Object Class. EX: LightSwitch Class. Basic Class Concepts: Parts. CS257 Computer Science I Kevin Sahr, PhD. Lecture 4: Writing Object Classes

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1 CS257 Computer Science I Kevin Sahr, PhD Lecture 4: Writing Object Classes 1 Object Class objects are the basic building blocks of programs in Object Oriented Programming (OOP) languages objects consist of attributes and behavior the attributes and behavior are defined in the object class attributes are represented by instance variables behavior consists of the constructors and instance methods the class is a blueprint or template for creating objects every object is an instance of some class 2 EX: LightSwitch Class the LightSwitch class is a template for creating objects that represent light switches the complete class source code, the API, and a program that uses the class are on the website 3 Basic Class Concepts: Parts the LightSwitch class illustrates the 3 major parts of an object class: instance variables constructors instance methods 4

2 - Basic Class Concepts: tostring the LightSwitch has a tostring method returns a String describing the current state of the object makes it easy to take a snapshot of the object state Note: tostring is automatically invoked on an object that is an argument to System.out.println/print or that is used in String concatenation EX: System.out.println(obj); is equivalent to: System.out.println(obj.toString()); 5 Basic Class Concepts: Encapsulation the LightSwitch class implements the important OOP concept of encapsulation encapsulation is an important concept in OOP languages, defined by: restricting access to the object internals instance variables are private providing an external interface to those internals as appropriate using service methods public getters/setters (accessors/mutators) provided as appropriate 6 EX: No ison Setter note that the API does not contain a setter for the ison instance variable this means that the designer of the LightSwitch class decided not to allow external users to directly change that value 7 Variable Scoping all variables are defined at the smallest possible scope Instance variables are only created for object state that requires a value for the entire existence of the object variables are declared as local variables whenever possible EX: String s in tostring 8

3 public ison flipcount ison private public New Features now let s use some new Java syntax to improve our basic LightSwitch class to create the class LightSwitch2 instance variable initialization this reference helper/support methods class variables class methods 9 Instance Variable Initialization instance variables can be initialized in constructors if the value is known at compile-time, instance variables can also be initialized when declared allows specifying default values for instance variables no need to duplicate the initialization in multiple constructors good idea to use instance variable initialization when possible ensures that the variable has a value (without relying on the constructor) ensures there is only one consistent default value 10 Example LightSwitch instance variables and first two constructors: ///// instance data variables private boolean ison; private int flipcount; ///// constructors LightSwitch () = false; // originally "off" = 0; // not switched yet } // method LightSwitch public LightSwitch (int flipcountin) flipcount = flipcountin; = false; // originally "off" } // method LightSwitch 11 Example can be rewritten using instance variable initialization in LightSwitch2: ///// instance data variables private boolean ison = false; int flipcount = 0; ///// constructors LightSwitch2 () } public LightSwitch2 (int flipcountin) flipcount = flipcountin; } // method LightSwitch2 12

4 The this Reference The this reference allows an object to refer to itself That is, the this reference, used inside a method, refers to the object through which the method is being executed Suppose the this reference is used inside a method called tryme, which is invoked as follows: obj1.tryme(); obj2.tryme(); In the first invocation, the this reference refers to obj1; in the second it refers to obj2 13 Avoiding Naming Conflicts public LightSwitch (boolean ison, int flipcount) Constructor for a LightSwitch with the specified on-state and number of times flipped. public LightSwitch (boolean isonin, int flipcountin) compiler must ison = isonin; be able to flipcount = flipcountin; distinguish between } // method LightSwitch argument and instance variable The this reference and Naming Conflicts The this reference can be used to distinguish the instance variables of a class from corresponding method arguments with the same names common in constructors and setters EX: public LightSwitch2 (boolean ison, int flipcount) this.ison = ison; this.flipcount = flipcount; } // method LightSwitch2 15 Helper/Support Methods all of the methods we have seen so far have been service methods public visibility meant to provide capabilities to external users of our class often it is useful to define methods that are used internally by methods in the class can replace code that is duplicated in multiple service methods can simplify the logic of the service methods we call these methods helper or support methods private visibility not available to external users of our class not listed in the class API 16

5 s return " String return Example the LightSwitch tostring method includes some code that assigns a String to the ison state: public String tostring() String s = new String("The switch is "); if (ison) s = s + "ON"; else s = s + "OFF"; = s + " and has been flipped " + flipcount + " times."; s; } // method tostring 17 Example in LightSwitch2 let s create a helper method to do that part: private String isonstring() s; if (ison) s = "ON"; else s = "OFF"; s; } // method isonstring 18 Example now the LightSwitch2 tostring method simplifies to: public String tostring() return "The switch is " + isonstring() + and has been flipped " + flipcount + " times."; } // method tostring 19 Static Class Members Recall that a class (or static) method is one that can be invoked through its class name For example, the methods of the Math class are static: result = Math.sqrt(25) Variables can be static as well often referred to as class variables Determining if a method or variable should be static is an important design decision 20

6 The static Modifier We declare class methods and variables using the static modifier It associates the method or variable with the class rather than with an object of that class Let's carefully consider the implications of each 21 Class Variables Normally, each object has its own data space, but if a variable is declared as static, only one copy of the variable exists private static float price; Memory space for a class variable is allocated when the class is first referenced recall that memory for instance variables is allocated for each object instance when it is instantiated All objects instantiated from the class share its class variables each object instance has its own copy of every instance variable Changing the value of a class variable in one object changes it for all others 22 Visibility of Class Variables to enforce encapsulation class variables should usually be private exception: if a class variable is a constant it can be public since the value can t be changed by external (or internal) users, this doesn t break encapsulation EX: the Math class contains a class variable declared: public static final double PI = ; external users can access this constant as Math.PI 23 Class Methods and Variables class methods and class variables often work together class methods cannot reference instance variables because instance variables don't exist until an object exists however, a class method can reference class variables or local variables instance methods can reference class variables (as well as instance and local variables) 24

7 Class Variables EX let s add a class variable to LightSwitch2 to keep track of how many light switches are currently on to enforce encapsulation the class variable will be private we ll define a public class method to provide the value to external users 25 Class Variables EX first declare and initialize the class variable: private static int numlightson = 0; define a service method to provide the value to external users: public static int getnumlightson () return numlightson; } 26 Class Variables EX now we have to find every place in LightSwitch where a light is turned on or off there are only two: the third constructor and the flip method 27 Class Variables EX change the third constructor to: public LightSwitch2 (boolean ison, int flipcount) this.ison = ison; if (this.ison) numlightson++; this.flipcount = flipcount; } // method LightSwitch2 28

8 Class Variables EX change the flip method to: public void flip () if (ison) ison = false; numlightson--; } else ison = true; numlightson++; } flipcount = flipcount + 1; } // method flip 29 LightSwitch2 EX the source code file LightSwitch2.java, the class API, and a program LightSwitchApp2.java that uses LightSwitch2 objects are available on the class website spend some time looking at this example 30 Account Class EX the Account class is a more complex object class it includes all of the new features discussed in this lecture the source code file Account.java and a program that uses the class (Transactions.java) are available on the class website spend some time understanding this class 31 UML Diagrams UML stands for the Unified Modeling Language UML allows us to design our classes before coding UML provides graphical documentation for our classes UML diagrams show what classes consist of and the relationships among classes and objects A UML class diagram consists of one or more classes, each with sections for the class name, attributes (data), and operations (methods) Lines between classes represent associations will discuss associations later 32

9 UML Diagrams Each class is represented by: 33 UML Diagrams Each variable has the form: visibility varname : type visibility can be either: + to indicate public, or - to indicate private class/static variables are indicated by underlining them 34 UML Diagrams Each method has the form: visibility methodname(args) : returntype as with variables: visibility can be + (for public) or - (for private) class/static methods should be underlined each argument has the same form as a variable (see previous slide), except that there is no visibility modifer separate arguments with commas a void return type is indicated by leaving the returntype blank Constructors are often left off the UML diagram 35 EX: Account Class UML Diagram 36

10 Lecture 4 Vocabulary encapsulation this reference helper/support methods class/static variables Unified Modeling Language/UML Diagrams 37