Java Generic Restrictions
In this chapter you will learn:
- Type Parameters Can't Be Instantiated
- What are restrictions on static members
- What are restrictions on generic array
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;//from w w w. j a v a2 s. c o m
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;
// ww w . ja va2 s. 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;//w ww . j av a 2 s . 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
}
}
Next chapter...
What you will learn in the next chapter: