3503 COP Computer Science II CLASS NOTES
INTRODUCTION TO OOP Terminology
DAY #2
Class Complete description of an object Provides the model, or pattern, from which an object is created. Example: An architect creates a blueprint when designing a house. The blueprint defines the important characteristics of the house: walls, windows, doors, electrical outlets, etc. Once the blueprints are created, several houses can be built using the same blueprint. In one sense the houses built from the same blueprint are different. They are in physically different locations, have different addresses, different furniture, and different people live in them. Yet in many ways they are the same. The layout of the rooms is the same, the electrical outlets are in the same locations as are the windows, etc. To create a completely different house we need a different blueprint. A class is a blueprint of an object. It defines the type of data that will be held in an object and defines the code for the methods. But a class is not an object any more than a blueprint is a house. Container for methods and data. Reserves no space for data each object has its own data space (reserved at time of instantiation) Constructors are used to instantiate an object. Instantiation Once a class is defined, an object can be created from it. The process of creating an object is called instantiation. Every object is an instance of a particular class. Example: Just as several houses can be created from a single blueprint, several objects can be instantiated from the same class. They are the same type of object with the same methods, but each object is unique because each has its own data space with possibly different values. Once an object has been instantiated, the dot operator allows access to its methods. (Like System.out.println) Method A group of programming statements that are given a name. Specific tasks to be performed on the objects data. A complete description of an objects actions.
Day 2 - 1
The behavior of an object. Form is: return-type method-name (parameter-list) { statement-list } May declare local variables that cannot be accessed outside of the method. Shared amongst all objects of the class. Object Contains variables and methods. The values of the variables define the state of the object and the methods define the behavior of the object A region in memory in which information in the form of a collection of data and methods are stored for use by the program. Data = state, Methods = behavior An instance of a class. Object-Oriented Paradigm EVERYTHING is an object. Computations are performed by objects (this represents the objects behavior) on the state of an object (its data) and by communication with other objects (method calls). Each object has its own memory. Each object is an instance of a class. Classes are organized in a tree structure (e.g. inheritance hierarchy). Structure of an OOP A collection of classes. Some are specific and developed for the problem and some are general and developed as polymorphic solutions (recall polymorphic solutions). Inheritance hierarchy. (IS-A). apple [An is-a fruit.] Object-Oriented Programming Modeling real world scenarios and events with software components. A way of thinking about the problem at hand. The focus is on the objects and the interaction between them. A means of implementing the Object Paradigm. A "real-world" system.
Day 2 - 2
Examples: School environment (small) Code developed by one programmer. Usually only understood by the programmer (hopefully the instructor too!) Solves a particular problem by design. The developed system has a very short life cycle. Biggest problem confronting the programmer is usually the due date. Real world (large) Code is typically developed by a team of programmers with management input. Usually there is no one who understands what is going on in all parts of the project. Typically designed to solve a "systems" level problem. Typically designed to have a long life cycle. Biggest problem is commonly communication amongst the developers. Solution is modular (divide and conquer strategy at the macro level). Enforced encapsulation. Designed with a high degree of reusability, compatibilty, continuity, and polymorphism (+ dynamic binding). Concurrency Object Oriented Design Patterns These are a set or recurring patterns of interactions between objects. Containers A container represents a group of objects, called elements. The elements may be ordered, unordered, duplicated or unique depending upon implementation. An object which holds other objects within it. A container has capacity and objects can be inserted and removed. All containers support the following operations: test for empty, return size, position at beginning, and position past the end. The first is a Boolean test, the second returns a number, and the last two return an iterator. Iterators An object which allows access to and iteration through all of the elements in a container. All iterators share a common interface and encapsulate the container.
Day 2 - 3
Constructors Similar to a method and is used to initialize an object. Has the same name as the class of object that it initializes. References A reference (shorthand for a reference variable) is a variable that stores the memory address of where an object resides (or null if it references no object). Java does not allow references to primitive types (byte, short, int, long, float, double, char, Boolean). Primitive types are handled by value, nonprimitive types are handled by reference. Accessors and Mutators An accessor is a method which examines but does not change the state of an object. Special cases of accessors examine only a single field and typically have names beginning with get (i.e., getMonth, getNext, etc.). A mutator is a method which can change the state of an object (it mutates the state of the object). Special cases of mutators change only a single field and typically have names beginning with set (i.e., setDay, setValue, etc.). The advantage a mutator has over making the field public is that the mutator can ensure that changes to the state of the object are consistent (an integrity check).
Day 2 - 4