Java Tutorial - What are restrictions for Java generic types

Type Parameters Can't Be Instantiated

It is not possible to create an instance of a type parameter. For example, consider this class:

// Can't create an instance of T. 
class Gen<T> {
 T ob;
 Gen() {
 ob = new T(); // Illegal!!!
 }
}

Restrictions on Static Members

No static member can use a type parameter declared by the enclosing class. For example, all of the static members of this class are illegal:

class Wrong<T> {
 // Wrong, no static variables of type T.
 static T ob;
/*www.java2s.com*/
 // Wrong, no static method can use T.
 static T getob() {
 return ob;
 }
 // Wrong, no static method can access object of type T.
 static void showob() {
 System.out.println(ob);
 }
}

You can declare static generic methods with their own type parameters.

Generic Array Restrictions

You cannot instantiate an array whose base type is a type parameter. You cannot create an array of type specific generic references.

The following short program shows both situations:

class MyClass<T extends Number> {
 T ob;/*fromwww.java2s.com*/
 T vals[];
 MyClass(T o, T[] nums) {
 ob = o;
 vals = nums;
 }
}
public class Main {
 public static void main(String args[]) {
 Integer n[] = { 1 };
 MyClass<Integer> iOb = new MyClass<Integer>(50, n);
 // Can't create an array of type-specific generic references.
 // Gen<Integer> gens[] = new Gen<Integer>[10]; 
 MyClass<?> gens[] = new MyClass<?>[10]; // OK
 }
}