Introduction to Programming Languages/Subtype Polymorphism

Subtype Polymorphism

The set of elements of a subtype is a subset of some existing set. The parameter definition of a function supports any argument of that type or a subtype. This way, if the parameters/operands of a function/operator have subtypes, that function/operator exhibits subtype polymorphism. The Java code below illustrates the use of this kind of polymorphism.

public class Sub {
  public static void print(Object o) {
    System.out.println(o);
  }
  public static void main(String[] a) {
    print(new String("dcc024"));
    print(new Integer(42));
    print(new Character('a'));
  }
}

In Java, the Object class is a superclass of all classes in Java. Every class in Java extends, directly or indirectly, from Object class. The Object class is the root of the class hierarchy in Java. In the code above, the line 2 defines a method print, which takes an Object as parameter and prints it using the method println of the System.out object. The lines from 6 to 8 show subtype polymorphic calls taking objects of String, Integer and Character as arguments. Every place where it is expected a class as parameter accepts a subclass of that class as parameter.