Inheritance in Programming Languages

Background

SIMULA introduced the concepts of object, class, inheritance, and polymorphism for describing discrete event simulations (Meyer, 1999). Subsequently, object-oriented programming languages such as Smalltalk, C++, Object Pascal., and so forth used these concepts for general purpose programming (Budd, 2002).

Object/Instance is a run-time structure with state and behavior. Object state is stored in its fields (variables) and behavior as its methods (functions). Class is a static description of objects. (In practice, a class itself can appear as a run-time structure manipulated using reflection APIs such as in Java, C#, CLOS, and so forth.) A class defines type of each field and code for each method, which inspects and/or transforms field values. (By field we mean instance field, and by method we mean instance method. The discussion of static fields and static methods, and access control primitives such as private, protected, and private, are beyond the scope of this chapter.) Inheritance is a binary relation between classes (say P and Q) that enables one to define a class (Q) in terms of another class (P) incrementally, by adding new fields, adding new methods, or modifying existing methods through overriding. A class Q is a subclass of class P if class Q inherits from class P.

class P { 
int i;
int f() { return 2;}
int f1() { return f() + i;}
void g() {}
}
class Q extends P { 
int j;
int f() { return 4;}
int h() { return i + j;}
}
class Main {
public static void main(String [] args) {
Q q = new Q();
P p = q;
p.f1();
}

}Every P-object has an int field i, and methods f(), f1() and g() defined on it. Every Q-object has int fields i and j, and methods f(), f1(), g() and h() defined on it. Q inherits i, f1(), and g() from P, and overrides f() from P.