Abstract Data Type (ADT)

Introduction

Think of the contacts app on your phone.

It stores data: names, phone numbers, email addresses. It gives you operations: add contact, delete contact, search by name.

Each operation has a defined behaviour. Add saves a new contact. Delete removes an existing one. Search finds a contact by name.

You use these operations without knowing how contacts are stored inside the phone's memory. That internal detail is hidden from you.

This is the idea behind an ADT: define what data is stored and what the operations do, without specifying how they are implemented.

Example: Stack ADT

A stack (covered in detail in a later lesson) stores a collection of elements. It supports these operations:

push(x)   -> insert element x onto the stack
pop()     -> remove and return the top element
peek()    -> return the top element without removing it
isEmpty() -> return true if the stack has no elements

The Stack ADT does not say whether to use an array or a linked list internally. Both can implement it. The ADT is only the specification, not the construction.

ADT vs implementation

An ADT describes what operations exist and what behaviour they provide. It says nothing about how they are implemented.

Quick recall

• An ADT defines data and operations, not implementation.
• The Stack ADT specifies push, pop, peek, and isEmpty, without saying how they are built.
• ADT describes behaviour; implementation describes the actual mechanism and code.