Extending Subtypes in Contexts

<aside> 💡 If S is compatible with T, S[] is compatible with T[]

</aside>

Example

ETicket[] etktarr = new ETicket[10];
Ticket[] tktarr = etktarr;
// Valid since ETicket is a subtype of Ticket

But now:

tktarr[5] = new Ticket()
//Invalid -> tktarr[5] refers to an ETicket and you trying to store Ticket which does not support all functionalities of ETicket.

• This error is only caught at runtime → because the above code is valid during compile time(string a Ticket object in a Ticket object array).

Thus

<aside> 💡 Java array typing is Covariant.

</aside>

• i.e. If S extends T, then S[] extends T[]

Generics and Subtypes

• Now, Generic classes are not Covariant.

  • i.e. LinkedList<String> is not compatible with LinkedList<Object>.

• The following will not work to print out an arbitrary LinkedList

  public class LinkedList<T>{...}
  
  public static void printlist(LinkedList<Object> l){
  	Object o;
  	Iterator i = l.get_iterator();
  	while(i.has_next()){
  		o = i.get_next();
  		System.out.println(o);
  	}
  }	
  

  • This will only work for LinkedList of type Object and not any other type.

• Solution: Make the above method generic by introducing type variable:

  public class LinkedList<T> { ... }
  
  public static <T> void printlist(LinkedList<T> l){
  		Object o;
  		Iterator i = l.get_iterator();
  		while(i.has_next()){
  			o = i.get_next();
  			System.out.println(o);
  	}
  }
  

  • Even though we can use Object o instead of T o also as above.

• Note that we are not using T anywhere inside our function → Instead we use Object o

Wildcards

• Instead of using T we can just use a wildcard ? as a type variable above.

  public class LinkedList<T>{ ... }
  
  public static void printlist(LinkedList<?> l){
  	Object o;
  	Iterator i = l.get_iterator();
  	while(i.has_next()){
  		o = i.get_next();
  		System.out.println(o);
  	}
  }
  

  • ? → Arbitrary Unknown Type

• We can define variables of the wildcard type(Only define the variable→ Cannot assign a new object of type ?):

  public class LinkedList<T>{ .. }
  
  LinkedList<?> l;   # LinkedList l of arbitrary type.