Object-Oriented Data Encapsulation & Types

Chapter 1: Foundations of Object-Oriented Programming

Data Encapsulation with Classes

The starting point of object-oriented programming is to provide a more faithful implementation of the notion of “type” in programming. The programming languages we are familiar with come with standard built-in types that we can assign to variables and values:

Built-in Programming Types

• In C, we can use int, float, double, char, etc.
• In Haskell, similarly, we can use Int, Float, Char, Bool, etc.

In addition, programming languages provide a way to assign a single name to a collection of values of the same type. The nature of this collection is determined by the underlying architecture assumed by the programming language.

Collections and Data Structures

Arrays in C: Random Access

Since C reflects the standard von Neumann stored program architecture, we have arrays that provide “random” (i.e., equal time) access to each element of the collection.

Lists in Haskell: Sequential Access

While Haskell, whose semantics is defined with respect to an idealized functional programming architecture, provides us with lists, which have “sequential access” (getting to the ith item takes time proportional to i) but which come with a number of useful decomposition functions that make inductive definitions easy to implement.

User-Defined Types and Abstract Data Types

However, no programming language can expect to predefine all the useful “types” that we might need in a particular application—we might want to work with stacks or binary search trees or directed acyclic graphs or... Typically, we define these in terms of the existing primitives available.

Stack Implementation Examples

For instance, in Haskell we might represent a stack as a list or, for efficiency, as a pair of lists, while in C we might represent a stack as an array or, if we want to let the stack grow arbitrarily large, as a linked list.

Stack Behavior and Abstraction

Regardless of the implementation, we expect a stack to be used in a particular way. Additions to the stack are done using a function “push”, deletions by a function “pop”, and the only inquiry that we can make about the state of the stack is the question “is-the-stack-empty”. Suppose the stack is defined as int s[100], top_of_stack = 0; in C and the stack grows as s[0], s[1], .... When the stack has 10 elements (i.e., top_of_st