meant to be a Type   E is meant to be an Element  List lt E gt   a list of Elements    K is Key  in a Map lt K V gt     V is Value  as a return value or mapped value      They are fully interchangeable  conflicts in the same declaration notwithstanding

User · Answer

A type variable   lt T   can be any non-primitive type you specify  any class type  any interface type  any array type  or even another type variable   The most commonly used type parameter names are    E - Element  used extensively by the Java Collections Framework  K - Key  N - Number  T - Type  V - Value    In Java 7 it is permitted to instantiate like this   Foo lt String  Integer gt  foo   new Foo lt  gt        Java 7 Foo lt String  Integer gt  foo   new Foo lt String  Integer gt        Java 6

[java] What is the difference between 'E', 'T', and '?' for Java generics?

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