23.9. Chapter Summary

[Page 703 ( continued )]

21.7. Important Facts

It is important to note that a generic class is shared by all its instances regardless of its actual concrete type. Suppose stack1 and stack2 are created as follows :

 GenericStack<String> stack1 =   new   GenericStack<String>(); GenericStack<Integer> stack2 =   new   GenericStack<Integer>(); 

Although GenericStack<String> and GenericStack<Integer> are two types, there is only one GenericStack class loaded into the JVM. stack1 and stack2 are both instances of GenericStack . So the following statements display true :

 System.out.println(stack1   instanceof   GenericStack); System.out.println(stack1   instanceof   GenericStack<?>); System.out.println(stack2   instanceof   GenericStack); System.out.println(stack2   instanceof   GenericStack<?>); 

But the expression stack1 instanceof GenericStack<String> is wrong. Since GenericStack<String> is not stored as a separate class in the JVM, it makes no sense to use it in casting or instanceof expression. The following expressions are illegal:

 stack1   instanceof   GenericStack<String> (GenericStack<String>)stack1 

Since all instances of a generic class have the same runtime class, the static variables and methods of a generic class are shared by all its instances. Therefore, it is illegal to refer to a generic type parameter for a class in a static method or initializer. For example, the following code is illegal:

   public class   Test  <E>  {   public      static      void   m(  E  o1) {  // illegal  }    static    {  E  o2;  // illegal  } }