Discovering Classes and Objects - Beginning Java 7 - page 148

}
class Subclass extends Superclass
{

@Override
SubReturnType createReturnType()
{

return new SubReturnType ();

}

}
class CovarDemo
{

public static void main(String[] args)
{

SuperReturnType suprt = new Super-

class().createReturnType();
System.out.println(suprt); // Output: superclass re-
turn type
SubReturnType subrt = new Sub-

class().createReturnType();
System.out.println(subrt); // Output: subclass return

type
}

}

Listing 2-40 declares SuperReturnType and Superclass superclasses, and
SubReturnType and Subclass subclasses;eachof Superclass and Subclass
declares a createReturnType() method. Superclass 's method has its return
typesetto SuperReturnType ,whereas Subclass 'soverridingmethodhasitsre-
turn type set to SubReturnType , a subclass of SuperReturnType .
Covariant return types minimize upcasting and downcasting. For example, Sub-
class 's createReturnType() method does not need to upcast its
SubReturnType instancetoits SubReturnType returntype.Furthermore,thisin-
stancedoesnotneedtobedowncastto SubReturnType whenassigningtovariable
subrt .
In the absence of covariant return types, you would end up with Listing 2-41 .

Listing 2-41. Upcasting and downcasting in the absence of covariant return types