Static method in a generic class

Question

In Java  I d like to have something as   class Clazz lt T gt      static void doIt T object                       But I get  Cannot make a static reference to the non-static type T  I don t understand generics beyond the basic uses and thus can t make much sense of that  It doesn t help that I wasn t able to find much info on the internet about the subject   Could someone clarify if such use is possible  by a similar manner  Also  why was my original attempt unsuccessful

User · Answer

Something like the following would get you closer

class Clazz
{
   public static <U extends Clazz> void doIt(U thing)
   {
   }
}

EDIT: Updated example with more detail

public abstract class Thingo 
{

    public static <U extends Thingo> void doIt(U p_thingo)
    {
        p_thingo.thing();
    }

    protected abstract void thing();

}

class SubThingoOne extends Thingo
{
    @Override
    protected void thing() 
    {
        System.out.println("SubThingoOne");
    }
}

class SubThingoTwo extends Thingo
{

    @Override
    protected void thing() 
    {
        System.out.println("SuThingoTwo");
    }

}

public class ThingoTest 
{

    @Test
    public void test() 
    {
        Thingo t1 = new SubThingoOne();
        Thingo t2 = new SubThingoTwo();

        Thingo.doIt(t1);
        Thingo.doIt(t2);

        // compile error -->  Thingo.doIt(new Object());
    }
}

User · Answer

It is correctly mentioned in the error  you cannot make a static reference to non-static type T  The reason is the type parameter T can be replaced by any of the type argument e g  Clazz lt String gt  or Clazz lt integer gt  etc  But static fields methods are shared by all non-static objects of the class    The following excerpt is taken from the doc      A class s static field is a class-level variable shared by all   non-static objects of the class  Hence  static fields of type   parameters are not allowed  Consider the following class   public class MobileDevice lt T gt        private static T os                      If static fields of type parameters were allowed  then the following code would be confused   MobileDevice lt Smartphone gt  phone   new MobileDevice lt  gt     MobileDevice lt Pager gt  pager   new MobileDevice lt  gt     MobileDevice lt TabletPC gt  pc   new MobileDevice lt  gt           Because the static field os is shared by phone  pager  and pc  what is the actual type of os  It cannot be Smartphone  Pager  and   TabletPC at the same time  You cannot  therefore  create static fields   of type parameters    As rightly pointed out by chris in his answer you need to use type parameter with the method and not with the class in this case  You can write it like   static  lt E gt  void doIt E object

User · Answer

I ran into this same problem   I found my answer by downloading the source code for Collections sort in the java framework   The answer I used was to put the  lt T gt  generic in the method  not in the class definition   So this worked   public class QuickSortArray        public static  lt T extends Comparable gt  void quickSort T   array  int bottom  int top     do it        Of course  after reading the answers above I realized that this would be an acceptable alternative without using a generic class   public static void quickSort Comparable   array  int bottom  int top     do it

User · Answer

You can t use a class s generic type parameters in static methods or static fields  The class s type parameters are only in scope for instance methods and instance fields  For static fields and static methods  they are shared among all instances of the class  even instances of different type parameters  so obviously they cannot depend on a particular type parameter   It doesn t seem like your problem should require using the class s type parameter  If you describe what you are trying to do in more detail  maybe we can help you find a better way to do it

User · Answer

Java doesn t know what T is until you instantiate a type   Maybe you can execute static methods by calling Clazz lt T gt  doit something  but it sounds like you can t   The other way to handle things is to put the type parameter in the method itself   static  lt U gt  void doIt U object    which doesn t get you the right restriction on U  but it s better than nothing

User · Answer

Since static variables are shared by all instances of the class  For example if you are having following code  class Class lt T gt      static void doIt T object           using T here          T is available only after an instance is created  But static methods can be used even before instances are available  So  Generic type parameters cannot be referenced inside static methods and variables

User · Answer

It is possible to do what you want by using the syntax for generic methods when declaring your doIt   method  notice the addition of  lt T gt  between static and void in the method signature of doIt      class Clazz lt T gt      static  lt T gt  void doIt T object           shake that booty         I got Eclipse editor to accept the above code without the Cannot make a static reference to the non-static type T error and then expanded it to the following working program  complete with somewhat age-appropriate cultural reference    public class Clazz lt T gt      static  lt T gt  void doIt T object        System out println  shake that booty      object getClass   toString                                             private static class KC          private static class SunshineBand          public static void main String args          KC kc   new KC        SunshineBand sunshineBand   new SunshineBand        Clazz doIt kc       Clazz doIt sunshineBand           Which prints these lines to the console when I run it      shake that booty  class com eclipseoptions datamanager Clazz KC        shake that booty  class com eclipseoptions datamanager Clazz SunshineBand

User · Answer

I think this syntax has not been mentionned yet  in the case you want a method without arguments     class Clazz     static  lt T gt  T doIt            shake that booty         And the call    String str   Clazz  lt String gt doIt      Hope this help someone

User · Answer

When you specify a generic type for your class  JVM know about it only having an instance of your class  not definition  Each definition has only parametrized type   Generics work like templates in C    so you should first instantiate your class  then use the function with the type being specified

User · Answer

Also to put it in simple terms  it happens because of the  Erasure  property of the generics Which means that although we define ArrayList lt Integer gt  and ArrayList lt String gt    at the compile time it stays as two different concrete types but at the runtime the JVM erases generic types and creates only one ArrayList class instead of two classes  So when we define a static type method or anything for a generic  it is shared by all instances of that generic  in my example it is shared by both ArrayList lt Integer gt  and ArrayList lt String gt   That s why you get the error A Generic Type Parameter of a Class Is Not Allowed in a Static Context

User · Answer

BD at Rivenhill  Since this old question has gotten renewed attention last year  let us go on a bit  just for the sake of discussion  The body of your doIt method does not do anything T-specific at all  Here it is   public class Clazz lt T gt      static  lt T gt  void doIt T object        System out println  shake that booty      object getClass   toString                                                     So you can entirely drop all type variables and just code  public class Clazz     static void doIt Object object        System out println  shake that booty      object getClass   toString                                                     Ok  But let s get back closer to the original problem  The first type variable on the class declaration is redundant  Only the second one on the method is needed  Here we go again  but it is not the final answer  yet                                                         public class Clazz      static  lt T extends Saying gt  void doIt T object        System out println  shake that booty    object say            public static void main String args          Clazz doIt new KC         Clazz doIt new SunshineBand              Output     KC    Sunshine  interface Saying         public String say       class KC implements Saying         public String say               return  KC              class SunshineBand implements Saying         public String say               return  Sunshine               However  it s all too much fuss about nothing  since the following version works just the same way  All it needs is the interface type on the method parameter  No type variables in sight anywhere  Was that really the original problem   public class Clazz      static void doIt Saying object        System out println  shake that booty    object say            public static void main String args          Clazz doIt new KC         Clazz doIt new SunshineBand            interface Saying         public String say       class KC implements Saying         public String say               return  KC              class SunshineBand implements Saying         public String say               return  Sunshine

[java] Static method in a generic class?

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