A Formal Presentation On Writing Classes With Rules and Examples CSC 123 Fall 2018 Howard Rosenthal

Transcription

1 A Formal Presentation On Writing Classes With Rules and Examples CSC 123 Fall 2018 Howard Rosenthal

2 Lesson Goals Defining a Class Defining Instance Variables Writing Methods The Object Reference this The tostring and equals Methods static Members of a Class Much of the information in this section is adapted from: Java Illuminated 5 TH Edition, Anderson, Julie and Franceschi Herve, Jones and Bartlett, 2019 Starting Out With Objects From Control Structures Through Objects, Gaddis, Tony, Pearson Publishing,

3 Why Do We Have User-Defined Classes The class methods are responsible for the validity of the data. Implementation details can be hidden. A class can be reused. Classes can emulate real world objects We move beyond primitive values and procedural calculations Note: The client of a class is a program that instantiates objects and calls the methods of the class 3

4 Basic class Syntax accessmodifier class ClassName // class definition goes here Conventions Use a noun for the class name. Begin the class name with a capital letter 4

5 Basic Terminology Fields Instance variables: the data for each object Class data: static data that all objects share Members Fields and methods Access Modifier Determines access rights for the class and its members Defines where the class and its members can be used 5

6 public vs. private Classes are usually declared to be public We need to be able to access them from other classes or they are useless Instance variables are usually declared to be private. It is good practice to make most instance variables private and only access them through meth0ds of the class We minimize the number of instance values and use methods to obtain derived values Methods that will be called by the client of the class are usually declared to be public. Methods that will be called only by other methods of the class are usually declared to be private. APIs (name and parameters of methods as well as return values) of methods are published (made known) so that clients will know how to instantiate objects and call the methods of the class. 6

7 Encapsulation And Information Hiding Another term that is often associated with encapsulation is information hiding. Many programmers regard encapsulation and information hiding as synonyms. However, object-oriented purists would define encapsulation as the technique that bundles data and methods into one unit and information hiding as the principle that hides the implementation of a class. Giving an instance variable private access has its advantages. Public access implies that the field is visible and can be changed from any other program with a simple assignment statement. Information hiding allows classes to be revised without affecting the code of its clients In general, information hiding is the principle that hides implementation details from a client class. When implementation details are hidden, all access to the attributes of a class is through its public methods. Remember, classes encapsulate but classes do not necessarily enforce information hiding. Restricting access within a class affords information hiding. 7

8 Instance Variables Each instance of a class has its own set of fields, which are known as instance fields. You can create several instances of a class and store different values in each instance s fields. The methods that operate on an instance of a class are known as instance methods. 8

9 The Syntax Of An Instance Variable Syntax: accessmodifier datatype identifierlist; datatype can be a primitive data type or a class type. identifierlist can contain: One or more variable names of the same data type Multiple variable names separated by commas Initialization values Optionally, instance variables can be declared as final. By convention capitalize any final variables. Conventions Begin the instance variable identifier with a lowercase letter and capitalize internal words as per our normal practices Define instance variables as private so that only the methods of the class will be able to set or change their values. 9

10 Methods In Classes When you write methods for any class follow all the same rules that are normally the case, including those related to overloading. If the method is going to be accessed from another class don t use the word static Special rules relating using static Static methods are convenient for many tasks because they can be called directly from the class, as needed. A static method is created by placing the key word static after the access specifier in the method header. When a class contains a static method, it isn t necessary for an instance of the class to be created in order to execute the method. The only limitation that static methods have is that they cannot refer to non-static members of the class. This means that any method called from a static method must also be static. It also means that if the method uses any of the class s fields, they must be static as well. 10

11 Constructors Constructors are special methods that are called automatically when an object is instantiated using the new keyword Constructors do not have any return value in the header (nor in the body) A constructor is automatically executed each time an object of the class is instantiated. A constructor method has the same name as the class with appendage.java. The only exception to this is when you place multiple classes into the class containing main, in which case the file has the same name as the class containing main. A class can have several constructors. Each constructor must have a different number of parameters or parameters of different types just like an overloaded method. The job of the class constructors is to initialize the instance variables of the new object. If and only if no constructor is specified a default constructor is used. The default constructor for a published class should be published as part of the class API 11

12 The UML For Auto - model : String - int : milesdriven - double : gallonsof Gas Auto + Auto(): + Auto(String : startmodel, int : startmilesdriven, double : startgallonsofgas): + getmodel() : String + getmilesdrivenl() : int + getgallonsofgas() : double + setmodel(string : model) : void + setmilesdrivenl(int : milesdriven) : void + setgallonsofgas(double : gallondnsofgas) : void + milespergallon() : double + tostring() : String + equals(object : obj) : boolean 12

13 Syntax Of The Constructor Syntax: public ClassName( parameter list ) // constructor body See AutoClientV1.java, AutoV1.java 13

14 Default Instance Values If the constructor does not assign values to the instance variables, they receive default values depending on the instance variable s data type. (just like arrays) The default constructor doesn t accept arguments. I. The only time that Java provides a default constructor is when you do not write your own constructor for a class. 14

15 A Note About Scope Instance variables have class scope A constructor or method of a class can directly refer to instance variables. We will review the use of the keyword this in a few slides Methods also have class scope A constructor or method of a class can call other methods of a class (without using an object reference). This is just what we have done up to now with methods A method's parameters have local scope A method can directly access its parameters, -but one method's parameters cannot be accessed by other methods. A method can define variables which also have local scope A method can access its local variables, - but one method's local variables cannot be accessed by other methods. 15

16 Accessor Methods Clients cannot directly access private instance variables, so classes provide public accessor methods with this standard form: public returntype getinstancevariable( ) return instancevariable; Where (returntype is the same data type as the instance variable.) A simple example looks like this public int getcounter( ) return counter; See AutoClientV2.java, AutoV2.java 16

17 Mutator Methods Mutator methods allow the client to change the values of instance variables. They have this general form: public void setinstancevariable( datatype newvalue ) // if newvalue is valid, // assign newvalue to the instance variable A simple example: public void setmilesdriven( int newmilesdriven ) if ( newmilesdriven >= 0 ) milesdriven = newmilesdriven; else System.out.printf( "Miles driven "cannot be negative.\n" ); System.out.printf( "Value not changed.\n" ); See AutoClientV3.java, AutoV3.java and AutoClientV4.java, AutoV4.java (adding some useful methods) 17

18 This Object Reference this How does a method know which object's data to use? The reference this refers to the current instance of a class, the object currently being used. this is an implicit parameter sent to methods. this is an object reference to the object for which the method was called. When a method refers to an instance variable name, this is implied. Thus, variablename is understood to be this.variablename this can be used in any method of the class, including any constructor. Example: public void setinstancevariable(datatype instancevariablename ) this.instancevariablename = instancevariablename; In the case above the true instance variable gets the value of the parameter 18

19 Using this (1) public void setinstancevariable(datatype instancevariablename ) this.instancevariablename = instancevariablename; Or in a constructor public newobject (datatype1 IVN1, datatype2 IVN2) this.ivn1 = IVN1; this.ivn2 = IVN2; See AutoClientV5.java, AutoV5.java 19

20 Using this (2) Some programmers like to use this pattern: public ClassName setinstancevariable( datatype instancevariablename ) this.instancevariablename = instancevariablename; return this; Example: public Auto setmodel( String model ) this.model = model; return this; See AutoClientV5.java, AutoV5.java 20

21 Calling A Constructor From A Constructor (1) If one constructor calls another constructor, no other statements can precede that call. So look at the following class: public class Room private int length; private int width; private int height; private double gallonsofpaint; public Room() length = 9; width =12; height = 8; public Room(int length,int width,int height) // three-argument constructor this.length = length; this.width = width; this.height = height; You can write it alternatively as shown on the next slide 21

22 Calling A Constructor From A Constructor (2) So look at the following rewritten class: public class Room private int length; private int width; private int height; private double gallonsofpaint; public Room() this(9. 12, 8);//does the same thing public Room(int length,int width,int height) // three-argument constructor this.length = length; this.width = width; this.height = height; 22

23 tostring (1) The tostring() method returns a String representing the data of an object Clients can call tostring() explicitly by coding the method call. Clients can call tostring() implicitly by using an object reference where a String is expected. Example client code: Auto compact = new Auto( ); // instantiate an object // explicit tostring call System.out.printf( %s\n, compact.tostring( ) ); // implicit tostring call System.out.printf( %s\n, compact ); 23

24 tostring (2) Most classes can benefit from having a method named tostring, which is implicitly called under certain circumstances. Typically, the method returns a string that represents the state of an object. Every class has a default tostring() if one isn t provided In general the default it is not all that useful, so we write what is called an overriding method When we look at inheritance we will be understand what is inherited and what is new in tostring() See AutoClientV6.java, AutoV6.java 24

25 Using equals() In Classes The purpose of equals() is to determine if the data in another object is equal to the data in this object. You cannot determine whether two objects contain the same data by comparing them with the == operator. Instead, the class must have a method such as equals for comparing the contents of objects. Like tostring(), equals(object obj) is inherited by every object. However, we almost always want to write an overriding method for your class, based on your definition of equality See AutoClientV6.java, AutoV6.java 25

26 instanceof Is A Special Binary Operator We can use the instanceof operator to determine if an object reference refers to an object of a particular class. instanceof is not really a method, rather it is a special binary operator Syntax: objectreference instanceof ClassName evaluates to true if objectreference is of ClassName type; false otherwise. if (! ( obj instanceof AutoV6 ) ) return false; else o o 0 See AutoClientV6.java, AutoV6.java 26

27 What Does Overriding Mean In any object-oriented programming language, Overriding is a feature that allows a subclass or child class to provide a specific implementation of a method that is already provided by one of its super-classes or parent classes. We will learn about this in greater detail when we discuss classes and subclasses Some programmer use annotation, (see V6) but it is not required 27

28 Rules For Overriding In Java The argument list should be exactly the same as that of the overridden method. The return type should be the same or a subtype of the return type declared in the original overridden method in the superclass. The access level cannot be more restrictive than the overridden method's access level. For example: If the superclass method is declared public then the overriding method in the sub class cannot be either private or protected. Instance() methods can be overridden only if they are inherited by the subclass. A method declared final cannot be overridden. A method declared static cannot be overridden but can be re-declared. If a method cannot be inherited, then it cannot be overridden. A subclass within the same package as the instance's superclass can override any superclass method that is not declared private or final. A subclass in a different package can only override the non-final methods declared public or protected. An overriding method can throw any uncheck exceptions, regardless of whether the overridden method throws exceptions or not. However, the overriding method should not throw checked exceptions that are new or broader than the ones declared by the overridden method. The overriding method can throw narrower or fewer exceptions than the overridden method. Constructors cannot be overridden. 28

29 static Variables static variables are also known as class variables. A static variable belongs to the class and not to any particular object; a class or static variable is shared by all objects of the class. One copy of a static variable is created per class. static variables are not associated with an object. static constants are often declared as public. When they are they are accessed as classname.variablename i.e. Math.PI To define static data, include the keyword static in its definition: Syntax: accessspecifier static datatype variablename ; Example: private static int countautos = 0; 29

30 Some Uses of static Variables If the class is employee, keep track of the total number of employees and total payroll with static variables If the class is car, use a static variable to keep track of the total number of cars sold Keep track of the total number of letters going though a post office with a static variable 30

31 static Methods static methods are also called class methods Often defined to access and change static variables static methods cannot access instance variables: static methods are associated with the class, not with any object. static methods do not have access to the implicit parameter this remember this is a reference to the object. static methods cannot access non-static methods To be accessed outside the class the method must be public You access a class method as follows classname.method(parameter list) classname is not needed within the class, only for external access Example: Math.sqrt(value) See AutoClientV7.java, AutoV7.java 31

32 Access Restrictions For static And Non-static Methods Exception: A static method may be called whether or not an object of the class exists, but a static method cannot invoke an instance method except via an object. For example, main is static, but it can invoke nonstatic method of a class via the object reference. 32

33 Some Notes On Garbage Collection (1) The Problem If unreferenced objects accumulate, a gargantuan program with thousands of objects could run out of memory. Even if a program does not run out of memory, if too much memory is allocated, program performance can deteriorate. We have seen how immutable objects, such as String objects, can quickly create large numbers of unreferenced objects. Fortunately, Java manages memory automatically, and this helps alleviate any potential disaster. The Java Virtual Machine automatically reclaims all memory allocated to unreferenced objects for future use. In other words, if an object is no longer referenced and accessible, the memory allocated to that object is freed and made available for the creation of other objects. This clean-up process is called garbage collection. 33

34 Some Notes On Garbage Collection (2) Sometimes we create an object for short term use A memory leak occurs when an application maintains references to obsolete objects To avoid a memory leak, set any reference to an object equal to null when that object is no longer going to be used. A reference with value null refers to no object and holds no address; it is called a void reference. Once the object has no reference to it the JVM automatically frees up the space Remember: Managing memory use is an important part of a programmer s job. The programmer must work in tandem with Java s automatic garbage collection as described above to ensure that there are no memory leaks. 34

35 Programming Exercise 1 - Overview CheckingAccount We will follow a structured approach to creating a CheckingAccount class and a test driver for the class. Please don t turn to the next page until told to. There is a four step process that we will follow: ØRequirements Definition ØDesign ØImplementation ØTest 35

36 PE1 - Requirements Analysis - Defining The Core Data (1) Think of three data items that will be part of a checking account. 36

37 PE1 - Requirements Analysis - Defining The Core Data (2) Think of three data items that will be part of a checking account. Answer: For this example here are three basic variables: Account number Name of account holder Current balance These variables are also referred to as the state of the class Next Question: What behavior does a checking account have? 37

38 PE1 - Requirements Analysis - Identifying The Behaviors What behavior does a checking account have? Ø Accept a deposit Ø Process a check Ø Get the current balance So now we have the basic structure of the class Data Account number Name of account holder Current balance Constructor Create the object; initialize the three data items Methods Accept a deposit Process a check Get the current balance 38

39 PE1 - Requirements Analysis - Defining The Behaviors The requirements describe what each method does. The method to accept a deposit adds an amount to the current balance. The current balance can be negative or positive. The method to process a check subtracts the amount of the check from the current balance. Overdrafts are allowed, so the balance can become negative. However, if the balance is less than $ before the check is processed, $0.15 is charged for each check. To simplify the program, assume that all data is correct (so the methods do not check for errors). 39

40 PE1 - Implementing The Constructor(s) (1) The account holder and the account number should each be a String reference because it is not expected to take part in arithmetic operations. Sometimes account numbers contain dashes or other non-digit characters. The balance is kept in terms of dollars and cents, so should be a double. To enforce good modularity, remember that instance variables should be private. So far, the CheckingAccount class looks like this: public class CheckingAccount // instance variables private String accountnumber; private String accountholder; private double balance; constructors methods What does the constructor look like?? 40

41 PE1 - Implementing The Constructor(s) (2) The structure looks like this: CheckingAccount( String accnumber, String holder, double start ) initialization of data 41

42 PE1 - Implementing The Constructor(s) (3) The complete constructor looks like this: CheckingAccount( String accountnumber, String accountholder, double balance ) this.accountnumber = accountnumber ; this.accountholder = accountholder ; this.balance = balance ; 42

43 PE1 - Incremental Testing First Testing Program (1) At this point you may want to write an incremental tester to make sure that your constructor works, or at least compiles Create a simple tester class called CheckingAccountTester 43

44 PE1 - The Original Tester public class CheckingAccountTester public static void main( String[] args ) CheckingAccount account1 = new CheckingAccount( "123", "Bob", ); Now add a tostring method to the CheckingAccount class 44

45 PE1 - CheckingAccount With A tostring() (1) public class CheckingAccount // instance variables private String accountnumber; private String accountholder; private double balance; public CheckingAccount( String accountnumber, String accountholder, double balance ) this.accountnumber = accountnumber ; this.accountholder = accountholder ; this.balance = balance ; public String tostring() return String.format( Account: %s\n Owner: %s\n Balance: $%.2f\n, accountnumber, accountholder, balance) ; 45

46 PE1 - An Updated Tester With tostring() public class CheckingAccountTester public static void main( String[] args ) CheckingAccount account1 = new CheckingAccount( "123", "Bob", 100 ); System.out.printf( %s\n, account1 ); CheckingAccount account2 = new CheckingAccount( "007", "James", ); System.out.printf(( %s\n, account2 ); Now write the three methods in the requirements, and test them via the Tester program See CheckingAccount.java and CheckingAccountTester.java 46

47 Programming Exercise 2 CellPhone and CellPhoneTester Wireless Solutions, Inc., is a business that sells cell phones and wireless service. You are a programmer in the company s information technology (IT) department, and your team is designing a program to manage all of the cell phones that are in inventory. You have been asked to design a class that represents a cell phone. The data that should be kept as fields in the class are as follows: The name of the phone s manufacturer will be assigned to the manufact field. The phone s model number will be assigned to the model field. The phone s retail price will be assigned to the retailprice field. The class will also have the following methods: A constructor that accepts arguments for the manufacturer, model number and retail price. A setmanufact method that accepts an argument for the manufacturer. This method will allow us to change the value of the manufact field after the object has been created, if necessary. A setmodel method that accepts an argument for the model. This method will allow us to change the value of the model field after the object has been created, if necessary. A setretailprice method that accepts an argument for the retail price. This method will allow us to change the value of the retailprice field after the object has been created, if necessary. A getmanufact method that returns the phone s manufacturer. A getmodel method that returns the phone s model number. A getretailprice method that returns the phone s retail price. A tostring method that prints out the phone s manufacture, model and price. Write a program that creates a CellPhone object and tests all the methods 47

48 Programming Exercise 3 CheckingAccountTester2 Modify CheckingAccountTester to CheckingAccountTester2 The new tester creates an array of 5 checking accounts For each account Read in a account number, account owner and initial balance for each account Read in a deposit and make it Read in a check and process it Print out the account information 48