{small lecturenumber - hepage :} Objects{small lecturenumber - hepage :} Objects{small lecturenumber - hepage :} Classes and objects{small lecturenumber - hepage :} An example{small lecturenumber - hepage :} An example{small lecturenumber - hepage :} An example{small lecturenumber - hepage :} An example{small lecturenumber - hepage :} An example{small lecturenumber - hepage :} Methods{small lecturenumber - hepage :} Data hiding{small lecturenumber - hepage :} Example{small lecturenumber - hepage :} Visibility modifiers{small lecturenumber - hepage :} Visibility modifiers{small lecturenumber - hepage :} Example{small lecturenumber - hepage :} Example{small lecturenumber - hepage :} Strings in Java{small lecturenumber - hepage :} Overloaded operators{small lecturenumber - hepage :} Overloaded operators{small lecturenumber - hepage :} ExampleIntro to Programming IIObjectsChris BrooksDepartment of Computer ScienceUniversity of San FranciscoDepartment of Computer Science — University of San Francisco – p.1/??2-2: Objects•Java is an object-oriented language.•So what the heck is an object anyway?Department of Computer Science — University of San Francisco – p.2/??2-3: Objects•An object is a type of abstraction•It provides a way of grouping together related data andfunctionality.•Makes it easier to organize and extend your program.•Also gives a “black box” effect.◦Users of your objects don’t need to worry about how theywork internally, just how to use them.Department of Computer Science — University of San Francisco – p.3/??2-4: Classes and objects•A class is a template or category•An object is a particular instance of that class.◦CartoonAnimals might be a class◦BugsBunny, Tweety are instances of that class•Classes let us specify behavior common to a set of objects.Department of Computer Science — University of San Francisco – p.4/??2-5: An example•Say we’re making a drawing program, and we need torepresent a collection of circles.•A circle has an x and y coordinate for its center, plus a radius.•We could do:c1xval = 3;c1yval = 4;c1radius = 10;c2xval = 5;c2yval = 2;c2radius = 6;...•(to be less messy, we could also store these in arrays)Department of Computer Science — University of San Francisco – p.5/??2-6: An example•This is not a good solution, though.•Why not?Department of Computer Science — University of San Francisco – p.6/??2-7: An example•This is not a good solution, though.•No grouping of a circle’s components.•Users need to know all about the internals of a circle.•What if we don’t know the number of circles in advance?Department of Computer Science — University of San Francisco – p.7/??2-8: An example•A better solution:public class circle {public int xval;public int yval;public int radius;}•We have encapsulated the center and radius information insidethe circle.Department of Computer Science — University of San Francisco – p.8/??2-9: An example•But we have two related variables representing the center.•Perhaps we should group them as well.public class point {public int xval;public int yval;}public class circle {public point center;public int radius;}Department of Computer Science — University of San Francisco – p.9/??2-10: Methods•As we know, classes also contain methods.•Methods are pieces of code that can be invoked on an object.•The allow us to encapsulate both state and behavior.Department of Computer Science — University of San Francisco – p.10/??2-11: Data hiding•It’s also important to protect instance data from outside users.•One way to do this is by providing accessors and mutators◦“setters and getters”•Rather than the user modifying your object’s data directly, theyuse a method to do it.◦Reduces error◦Hides implementation from the user.Department of Computer Science — University of San Francisco – p.11/??2-12: Example•For the following classes, add setter and getter methods foreach of the instance variables.public class point {public int xval;public int yval;}public class circle {public point center;public int radius;}Department of Computer Science — University of San Francisco – p.12/??2-13: Visibility modifiers•public and private are used to indicate who can access avariable or method.◦public: anyone can use it.◦private: only available within that object.Department of Computer Science — University of San Francisco – p.13/??2-14: Visibility modifiers•Instance variables should be made private unless there’s acompelling reason not to.◦Use accessors and mutators to access and change data•public methods are available for everyone to use.•private methods can be used only within the object.◦These are nice for “helper” methods that you don’t want auser to call.Department of Computer Science — University of San Francisco – p.14/??2-15: Example•Design a book class.•It should have instance variables for title, author, genre,publishers, copyright date. They should be strings.•It should also have getters and setters for each of these.Department of Computer Science — University of San Francisco – p.15/??2-16: Example•Now create a Name class. It should have two members:firstName and lastName.•Modify Book so that author is of type Name.•Since you used setters and getters, users of your book classwill never know!Department of Computer Science — University of San Francisco – p.16/??2-17: Strings in Java•Strings in Java are objects•This mean that they have a set of methods they respond to:◦compareTo(), equals()◦indexOf()◦length()◦replace()◦startsWith(), endsWith()◦etcDepartment of Computer Science — University of San Francisco – p.17/??2-18: Overloaded operators•Unlike most other objects, Strings also have special behaviorfor creating string literals and for concatenation.•String literals are strings where the value is known at compiletime:◦String s1 = “hello world”◦String s2 = “USF”◦String s3 = “I love Java”•We can create a string without calling new.Department of Computer Science — University of San Francisco – p.18/??2-19: Overloaded operators•We can also use the ’+’ symbol to concatenate strings.◦String s1 = “hello”◦String s2 = “world”◦String s3 = s1 + s2 // s3 = “hello world”•This is a phenomenon called overloading; an operator isredifined to provide different functionality.Department of Computer Science —
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