COMP201 Java Programming

COMP201 Java Programming Topic 3: Classes and Objects Readings: Chapter 4 Especially “documentation and class design hints”

Outline • An Example: What does a Java class look like • Ingredients of a class: How to write a class • Instance fields • Initialization and constructors • Methods • Class modifiers • Packages: How classes fit together

An Example class Employee { // constructor public Employee(String n, double s, int year, int month, int day) { name = n; salary = s; GregorianCalendar calendar = newGregorianCalendar(year, month - 1, day); // GregorianCalendar uses 0 for January hireDay = calendar.getTime(); } // methods public String getName() { return name; }

An Example public double getSalary() { return salary; } public Date getHireDay() { return hireDay; } public void raiseSalary(double byPercent) { double raise = salary * byPercent / 100; salary += raise; } private String name; private double salary; private Date hireDay; }

An Example import java.util.*; public class EmployeeTest { public static void main(String[] args) { // fill the staff array with three Employee objects Employee[] staff = new Employee[3]; staff[0] = new Employee("Carl Cracker", 75000, 1987, 12, 15); staff[1] = new Employee("Harry Hacker", 50000, 1989, 10, 1); staff[2] = new Employee("Tony Tester", 40000, 1990, 3, 15);

An Example // raise everyone's salary by 5% for (int i = 0; i < staff.length; i++) staff[i].raiseSalary(5); // print out information about all Employee objects for (int i = 0; i < staff.length; i++) { Employee e = staff[i]; System.out.println("name=" + e.getName() + ",salary=" + e.getSalary() + ",hireDay=" + e.getHireDay()); } } } //EmployeeTest.java

Date and GregorianCalendar • Date: a class belong to the standard Java library. An instance of the Date class has a state, means a particular point in time, represented by the number of milliseconds from a fixed point. • GregorianCalendar: expresses dates in the familiar calendar notation. new GregorianCalendar() //date & time when constructed //midnight of Chinese New Year eve GregorianCalendar now=new GregorianCalendar(2002,2, 11) int month = now.get(Calendar.MONTH); int day = now.get(Calendar.DAY_OF_WEEK); • Converting betweenDateandGregorianCalendar Date time = now.getTime(); now.setTime(time);

Output of the Example name=Carl Cracker,salary=78750.0,hireDay=Tue Dec 15 00:00:00 GMT+08:00 1987 name=Harry Hacker,salary=52500.0,hireDay=Sun Oct 01 00:00:00 GMT+08:00 1989 name=Tony Tester,salary=42000.0,hireDay=Thu Mar 15 00:00:00 GMT+08:00 1990

Ingredients of a Class • A class is a template or blueprint from which objects are created. class NameOfClass { constructor1 // construction of object constructor2 … method1 // behavior of object method2 … field1 // state of object field2 … }

Instance Fields • Instance fields describe state of object • Plan: • Various types of instance fields • Initialization of instance fields

Instance Fields • Access modifiers: • private: visible only within this class • Default (no modifier): visible in package • protected: visible in package and subclasses • public: visible everywhere • It is never a good idea to have public instance fields because everyone can modify it. Normally, we want to make fields private. OOP principle.

Final Instance Fields • Declared with modifier final. • Must be initialized when object is created . • Cannot be modified afterwards, • Example: class Employee { … private final String name; }

Static Instance Fields • Declared with modifier static. • It belongs to class rather than any individual object. • Usage: className.staticField NOT objectName.staticField • Examples:System.out, System.in

Static Instance Field class Employee {public Employee(String n, double s, int year, int month, int day) { name = n; salary = s; GregorianCalendar calendar = new GregorianCalendar(year, month - 1, day); hireDay = calendar.getTime() numOfEmpoyees ++; } … public static int numOfEmployees = 0; …} Employee.numOfEmpoyees; // ok Employee hacker = new Employee(…); hacker.numOfEmployee; // wrong

Static Final Instance Fields • Constants: • Declared with static final. • Initialized at declaration and cannot be modified. • Example: Public class Math { … public static final double PI = 3.141592; … } • Notes: • Static fields are rare, static constants are more common. • Although we should avoid public fields as a principle, public constants are ok.

Initialization of Instance Fields • Several ways: • Explicit initialization • Initialization block • Default initialization • Constructors

Initialization of Instance Fields • Explicit initialization: initialization at declaration. public static int numOfEmployees = 0; public static final double PI = 3.141592; • Initialization value does not have to be a constant value. class Employee { … static int assignId() { int r = nextId; nextId++; return r; } … private int id = assignId(); private static int nextId=1; }

Initialization of Instance Fields • Initialization block: • Class declaration can contain arbitrary blocks of codes. • Those blocks are executed when object are being constructed. class Employee { … // object initialization block { id = nextId; nextId++; } … private int id; private static int nextId=1; }

Initialization of Instance Fields class Employee {public Employee(String n, double s, int year, int month, int day) { name = n; salary = s; GregorianCalendar calendar = newGregorianCalendar(year, month - 1, day); // GregorianCalendar uses 0 for January hireDay = calendar.getTime(); } private String name; private double salary; private Date hireDay; } new Employee("Harry Hacker", 35000, 1989,10,1);

Default Initialization If programmer provides no constructors, Java provides an a default constructor that set all fields to default values • Numeric fields, 0 • Boolean fields, false • Object variables, null Note: If a programmer supplies at least one constructor but does not supply a default constructor, it is illegal to call the default constructor. In our example, the following should be wrong if we don’t have the constructor shown on the previous slide: NewEmployee();// not ok

Initialization • Java initializes instance fieldsto a default (0, nullfor objects, false) if you don’t initialize them explicitly using a constructor. • If there is no constructor, a default constructor is invoked to do the initialization. • Unlike in C++, you can initialize instance fields directly in the class definition. class Customer{ ……… private int nextOrder =1; }

Constructors • A class can have one or more constructors. • A constructor • Has the same name as the class • May take zero, one, or more parameters • Has no return value • Always called with the newoperator

Default Constructor • Constructors with no parameters. • Written by programmer or supplied automatically by Java. public Employee() { name = “”; salary = 0; hireDay = null; }

Order of initialization • Default values assigned or properties set to default value • If the first line of the constructor calls another constructor then that constructor is executed. • All field initializers and initialization blocks are executed, in the order they occur in the class declaration. • The body of the constructor is executed.

Using this in Constructors • thisrefers to the current object. • More meaningful parameter names for constructors public Employee(String name, double salary, int year, int month, int day) { this.name = name; this.salary = salary; 本 … } • No copy constructor in Java. To copy objects, use the clone method, which will be discussed later.

Object Creation and Destruction • Must usenewto create an object instance Employee hacker = new Employee("Harry Hacker", 35000, 1989,10,1); This is illegal: Employee number007(“Bond”, 1000, 2002, 2, 7); • No delete operator. Objects are destroyed automatically by garbage collector • To timely reclaim resources, add a finalize method to your class. This method is called usually before garbage collect sweep away your object. But you never know when. • A better way is to add a dispose method to your class and call it manually in your code.

Methods • Methods are functions that determine what we can do to objects • Parameter (argument) syntax same as in C • Parameters are all passed by value; • But note that identifiers to objects (array, strings, etc.) are actually references. While the references cannot be modified, contents of objects might. • Return value can be anything, including an array or an object. • Plan: • Types of methods • Parameters of methods • Function overloading • Access rights of methods

Type of Methods • Accessor methods: public String getName() { return name; } • Mutator methods: Public void setSalary(double newSalary) { salary = newSalary; } • Factory methods: generate objects of the same class. Will discuss later.

Static Methods • Declared with modifier static. • It belongs to class rather than any individual object. • Usage: className.staticMethod()NOTobjectName.staticMethod() • Examples: • public static void main(String args[]) Because we don’t have any objects at the beginning.

Methods class Employee {public Employee(String n, double s, int year, int month, int day) { name = n; salary = s; GregorianCalendar calendar = new GregorianCalendar(year,month - 1, day); hireDay = calendar.getTime(); numCreated++; } … public staticint getNumOfEmployees() { return numOfEmpolyees;} private static int numOfEmployees = 0; …} Employee.getNumOfEmployee(); // ok Harry.getNumOfEmployee(); // not ok

A Diversion/Command-Line arguments public static void main(String args[]) { for (int i=0; i<args.length; i++) System.out.print(args[i]+“ ”); System.out.print(“\n”); } // note that the first element args[0] is not // the name of the class, but the first // argument //CommandLine.java

Parameters • Parameter (argument) syntax same as in C • Parameters are all passed by value; • A method cannot modify the values of parameter variables. • But note that values of object variables are actually references (locations) of objects. While the references cannot be modified, states of objects might.

Parameters/Pass by Value Employee a = new Employee(“Alice”, ….); Employee b = new Employee(“Bob”, ….); swap(a, b); …. // a swap function with no effect void swap(Object x, Object y) { Object temp = x; x = y; y = temp; } //The x and y parameters of the swap method are initialized with copies of a,b. After swap, x refers to Bob and y to Alice, but the original variables a and b still refer to the same objects as they did before the method call. ( ParamTest.java)

Parameters/Pass by Value // a function that modify objects void bonus(Employee a, double x) { a.raiseSalary(x);//a.salary modified although a is not } //ParamTest.java

Function Overloading • Can re-use names for functions with different parameter types void sort (int[] array); void sort (double[] array); • Can have different numbers of arguments void indexof (char ch); void indexof (String s, int startPosition); • Cannot overload solely on return type void sort (int[] array); boolean sort (int[] array); // not ok

Resolution of Overloading • Compiler finds best match • Prefers exact type match over all others • Finds “closest” approximation • Only considers widening conversions, not narrowing • Process is called “resolution” void binky (int i, int j); void binky (double d, double e); binky(10, 8) //will use (int, int) binky(3.5, 4) //will use (double, double)

Access rights of methods • A method can access the public fields and methods of any classes that are visible to it (see next slide). • A method of a class can access the private fields of any objects of the same class: class Employee { … Boolean equals(Employee other) { return name.equal( other.name ); } } Inif (harry.equals( boss) …) method equalsaccesses the private field name of both harry and boss.

Class Modifiers • Default (no modifier): visible in package • public: visible everywhere • private: only for inner classes, visible in the outer class (more on this later) • Each source file can contain at most one publicclass, which must have the same name as the file. File:EmployeeTest.java public class EmployeeTest {…} class Employee {…}

Packages • Classes are grouped into packages. • A program involves multiple packages. • Next: • Finding information about existing packages • Creating your own package • Using packages (to build a program) • Informing Java compiler where packages are located.

Information About Existing Packages • Information about existing packages can be found online http://java.sun.com/products/jdk/1.3/docs/api/ Linked to course page. • Examples: • java.lang, java,lang.ref, java,lang.reflect • Java.util, java.util.jar, java.util.zip • Packages are organized hierarchically.

Creating Your Own Package • To create a package named foo, • Create a directory with that name: classDir/foo • Add line “package foo;” to the top of all class files of the package • Place the class files under foo or its subdirectories. • To create a sub-package of foo named bar, • Create a subdirectory named bar: classDir/foo/bar • Add line “package bar;” to the top of all class files of the package • Place the class files under bar or its subdirectories. • The sub-package is known as “foo.bar”.

Creating Your Own Packages • If a class file has no package specification, it belongs to the default package located at the current directory. • If a class file under baseDire/foo/bar does not contain the “package …” line, it belongs to foo.bar by default. • If you want it to be part of foo, you must add line “package foo;” to the top. //PackageTest.java

import Packages • Full name for a class: packageName.className Java.util.Date today = new Java.util.Date(); • Use import so as to to use shorthand reference import java.util.*; Date today = newDate(); • Can import all classes in a package with wildcard • import java.util.*; • Makes everything in the java.util package accessible by shorthand name: Date, Hashtable, etc. • Everything in java.lang already available by short name, no import necessary Differ from “include” directive in C++. Merely a convenience.

Resolving Name Conflict • Both java.util and java.sql contain a Date class import java.util.*; import java.sql.*; Date today; //ERROR--java.util.Date or java.sql.Date? • Solution: import java.util.*; import java.sql.*; import java.util.Date; • What if we need both? Use full name java.util.Date today = new java.util.Date(); java.sql.Date deadline = new java.sql.Date();

Using Packages • Java compiler has “make” facility built-in, • It automatically searches for classes used • Runtime library files (under jre/lib and jre/lib/ext) • In files under current directory • In imported packages. • No makefile necessary

Informing Compiler Locations of Packages • Can do from command line, but inconvenient • Set the CLASSPATH environment variable: • On UNIX/Linux: Add a line such as the following to .cshrc setenv CLASSPATH /home/user/classDir1:/home/user/classDir2:. • The separator “:” allows you to indicate several base directories where packages are located. • The last “.” means the current directory. Must be there. • On Windows 95/98: Add a line such as the following to the autoexec.bat file SET CLASSPATH=c:\user\classDir1;\user\classDir2;. • Now, the separator is “;”. • On Windows NT/2000: Do the above from control panel

Informing Compiler Locations of Packages • Example: setenv CLASSPATH /homes/lzhang/DOS/teach/201/code/:/appl/Web/HomePages/faculty/lzhang/teach/201/codes/servlet/jswdk/lib/servlet.jar:/appl/Web/HomePages/faculty/lzhang/teach/201/codes/servlet/jswdk/webserver.jar:. • jar files: archive files that contain packages. Will discuss later.

COMP201 Java Programming