Generic in Hack Language Facebook

Hack Language introduces the concept of generics to PHP. It allows a class or function or interface to define the type of it’s at the time of creation of its objects.
The rules that should be taken into consideration while writing generic methods are:

• The generic parameters must start with a Capital letter T.
• The generic method name should not be similar to any non-generic method name.
• Nullable T is supported ( i.e. ?T).
• Hack does not casting to T type or creating object of typeT.

Generic Class:

<?hh
 class GenericClass {
       private T $val;
       public function __construct(T $x) {
       $this->val = $x;
    }
    public function getValue(): T {
        return $this->val;
    } 
}

Craeting generic class objects:

<?hh
function main_func() {
    $objInt = new GenericClass(3);
    $objStr = new GenericClass ("Hi");
    $objArray = new GenericClass(array());
    echo $objInt->getValue()."\n";
    echo $objStr->getValue()."\n";
    echo $objArray->getValue()."\n";
}
main_func();

Generic Class:

<?hh
// generic interface
interface GenericInterface {
  public function addValue(T$item): void;
  public function removeValue(): T;
}
class UseClass implements Box {
   //Do some code
}