Object-Oriented Programming Basics Reading for this week†: BG 1-4.10 UNIX 1,2,4,9 JAVA 0,1 †BG = Beginner’s Guide UNIX = Harley Hahn’s Student guide to Unix, Harley Hahn JAVA = An Introduction to Object Oriented Programming with Java, Thomas Wu
How can one design a program? • Top-down structured design: uses algorithmic decomposition where each module denotes a major step in some overall process • Object-oriented design: divides the problem into a set of objects that interacts to solve the problem. Your program’s properties and behaviors are modelled based upon real objects like cars, books, houses, etc.
Why OOD? • Software is complex (too many people is doing too many things – the mess is inevitable ) • One of the main goals in programming is to avoid the redundancy and objects can help to do this (inheritance) • Objects can help increase modularity through data hiding (encapsulation)
OK, but what is object ? • An object is a thing, either tangible or intangible. • Anobject-oriented program consists of many objects. • An object is composed of identity, state (attributes, data, and their current values) and behavior (operations) . • Identity? State? Behavior? Too many new expressions – I’ll get a headache!!!
Identity, State, Behavior • Identity is the property of an object that distinguishes it from all other objects. • The failure to recognize the difference between the name of the object and the object itself is the source of many errors in object-oriented (OO) programming.
Identity, State, Behavior • The state of an object encompasses all of the (static) properties of the object plus the current (dynamic) values of each of these properties • A property is an inherent or distinctive characteristic, trait, quality, or feature that contribute to making an object uniquely that object • We will use the word attribute, or data member, to refer to the state of an object
Examples of State • Properties • Elevators travel up or down • Vending machines accept coins • Clocks indicate the current time • Values • Current floor • Number of coins deposited • The number of minutes since the last hour
Identity, State,Behavior • Behavior is how an object acts and reacts, in terms of state changes and interactions with other objects. • An operation is some action that one object performs upon another in order to elicit a reaction. • We will use the word method to describe object behavior in java. • Invoking a method causes the behavior to take place.
A graphical representation of an object: < Object name > A graphical representation of a class: < Class name > Objects and Classes – graphical representation
Object example Audi 6 BMW Z3 Corvette • Notice that all objects are of the same type. All objects are cars!
Classes • Classes are the definitions (or blueprints) used to create objects. I’d say: descriptions of objects. • To make acar the manufacturer must first have a design from which to build the first car. Then, once all the problems are worked out, the design is used to build all the cars of that model.
Objects • An object is an instance of a class. • If we have a class definition called Car, then we can think of Audi, BMW, and Corvette as each being an instance (object) of the class Car, i.e., they are each a type of car.
Object example Audi 6 BMW Z3 Corvette Car Car Car • Notice that all objects are of the same type. All objects are cars!
Classes and Objects • An object is an instance of exactly one class!!! • Corvette can not be an instance of a car class and an instance of a plane class at the same time. • An instance of a class, an object, belongs to that particular class. • A Corvette is a car Corvette belongs to the class Car.
Classes • Once a class is defined you can create as many instances of the class (objects from the class) as you would like. • Once a blue print is completed for the 2003 Porsche 911, Porsche will use an assembly line to build as many instances of the 2003 Porsche 911 as they wish.
Class car and objects- graphically Car This line shows an instance-ofrelationship. Audi 6 BMW Z3 Corvette Car Car Car
Defining a class • Properties are variables which describe the essential characteristics of an object. • Properties of a car: color, model, make, how many doors, transmission type, direction of movement, etc. • Behaviors are methods that describe how the object behaves and how the properties may be modified. • Behavior of a car: braking, changing gears, opening doors, moving forwards or backwards, etc.
Defining a class Behavior or Method 1 Behavior Behavior or or Method 4 Method 2 Behavior or Method 3 Properties or Instance Variables
Instance variables • The class definition will include parameter definitions (properties) that represent data about a particular object, instance variables. • Example, Jessica's car may have 4 gallons of gas in it while Clint's car has 10 gallons. • The amount of gas in each car may change without affecting the amount of gas in the any other cars. • All instances (objects) of a class will have a set of instance variables that are specific to that individual object. • The combination of the values of these instance variables is known as the object’s state.
Instance variables Car MaxSpeed Audi 6 BMW Z3 Corvette Car Car Car MaxSpeed = 155 MaxSpeed = 165 MaxSpeed = 145
Class variables • The class definitions may also include parameter definitions that represent data that is shared by all class instances (objects), called class variables. • In the case of the car class, we will define a maximum allowed speed, by the law (variable LMaxSpeed). This will be the same for each individual car (that’s why your car have those annoying speed limiters ).
Class variables Car MaxSpeed LMaxSpeed=155 Audi 6 BMW Z3 Corvette Car Car Car MaxSpeed = 155 MaxSpeed = 165 MaxSpeed = 145
Class variables • Class variables may also be used to keep track of things such as how many instances of a class exist. • Example: let’s create a counter the records how many cars are in the garage.
Class variables Car MaxSpeed LMaxSpeed=155 NumCars = 3 Audi 6 BMW Z3 Corvette Car Car Car MaxSpeed = 155 MaxSpeed = 165 MaxSpeed = 145
Constant parameters • If it was variables instead of parameters, that would be oxymoron. • If there is a parameter whose value should not change while your program is running, that parameter type is called a constant. • The LMaxSpeed parameter that we defined for the Car class is a constant. The maximal speed is limited by the low, and it cannot be changed once the car has been built. • What about Z3 and 165 mph? Well, the owner has just made an illegal modification !!!
For Objects The object to whom the message is being sent. The name of the method (behavior) that object is to execute. Any parameters(variables) needed by that method. For Humans Who the message is for. What we want the person to do. What information is needed to do it. Messages Audi 6 • turnOnHazard()
Messages and Methods • In order to process a message, an object needs to have a method defined for the requested task. • A method is a small, well-defined piece of code that completes a specific task. • For our previous example, we need to define a method to turn on the car's hazard lights.
Messages and Methods Car MaxSpeed LMaxSpeed=155 NumCars = 3 turnOnHazard() Audi 6 BMW Z3 Corvette Car Car Car MaxSpeed = 155 turnOnHazard() MaxSpeed = 165 turnOnHazard() MaxSpeed = 145 turnOnHazard()
Instance methods • Each class can have methods that are specific to each object, called instance methods. • These can only affect that object's parameters, i.e., it’s instance variables. • Example: If BMW has 4 gallons of gas and someone puts 6 more gallons of gas in his/her car, the car now has 10 gallons. The amount of gas in Audi and Corvette is unchanged.
Messages and Methods Car MaxSpeed LMaxSpeed=155 NumCars = 3 turnOnHazard() addGass(amount) Audi 6 BMW Z3 Corvette Car Car Car MaxSpeed = 155 turnOnHazard() addGass(amount) MaxSpeed = 165 turnOnHazard() addGass(amount) MaxSpeed = 145 turnOnHazard() addGass(amount)
Methods • It is also possible that you want information from an object; in this case, you would define a method that sends (returns) a message back to the requester containing that information. • We need to know how much gas is in our cars, so we will create a new method that returns the value of GasLevel variable for our car.
turnOnHazard() turnOnHazard() addGass(amount) addGass(amount) getGasLevel():GasLevel getGasLevel():GasLevel Messages and Methods Car MaxSpeed GasLevel LMaxSpeed=155 NumCars = 3 Audi 6 Corvette BMW Z3 Car Car MaxSpeed = 155 GasLevel = 4 turnOnHazard() addGass(amount) getGasLevel():GasLevel Car MaxSpeed = 145 GasLevel = 6 turnOnHazard() addGass(amount) getGasLevel():GasLevel MaxSpeed = 165 GasLevel = 10
Class methods • Class methods are used to get or manipulate information about all objects created from the class. • Typically, class methods are changing class variables. For example: • Each time we move the car in or out of the garage, we need to add/subtract one to the number of cars: carIn( ) & carOut( ) • Also, we may want to know how many cars are actually in the garage: getNumCars( )
MaxSpeed GasLevel LMaxSpeed=155 NumCars = 3 Messages and Methods Car carIn() carOut() turnOnHazard() getNumCars():NumCars addGass(amount) getGasLevel():GasLevel Audi 6 BMW Z3 Corvette Car Car Car MaxSpeed = 155 GasLevel = 4 turnOnHazard() addGass(amount) getGasLevel():GasLevel MaxSpeed = 165 GasLevel = 10 turnOnHazard() addGass(amount) getGasLevel():GasLevel MaxSpeed = 145 GasLevel = 6 turnOnHazard() addGass(amount) getGasLevel():GasLevel
Types of methods There are 4 basic types of methods: • Modifier (sometimes called a mutator) • Changes the value associated with an attribute of the object (or class) • Accessor • Returns the value associated with an attribute of the object (or class) • Constructor • Called once when the object is created (before any other method will be invoked) • Destructor • Called when the object is destroyed
OOP • When writing object-oriented programs, first one must define the classes (like Car). • Then, while the program is running, the instances of the classes (objects) (such as Audi, BMW, Corvette in our example) are created.
OOP • Object-oriented programming allows the programmer to hide the implementation details from the other objects and the users. • In other words the implementation is transparent to the other objects or the user. • Example: Although our computers all are different “under the hood”, we don’t need to know what’s there to be able to use them.
OOP - benefits • An object can be written and maintained separately from the rest of the program, modularity. • An object has a “public face” that it uses to communicate with other objects, but other objects can not directly access its instance variables, information hiding.
Inheritance • All classes in Java are organized into a class hierarchy. • The highest level classes are very general and the lower level classes are more specific. • The lower level classes are based upon the higher level classes and inherit instance variables and methods from those higher level class. They also may contain their own (new) instance variables and methods beyond the higher level class definition.
Inheritance • A higher level class is called a superclass; a lower level class is called a subclass. • A subclass may also be a superclass • Inheritance allows you to define certain behaviors once and then to reuse those behaviors over and over again in the subclasses. This is called reusability.
Inheritance • Our Car class is very general. • Let's define a new class called BMW that contains the parameters: model, color, engine size.
MaxSpeed GasLevel LMaxSpeed=155 turnOnHazard() addGass(amount) getGasLevel():GasLevel Model Color EngineSize MaxSpeed GasLevel turnOnHazard() addGass(amount) getGasLevel():GasLevel Inheritance Car BMW
Inheritance • Now let's define two new classes. One for the Z3 and another for the 3 Series Sedan. • What might be some of the differences between the two classes? • Number of doors (3, 5) • Roof (soft or hardtop) • Therefore, we add variables NumDoors and Roof
BMW Z3 3 series MaxSpeed GasLevel LMaxSpeed=155 turnOnHazard() addGass(amount) getGasLevel():GasLevel Model Color EngineSize MaxSpeed GasLevel turnOnHazard() addGass(amount) getGasLevel():GasLevel Model Color EngineSize NumDoors MaxSpeed GasLevel turnOnHazard() addGass(amount) getGasLevel():GasLevel Model Color EngineSize Roof MaxSpeed GasLevel turnOnHazard() addGass(amount) getGasLevel():GasLevel Inheritance Car
Views of the class • A class can be viewed as a sort of contract that specifies what instances of the class can, and cannot do • It is possible to distinguish between the outside and inside view of a class • The interface of a class provides its outside view and emphasizes the abstraction • The implementation of a class is its inside view
Access • Most classes provide three levels of access to their members (state and behavior): • Public • The part of the class that is visible to all clients of the class • Protected • The part of the class that is only visible to subclasses of the class • Private • A part of the class that is not visible to any other classes
Primitive data types • What ?!!! Primitive? What is it? • Certain basic data types are built into Java: • Alphabetical: char • Integral: int, byte, short, long • Real: float, double • Logical: boolean • Java supports 8 primitive types (i.e. 4 basic types)
Primitive data types • When typing integer into a program, the number must have an ‘L’ at the end: 3932354162L • Real numbers are represented in scientific notation: • mantissa [-1,1] • exponent (powers of two)
Real data type • Characterized by precision and range • Precision: represents number of significant digits that is represented in the number. Depends upon the number of bits in the mantissa. • Range: represents the difference between the largest and smallest numbers that can be represented. Depends upon the number of bits in the exponent. • float: • 24b mantissa + 8b exponent • 6-7 decimal digits of precision • double: • 53b mantissa + 11b exponent • 15-16 decimal digits of precision