How to make a Java Generic method static

Question

The following is a snippet on how to make a java generic class to append a single item to an array   How can I make appendToArray a static method   Adding static to the method signature results in compile errors   public class ArrayUtils lt E gt             public E   appendToArray E   array  E item                E   result    E   new Object array length 1               result array length    item              return result

User · Answer

public static  lt E gt  E   appendToArray E   array  E item          Note the  lt E gt    Static generic methods need their own generic declaration  public static  lt E gt   separate from the class s generic declaration  public class ArrayUtils lt E gt     If the compiler complains about a type ambiguity in invoking a static generic method  again not likely in your case  but  generally speaking  just in case   here s how to explicitly invoke a static generic method using a specific type   class   lt  generictypeparams  gt  methodname     String   newStrings   ArrayUtils  lt String gt appendToArray strings   another string      This would only happen if the compiler can t determine the generic type because  e g  the generic type isn t related to the method arguments

User · Answer

I ll explain it in a simple way   Generics defined at Class level are completely separate from the generics defined at the  static  method level    class Greet lt T gt         public static  lt T gt  void sayHello T obj            System out println  Hello     obj             When you see the above code anywhere  please note that the T defined at the class level has nothing to do with the T defined in the static method  The following code is also completely valid and equivalent to the above code   class Greet lt T gt         public static  lt E gt  void sayHello E obj            System out println  Hello     obj             Why the static method needs to have its own generics separate from those of the Class      This is because  the static method can be called without even   instantiating the Class  So if the Class is not yet instantiated  we   do not yet know what is T  This is the reason why the static methods   needs to have its own generics    So  whenever you are calling the static method   Greet sayHello  Bob    Greet sayHello 123     JVM interprets it as the following   Greet  lt String gt sayHello  Bob    Greet  lt Integer gt sayHello 123     Both giving the same outputs   Hello Bob Hello 123

User · Answer

the only thing you can do is to change your signature to  public static  lt E gt  E   appendToArray E   array  E item    Important details   Generic expressions preceding the return value always introduce  declare  a new generic type variable   Additionally  type variables between types  ArrayUtils  and static methods  appendToArray  never interfere with each other   So  what does this mean  In my answer  lt E gt  would hide the E from ArrayUtils lt E gt  if the method wouldn t be static  AND  lt E gt  has nothing to do with the E from ArrayUtils lt E gt    To reflect this fact better  a more correct answer would be   public static  lt I gt  I   appendToArray I   array  I item

User · Answer

You need to move type parameter to the method level to indicate that you have a generic method rather than generic class   public class ArrayUtils       public static  lt T gt  E   appendToArray E   array  E item            E   result    E   new Object array length 1           result array length    item          return result

[java] How to make a Java Generic method static?

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