Object-Oriented Programming Concepts: Classes, Objects, and Access Modifiers, Exercises of Java Programming

Download Object-Oriented Programming Concepts: Classes, Objects, and Access Modifiers and more Exercises Java Programming in PDF only on Docsity! at Basic Object-Oriented Concepts py - 3-Jul-12 docsity.com 2 Concept: An object has behaviors  In old style programming, you had:  data, which was completely passive  functions, which could manipulate any data  An object contains both data and methods that manipulate that data  An object is active, not passive; it does things  An object is responsible for its own data  But: it can expose that data to other objects docsity.com 5 Concept: Classes describe objects  Every object belongs to (is an instance of) a class  An object may have fields, or variables  The class describes those fields  An object may have methods  The class describes those methods  A class is like a template, or cookie cutter  You use the class’s constructor to make objects docsity.com 6 Concept: Classes are like Abstract Data Types  An Abstract Data Type (ADT) bundles together:  some data, representing an object or "thing"  the operations on that data  The operations defined by the ADT are the only operations permitted on its data  Example: a CheckingAccount, with operations deposit, withdraw, getBalance, etc.  Classes enforce this bundling together  If all data values are private, a class can also enforce the rule that its defined operations are the only ones permitted on the data docsity.com 7 Example of a class class Employee { // Fields private String name; //Can get but not change private double salary; // Cannot get or set // Constructor Employee(String n, double s) { name = n; salary = s; } // Methods void pay () { System.out.println("Pay to the order of " + name + " $" + salary); } public String getName() { return name; } // getter } docsity.com 10 Example of (part of) a hierarchy A FileDialog is a Dialog is a Window is a Container Container Panel ScrollPane Window Dialog Frame FileDialog docsity.com 11 C++ is different  In C++ there may be more than one root  but not in Java!  In C++ an object may have more than one parent (immediate superclass)  but not in Java!  Java has a single, strict hierarchy docsity.com 12 Concept: Objects inherit from superclasses  A class describes fields and methods  Objects of that class have those fields and methods  But an object also inherits:  the fields described in the class's superclasses  the methods described in the class's superclasses  A class is not a complete description of its objects! docsity.com 15 Notation: How to declare and create objects Employee secretary; // declares secretary secretary = new Employee (); // allocates space Employee secretary = new Employee(); // does both  But the secretary is still "blank" (null) secretary.name = "Adele"; // dot notation secretary.birthday (); // sends a message docsity.com 16 Notation: How to reference a field or method  Inside a class, no dots are necessary class Person { ... age = age + 1; ...}  Outside a class, you need to say which object you are talking to if (john.age < 75) john.birthday ();  If you don't have an object, you cannot use its fields or methods! docsity.com 17 Concept: this object  Inside a class, no dots are necessary, because  you are working on this object  If you wish, you can make it explicit: class Person { ... this.age = this.age + 1; ...}  this is like an extra parameter to the method  You usually don't need to use this docsity.com 20 Concept: Methods can be overridden  So birds can fly. Except penguins. class Bird extends Animal { void fly (String destination) { location = destination; } } class Penguin extends Bird { void fly (String whatever) { } } docsity.com 21 Concept: Don't call functions, send messages Bird someBird = pingu; someBird.fly ("South America");  Did pingu actually go anywhere?  You sent the message fly(...) to pingu  If pingu is a penguin, he ignored it  Otherwise he used the method defined in Bird  You did not directly call any method  You cannot tell, without studying the program, which method actually gets used  The same statement may result in different methods being used at different times docsity.com 22 Sneaky trick: How to use overridden methods class FamilyMember extends Person { void birthday () { // override birthday() in Person super.birthday (); // call overridden method givePresent (); // and add your new stuff } } docsity.com 25 Trick: Give field and parameter the same name  A parameter overrides a field with the same name  But you can use this.name to refer to the field  class Person { String name; int age; Person (String name, int age) { this.name = name; this.age = age; } }  Using the same name is a common and useful convention docsity.com 26 Internal workings: Constructor chaining  If an Employee is a Person, and a Person is an Object, then when you say new Employee ()  The Employee constructor calls the Person constructor  The Person constructor calls the Object constructor  The Object constructor creates a new Object  The Person constructor adds its own stuff to the Object  The Employee constructor adds its own stuff to the Person docsity.com 27 The case of the vanishing constructor  If you don't write a constructor for a class, Java provides one (the default constructor)  The one Java provides has no arguments  If you write any constructor for a class, Java does not provide a default constructor  Adding a perfectly good constructor can break a constructor chain  You may need to fix the chain docsity.com 30 Trick: one constructor calling another  this(...) calls another constructor for this same class  It is poor style to have the same code more than once  If you call this(...), that call must be the first thing in your constructor class Something { Something (int x, int y, int z) { // do a lot of work here } Something ( ) { this (0, 0, 0); } } docsity.com 31 Concept: You can control access class Person { public String name; private String age; protected double salary; public void birthday { age++; } }  Each object is responsible for its own data  Access control lets an object protect its data and its methods  Access control is the subject of a different lecture docsity.com 32 Concept: Classes can have fields and methods  Usually a class describes fields (variables) and methods for its objects (instances)  These are called instance variables and instance methods  A class can have its own fields and methods  These are called class variables and class methods  There is exactly one copy of a class variable, not one per object  Use the special keyword static to say that a field or method belongs to the class instead of to objects docsity.com 35 Advice: Use setters and getters  This way the object maintains control  Setters and getters have conventional names: setDataName, getDataName, isDataName (booleans only) class Employee extends Person { private double salary; private boolean male; public void setSalary (double newSalary) { salary = newSalary; } public double getSalary () { return salary; } public boolean isMale() { return male; } } docsity.com 36 Kinds of access  Java provides four levels of access:  public: available everywhere  protected: available within the package (in the same subdirectory) and to all subclasses  [default]: available within the package  private: only available within the class itself  The default is called package visibility  In small programs this isn't important...right? docsity.com oft The End 37 docsity.com