Classes and Objects in Java Jan 14 2019 Classes and Objects A Java program consists of one or more classes A class is an abstract description of objects Here is an example class class Dog description of a dog goes here Here are some objects of that class 2 More Objects Here is another example of a class class Window Here are some examples of Windows 3 Classes contain data definitions Classes describe the data held by each of its objects Example Data usually goes first in a class A class may describe any number of objects class Dog String name int age rest of the class Examples Fido 3 Rover 5 Spot 3 A class may describe a single object or even no objects at all 4 Classes contain methods A class may contain methods that describe the behavior of objects Example Methods usually go after the data class Dog void bark System out println Woof When we ask a particular Dog to bark it says Woof Only Dog objects can bark the class Dog cannot bark 5 Methods contain statements A statement causes the object to do something A better word would be command but it isn t Example System out println Woof This causes the particular Dog to print actually display on the screen the characters Woof 6 Methods may contain temporary data Data described in a class exists in all objects of that class Example Every Dog has its own name and age A method may contain local temporary data that exists only until the method finishes Example void wakeTheNeighbors int i 50 i is a temporary variable while i 0 bark i i 1 7 Classes always contain constructors A constructor is a piece of code that constructs or creates a new object of that class If you don t write a constructor Java defines one for you behind the scenes You can write your own constructors Example This part is the class Dog constructor String name int age Dog String n int age name n this age age 8 Diagram of program structure Progra m Fil e Fil e Clas s Variable s Constructo rs Method s Variable s Statements Variable s Statement s Fil e Fil e A program consists of one or more classes Typically each class is in a separate java file 9 Summary A program consists of one or more classes A class is a description of a kind of object A class describes data constructors and methods In most cases it is the objects that do the actual work An object s data is information about that object An object s methods describe how the object behaves A constructor is used to create objects of the class Methods and constructors may contain temporary data and statements commands 10 Writing and running programs When you write a program you are writing classes and all the things that go into classes Your program typically contains commands to create objects that is calls to constructors When you run a program it creates objects and those objects interact with one another and do whatever they do to cause something to happen Analogy A class is like a cookie cutter objects are like cookies Analogy Writing a program is like writing the rules to a game running a program is like actually playing the game You never know how well the rules are going to work until you try them out 11 Getting started Question Where do objects come from Question Where does the first object come from Answer They are created by other objects Answer Programs have a special main method not part of any object that is executed in order to get things started public static void main String args Dog fido new Dog Fido 5 creates a Dog The special keyword static says that the main method belongs to the class itself not to objects of the class Hence the main method can be called before any objects are created Usually the main method gets things started by creating one or more objects and telling them what to do 12 A bad program public class Dog String name int age Dog String name int age this name name this age age public static void main String args bark new Dog Fido 5 bark void bark System out println Woof non static method bark cannot be referenced from a static context 13 A complete program class Dog String name int age Dog String n int age name n this age age void bark System out println Woo f void wakeTheNeighbors int i 50 while i 0 bark i i 1 public static void main String args Dog fido new Dog Fido 5 fido wakeTheNeighbors ends the class 14 The End I invented the term Object Oriented and I can tell you I did not have C in mind Alan Kay creator of Smalltalk 15
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