IE582 Spring-2004 1
Java Tutorial for Beginners
http://www.personal.psu.edu/kxt179/teaching/index.html
1 Introduction to Java Programming... 3
1.1 What is Java? ... 3
1.2 Features of Java Programming Language ... 3
1.3 Why Java? ... 3
1.4 Different Types of Java Programs ... 3
1.5 Getting stated with Java Programming ... 4
2 Important Java Concepts ... 6
2.1 Object ... 6 2.2 Message ... 6 2.3 Class... 7 2.4 Inheritance... 7 2.5 Interface ... 8 2.6 Package ... 8
3 Java Language Basics ... 10
3.1 Statements and Expressions ... 10
3.2 Variables and Data Types ... 10
3.3 Expressions and Operators... 11
3.4 String Arithmetic ... 12
3.5 Modifiers ... 12
3.6 Conditional Statement... 12
3.6.1 if conditional statement ... 12
3.6.2 switch Conditional Statement ... 12
3.7 Loop statement ... 13
3.7.1 for Loops ... 13
3.7.2 while and do Loops... 13
3.7.3 Breaking Out of Loops... 14
4 Working with Objects ... 16
4.1 Package ... 16 4.1.1 Defining package ... 16 4.1.2 Using package ... 16 4.2 Class... 16 4.2.1 Defining classes ... 16 4.2.2 Defining variables... 16 4.2.3 Defining methods ... 16
4.2.4 Java access specifiers... 17
4.3 Inheritance... 17
4.4 Interface ... 17
4.5 Creating New Objects ... 18
4.6 Accessing and Setting Class and Instance Variables ... 19
4.7 Calling Methods ... 19
4.8 References to Objects... 20
4.9 Casting and Converting Objects and Primitive Types ... 20
4.11 The Java Class Library... 21
5 Array and Container Objects... 22
5.1 Array Objects ... 22
5.1.1 Arrays in Java ... 22
5.1.2 Declaring Array Variables ... 22
5.1.3 Creating Array Objects ... 22
5.1.4 Accessing Array Element s ... 22
5.1.5 Changing Array Elements ... 22
5.2 Container Objects... 23
5.2.1 Container taxonomy... 23
5.2.2 Some Useful Container Objects ... 23
5.2.3 More on LinkedList ... 23
6 Input and Output ... 25
6.1 Stream? ... 25
6.2 Character Streams and Byte Streams ... 25
6.3 Reading Console Input... 26
6.4 Writing Console Output ... 27
6.5 Reading and Writing Files ... 27
6.6 Parsing String Input using Tokenizers ... 27
7 Java Application... 29
7.1 Creating Java Applications ... 29
7.2 Java Applications and Command-Line Arguments ... 29
8 What you need for (serious) Java Programming... 31
8.1 Buy a Java book (or use online tutorials)... 31
8.2 Use (hyperlinked) Java APT specification... 31
8.3 Use RAD Tools (especially for GUI programming) if possible ... 31
IE582 Spring-2004 3
Java Tutorial for Beginners
Kaizhi Tang
1 Introduction to Java Programming
1.1 What is Java?
l Java is a Programming Language: Full-fledged general-purpose programming language, especially object-oriented programming
l Java is a Platform: Java Virtual Machine (JVM) and Java API (Application Programming Interface)
1.2 Features of Java Programming Language
l Platform Independent:
n Java program can run on any computer system for which a JVM has been installed
n Java bytecode / Compiler / Interpreter l Object Oriented
n Java is a pure object oriented programming langauge l Easy to Learn
n Similar to C and C++, but most of more complex parts of them have been excluded
l Performance
n Exception handling mechanism l Robust
n Interpreted but still high performance (using Just In Time Compiler technology)
l And, Network-Savvy, Secure, Multi-threaded, Dynamic, etc.
1.3 Why Java?
l Shorten development time
n Easy to use, Automatic garbage collection, using the third party’s APIs l Shorten deployment time
n Platform independent l Solve complicated problem
1.4 Different Types of Java Programs
l Console Application: Run in command line (*) l Applet: Run in the browser (IE or Netscape)
l Servlet: Run in the Java Servlet container (Jakarta Tomcat) l Beans: component strategy in Java, especially in J2EE
1.5 Getting stated with Java Programming
l Getting a Java Development Environment (JDK) and documentation n Go to the JavaSoft Web site
(http://java.sun.com/j2se/1.4.2/download.html) n Download SDK, JRE and documentation
l Install the JDK
n For example: C:\ j2sdk1.4.2
l Unzip the documentation: C:\ j2sdk1.4.2\docs l Creating a Java Application
n Creating the Source File: u using text editors
u file name extension: .java
u file name = class name (example: ‘HelloWorld.java’ for ‘class HelloWorld’)
n Compiling and Running the Source File
u javac HelloWorld.java (à bytecode: HelloWorld.class) u java HelloWorld
l Creating a Java Applet n Creating the Source File
n Compiling and Running the Source File import java.applet.Applet; import java.awt.Graphics;
public class HelloWorld extends Applet { public void paint(Graphics g) {
g.drawString("Hello world!", 50, 25); }
}
}
class HelloWorld {
public static void main(String[] args){ System.out.println("Hello World!"); }
IE582 Spring-2004 5 n Including the Applet in a Web page (Only for Netscape, IE is different)
n Execution à Web browser or appletviewer <HTML>
<HEAD>
<TITLE> A Simple Program </TITLE> </HEAD>
<BODY>
Here is the output of my program:
<APPLET CODE="HelloWorld.class" WIDTH=150 HEIGHT=25>
</APPLET> </BODY>
2 Important Java Concepts
2.1 Object
l Two characteristics of real-world objects: states and behaviors
§
Dogs: state (name, color, breed, hungry) and behavior (barking, fetching, and wagging tail)§
Bicycles: state (current gear, current pedal cadence, two wheels, number of gears) and behavior (braking, accelerating, slowing down, changing gears) l Software objects to model real-world objects: variables and methodsl Definition: An object is a software bundle of variables and related methods
l Encapsulation: package an object’s variable within the protective custody of its method => methods restrict and protect the variable access in an object
l Benefit of Encapsulation
n Modularity: write and maintain source code of other objects independently n Information hiding: only public method can communicate with other objects
2.2 Message
• Software objects interact and communicate with each other by sending messages to each other
• Objects will update states or send messages to other objects in the system
• Benefits
o Message parsing support all possible interactions between objects o Objects don’t need to be in the same processor or even on the same
machine => distributed computing is easy in Java • Java Program = Objects + Message interactions
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2.3 Class
l Definition: A class is a blueprint, or prototype, that defines the variables and the methods common to all objects of a certain kind.
o All the bicycles share the same characteristics and they can be defined as one class
l A class is an abstract of a group of objects
• Subclasses: Mountain Bike, Racing Bike, Tandem Bike • Superclass: Bicycle
• The class variables and methods in the subclasses are the subsets of those in the superclass
o Mountain bikes, racing bikes, and tandems share some states: cadence, speed, and the like. Also, each subclass inherits methods from the superclass. Mountain bikes, racing bikes, and tandems share some behaviors: braking and changing pedaling speed
o Tandem bicycles have two seats and two sets of handle bars; some mountain bikes have an extra set of gears with a lower gear ratio • Subclass can overrides some methods in the superclass
o For example, if you had a mountain bike with an extra set of gears, you would override the "change gears" method so that the rider could use those new gears.
• Benefits:
o Reuse the code in the superclass many times
o Programmers can implement superclasses called abstract classes that define "generic" behaviors . The abstract superclass defines and may partially implement the behavior, but much of the class is undefined and unimplemented. Other programmers fill in the details with specialized subclasses
2.5 Interface
• Definition: a device that unrelated objects use to interact with each other o Analogous to a protocol (an agreed on behavior)
• Example: Inventory Interface
o Bicycle class hierarchy defines different bicycle classes o They needs to be used in an inventory program
o Inventory program only needs to know tracking numbers and retail prices o An inventory interface is defined to connect bicycles and the inventory
programs
o This interface can also be used by other class that needs to be used in the inventory program
• Benefits
o Capturing similarities among unrelated classes without artificially forcing a class relationship (inheritance)
o Declaring methods that one or more classes are expected to implement
o Revealing an object's programming interface without revealing its class
2.6 Package
• Definitions: a collection of classes
• Without explicit declaration, access to the classes in a package is not allowed o import java.io.*;
IE582 Spring-2004 9 • Benefits
o Easy to manage large projects
3 Java Language Basics
3.1 Statements and Expressions
• Statement : forms a single Java operation. They must end with semi-colons(;). Example:
int i = 1;
import java.awt.Font;
System.out.println("This car is a "+ color + " " + make); m.engineState = true;
• Expressions : statements which return values
• Block: compound statements. They are surrounded by braces({}).
3.2 Variables and Data Types
• Declaring Variables
§ Variable definition can go anywhere in a method definition § Instance variables vs. Class variables
Example:
String firstName; // instance variable static String lastName; // class variable
§ Initializing: Example:
String myName = "Yong-Han";
§ Finalizing: (constant) Example:
final String myName = "Kumara";
• Variable Names Example:
int _number, $money, 5five // à O.K
button theButton; // By convention, Java variables long reallyBigNumber; // have meaningful names, often
boolean currentWeatherState; // are made up of several // words combined.
• Variable Types § Primitive types:
Category Type Size / Format Description byte 8-bit two's complement Byte-length integer Integer
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int 32-bit two's complement Integer long 64-bit two's complement Long integer float 32-bit IEEE 754 Single -precision
floating point Real Number
double 64-bit IEEE 754 Double-precision floating point char 16-bit Unicode character A single character Other Types boolean true or false A boolean value
(true or false)
§ Class types: variables in Java can also be declared to hold an instance of a particular class. These variables can hold instances of the named class or of any of its subclasses.
Example:
String myName;
Object theObject;//this variable can hold any object.
•
Assigning Values to Variables: using operator ‘=’. Example: myName = "Mahima" ; x = y = z = 0; x += y; // x = x + y x -= y; // x = x - y x *= y; // x = x * y x /= y; // x = x / y • Comments: • /* and */ • //• /** and */ ß used by the javadoc system to generate API documents • Literals
Example:
Number Literals: -45, 4L, 0777, 0XFF, 2.56F, 10e45, .36E-2, .. Boolean Literals: true, false
Character Literals: 'a', '#', '3', \n, \\, \", .. String Literals: "A string with a \t tab in it.", ..
3.3 Expressions and Operators
• Arithmetic : +, -, *, /, % (modulus operator) • Incrementing and Decrementing
Example:
y = x++; // y = x; x = x + 1; y = ++x; // x = x + 1; y = x;
• Logical operators: &, &&, |, ||, ^, !
3.4 String Arithmetic
• Additional operator (+) to create and concatenate strings. Example:
System.out.println(name + " is a " + color + " beetle"); myName += " Jr."; // myName = myName + " Jr.";
3.5 Modifiers
Modifiers are special keywords that modify the definition of a class, method, or variable.
• Access Control: public, protected, private • Making class methods, variables: static
• Finalizing: final
• Others: abstract, synchronized, volatile, native
3.6 Conditional Statement
3.6.1 if conditional statement
if (expression) if (expression) statement ,or statement-1
else
statement-2
• The conditional expression returns boolean value (true or false) Example:
if (engineState == true)
System.out.println("The engine is already on."); else {
engineState = true;
System.out.println("The engine is now on."); }
3.6.2 switch Conditional Statement
• When the result of a test can be represented by byte, char, short, or int.
Example:
int month; int numDays; . . .
IE582 Spring-2004 13 switch (month) { case 1: case 3: case 5: case 7: case 8: case 10: case 12: numDays = 31; break; case 4: case 6: case 9: case 11: numDays = 30; break; case 2:
if (((year%4 == 0) && !(year%100 == 0)) || (year%400 == 0))
numDays = 29; else
numDays = 28; break;
} // end of switch (month)
3.7 Loop statement
3.7.1 for Loops
for (initialization; test; increment) statement;
Example:
String strArray[] = new String[10]; int i;
for (i = 0; i < strArray.length; i++) strArray[i]= “a”;
3.7.2 while and do Loops
• To repeat a statement or block of statements as long as particular condition is true.
Example:
while (condition) { bodyOfLoop;
int x = 1; while (x <= 10) { System.out.println(“looping, round “ + x); x++; } Example: int x = 1; do { System.out.println(“looping, round “ + x); x++; } while (x <= 10)
3.7.3 Breaking Out of Loops
• break: when used in a loop, it immediately halts execution of the current loop. Example:
// Loop to copy elements from array1 into array2 // until the end of array1 or until a 0 is reached. int count = 0;
while (count < array1.length) { if (array1[count] == 0) { break;
}
array2[count] = array1[count++]; }
• continue: when used in a loop, it immediately skip the current iteration, and the loop starts over at the next iteration.
Example:
//Loop to copy non-zero elements from array1 into array2 // until the end of the array1. (by skipping zero-elements) int count1 = 0;
int count2 = 0;
while (count < array1.length) { if (array1[count] == 0) { continue; count1++; } array2[count2++] = array1[count1++]; }
• Labeled loops: let the program break to specific points outside nested loops or continue a loop outside the current loop.
do { // check the condition after the body of loop bodyOfLoop;
IE582 Spring-2004 15 Example:
// When the condition (i * x == 400), the ‘break’ causes // the execution to break out of both loops (for and
// while) and continue executing any code after both loops. out: // a label
for (int i = 0; i < 10; I++) { while (x < 50) { if (i * x == 400) break out; ... } ... }
4 Working with Objects
4.1 Package
4.1.1 Defining package
• Put package name at the beginning of a java file Example:
//Bicycle.java package myapp.test;
• The directory structure has to be the same as the package organization Example:
Bicycle.java has to be in the directory of .\ myapp\test\Bicyle.java
• If no explicit package, default package uses the current directory
4.1.2 Using package
• Classes from packages other than java.lang must be explicitly imported or refereed to by full package name. (* Packages do not limit the inheritance, i.e. a class can be inherited from a class in other packages)
Example:
import java.awt.Graphics; import java.awt.Font; import java.awt.*;
import myapp.util.*;
// The directory .\myapp\util needs to be set in the // classpath of the system variable
public class .. {
Font f; // java.awt.Font f;
...
4.2 Class
4.2.1 Defining classes
• Java access specifiers + “class” + class name + variables + methods
4.2.2 Defining variables
• Java access specifiers + Type (Class) + variable name
4.2.3 Defining methods
• Java access specifiers + return type(class) + method name + parameter list
Example:
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public class Bicycle{ double currentSpeed; int currentCadence; int numberOfGears; int currentGear; public void brake();
public void changeCadence(int cadence); public void changeGear(int gear); }
4.2.4 Java access specifiers
• “Friendly” (no specifier): public to the same package and private to other packages
• public: everybody, everywhere can access it • protected: only inheriting classes can access it • private: only the class itself can access it
4.3 Inheritance
• Use keyword “extends” Example:
//MountainBicycle.java
class MountainBicycle extends Bicycle{ public void climbUp();
public void quickDown(); }
• The friendly members in the same package can be accessed by subclasses
4.4 Interface
• Use keyword “interface” to define an Interface Example: //Inventory.java ｉｎｔｅｒｆａｃｅ Ｉｎｖｅｎｔｏｒｙ ｛ ｉｎｔ ｇｅｔＴｒａｃｋＮｕｍｂｅｒ（）； ｄｏｕｂｌｅ ｇｅｔＲｅｔａｉｌＰｒｉｃｅ（）； ｖｏｉｄ ｓｅｔＴｒａｃｋＮｕｍｂｅｒ（ｉｎｔ ｎｕｍｂｅｒ）； ｖｏｉｄ ｓｅｔＲｅｔａｉｌＰｒｉｃｅ（ｉｎｔ ｐｒｉｃｅ）； ｝
• Use keyword “implements” to agree the interface //Bicycle.java will be modified in the following way
public class Bicycle implements Intentory{ double currentSpeed;
int currentCadence; int numberOfGears; int currentGear; public void brake();
public void changeCadence(int cadence); public void changeGear(int gear); //implements the following methods
ｉｎｔ ｇｅｔＴｒａｃｋＮｕｍｂｅｒ（）； ｄｏｕｂｌｅ ｇｅｔＲｅｔａｉｌＰｒｉｃｅ（）；
ｖｏｉｄ ｓｅｔＴｒａｃｋＮｕｍｂｅｒ（ｉｎｔ ｎｕｍｂｅｒ）；
void setRetailPrice(int price);
}
• All the methods defined in the interface needs to be implemented
4.5 Creating New Objects
• Using new
§ To create a new object, use the new operator with the name of the class (exception for String object creation using string literal).
Example:
String str = new String(); Random r = new Random(); Tree t = new Tree();
§ The number and type of arguments are defined by the special method – constructor.
Example:
Date d1, d2, d3; // class in the java.util package d1 = new Date(); // current time
d2 = new Date(98, 8, 22, 18, 0); // 1998 Sep. 22, 6:00 pm d3 = new Date(“September 22 1998 6:00 PM”);
§ constructors: special methods that initialize a new object, set its variables, create any other objects that object needs, and generally perform any other operations the object needs to initialize itself.
• What new Does
§ New instance of the given class is created. § Memory is allocated.
§ The constructor method is called. Example:
public class Car {
String make = “Nissan”; String color = “red”;
boolean engineState = false; Car() {
System.out.println(“a Car object created”); }
Car(String mk, String cl) { this();
make = mk; color = cl;
IE582 Spring-2004 19
}
Car(String mk, String cl, boolean es) { this(mk,cl);
engineState = es; }
}
4.6 Accessing and Setting Class and Instance Variables
• à use dot notation. Eexample:
myCar.make, myCar.color
• Changing values Example:
myCar.engineState = true;
• Class Variables: variables that are defined and stored in the class itself (static keyword), only one copy of the variable exists among all its instantiations.
4.7 Calling Methods
• à use dot notation Example:
myCar.startEngine();
myCar.changeColor(“blue”); theMake = myCar.getColor();
More complicated example:
public class Car { Engine engine; ... Engine GetEngine(){ Return engine; } ... }
public class Engine { long CC; ... void getCC(){ Return CC; } ... }
long theCC = myCar.GetEngine().getCC(); //
long theCC = myCar.engine.getCC(); // the same. long theCC = myCar.engine.CC; //
• Class methods: method that can be called without any instantiation of its class. Example:
int biggerOne = Math.max(x, y);
// We don’t need to create an object of class Math at all!
4.8 References to Objects
• Assigning objects to variables and passing objects as arguments to methods à passing references to those objects, not the objects themselves or copies of those objects.
Example:
Point pt1, pt2; // create two variables for class ‘Point’ pt1 = new Point(100,100);
pt2 = pt1;
pt1.x = pt1.y =150;
4.9 Casting and Converting Objects and Primitive Types
• Casting: converts the value of an object or primitive type into another type. • Casting Primitive Types: (typename) value
Example:
i = (int) (x / y); // x and y are floats, i is an integer.
• Casting Objects: (classname) object
§ The class and the object must be related by inheritance. Example:
Apple a = new Apple();
GreenApple aGreen = new GreenApple(); a = aGreen; // no casting needed
aGreen = (GreenApple)a; // casting needed
• Converting Primitive Types to Object and Vice Versa à direct casting not allowed!
§ Use constructors and special methods in the classes. Example:
Integer intObject = new Integer(35);
int theInt = intObject.intValue(); // return 35
x: 150 y: pt1 pt2 Point Apple GreenAppl
IE582 Spring-2004 21
4.10 Comparing Objects
• Cannot use the comparison operators except == and !=, which just test whether the two operands refer to exactly the same object in memory.
• Instead, in order to compare the contents of objects, you have to implement special methods in your class, and use them.
Example:
boolean equalOrNot = myString.equals(yourString); // equals() method was implemented in String class.
4.11 The Java Class Library
• Standard Java packages support a huge set of classes that are guaranteed to be available in any commercial Java environment.
• Those classes are in the java and javax package including:
§ java.lang.* – Classes that apply to the language itself, including the Object class, the String class, and the System class. It also contains the special classes fo r the primitive types (Integer, Character, Float, and so on).
§ java.util.* – Utility classes, such as Data, as well as simple collection classes, such as Vector and Hashtable.
§ java.io.* – Input and output classes for writing to and reading from streams (such as standard input and output) and for handling files.
§ java.net.* – Classes for networking support, including Socket and URL (a class to represent references to documents on the WWW).
§ java.awt.* – This is the Abstract Windowing Toolkit. It contains classes to implement graphical user interface features, including classes for Window, menu, Button, Font, CheckBox, and so on. It also includes mechanisms for managing system events and for processing images (in the java.awt.Image package).
§ java.applet.* – Classes to implement Java applets.
§ And a lot more: javax.swing.*, java.bean.*, java.math.*, java.rmi.*, java.security.*, java.sql.*,
java.text.*, ...
5 Array and Container Objects
5.1 Array Objects
5.1.1 Arrays in Java
• Each slot in the array can hold an object or a primitive value. • Arrays in Java are objects.
• Steps to create an array in Java:
• Declare a variable to hold the array.
• Create a new array object and assign it to the array variable. • Store things in that array.
• An array of objects in Java is an array of references to those objects!
5.1.2 Declaring Array Variables
• Use empty brackets ([]). Example: String studentNames[]; String[] studentNames; Point centerPts[]; Point[] centerPts; int temps[]; int[] temps;
5.1.3 Creating Array Objects
• Use new, or directly initialize the contents of that array. Example:
String[] names = new String[10]; int[] theNumbers = new int[100]; Car[] theCars = new Car[100];
String[] names = {“Yong-Han”, “Goutam”, “Shreesh”}; boolean[] isGoodGuy = {true, true, true};
5.1.4 Accessing Array Elements
• Use the array subscription expression ([]).
• Use the instance variable length to test the length of an array. Example:
currentString = theStrings[i++]; int theFirst = theNumbers[0];
int theLast = theNumbers[theNumbers.length-1];
5.1.5 Changing Array Elements
• Just use assignment statements. Example:
theStrings[10] = “Hello!”; theNumbers[0] = 15;
IE582 Spring-2004 23
Car[2] = Car[3]; // Object type // what is the difference between them?
5.2 Container Objects
5.2.1 Container taxonomy
5.2.2 Some Useful Container Objects
• Vector: a growable array of objects.
• ArrayList: resizable-array implementation of the List interface.
• Hashtable: This class implements a hashtable, which maps keys to values • Stack: a growable array implementing first in last out mechanism
• HashSet: implements a set in a hash table
• TreeSet: implements a set in a tree structure, it can be sorted • HashMap: implements a map in a hash table
• TreeMap: implements a map in a tree structure • LinkedList: implements a linked list
5.2.3 More on LinkedList
Linked list is a simple but powerful data structure for the dynamic memory allocation. Java has fully implemented it.
import java.util.*; ...
List list = new LinkedList(); • Iterate the list
Iterator it = list.iterator(); while(it.hasNext()){ Object o = it.next(); } • Add an item list.add(o); • Remove an item
List.remove(o); // o is an object in the list
list.remove(i); // i is the index of an object in the list • Update an item
(1) use iterator to locate the object (2) update the object using the reference
IE582 Spring-2004 25
6 Input and Output
6.1 Stream?
• A stream is a path of communication between the source of some information and its destination.
• Procedure for reading and writing data
Reading Writing
open a stream
while more information read information close the stream
open a stream
while more information write information close the stream
6.2 Character Streams and Byte Streams
• Character Streams: read and write a stream of characters (16 bits)
o Reader and Writer are the abstract super classes for all character streams.
• Byte Streams: read and write a stream of bytes (8 bits).
o InputStream and OutputStream are the abstract super classes for all byte streams.
o These streams are typically used to read and write binary data such as images and sounds
6.3 Reading Console Input
• Use predefined stream variable System.in and BufferedReader object. Example:
char ch; String str;
BufferedReader br = new BufferedReader(new
InputStreamReader(System.in)); try { // read a character.
c = (char) br.read(); } catch (IOException e) {
System.out.println(e.getMessage()); }
try { // read a String. str = br.readLine(); } catch (IOException e) {
System.out.println(e.getMessage()); }
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6.4 Writing Console Output
• Use predefined print and println methods of System.out object. Example:
String str = “a string line”; System.out.println(str); System.out.print(str + “\n”);
6.5 Reading and Writing Files
• Use two stream classes, FileInputStream and FileOutputStream. Example:
try { // read String lines from the file “input.txt”
FileInputStream fis = new FileInputStream(“input.txt”); BufferedReader buff = new BufferedReader(new
InputStreamReader(fis)); String line = new String();
while((line = buff.readLine()) != null) System.out.println(line);
} catch (FileNotFoundException e){ System.out.println(e.getMessage()); } catch (IOException e){
System.out.println(e.getMessage()); }
try { // write a String array to the file “output.txt”
FileOutputStream fos = new FileOutputStream(“output.txt”); BufferedWriter buff = new BufferedWriter(new
OutputStreamWriter(fos));
String[] line = {“line 1”, “line 2”, “line 3”, “line 4”}; for (int i=0; i<4; i++) {
buff.write(line[i],0,line[i].length()); buff.newLine();
}
fos.flush(); fos.close();
} catch (FileNotFoundException e){ System.out.println(e.getMessage()); } catch (IOException e){
System.out.println(e.getMessage()); }
6.6 Parsing String Input using Tokenizers
• The StringTokenizer class allows an application to break a string into tokens. Example:
StringTokenizer st = new StringTokenizer("this is a test"); while (st.hasMoreTokens()) {
System.out.println(st.nextToken()); }
The output result is: this
is a test
In the above example, the delimiters are in the string " \t\n\r\f". You can also customize the delimiters.
Example: StringTokenizer st = new StringTokenizer("kaizhi:student:2002", “:\n\f”); while (st.hasMoreTokens()) { System.out.println(st.nextToken()); }
The output result is:
kaizhi student 2002
• The StreamTokenizer class takes an input character stream and parses it into tokens, allowing the tokens to be read one at a time.
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7 Java Application
7.1 Creating Java Applications
• Applications are Java programs that run on their own, while applets are Java programs that run on the enabled browser and servlets run on the Java-enabled web server. A main() method must exist in a
application.
• A typical example of the main() method: Public static void main(String args[]) { ...
}
• Helper Classes: you can create as many classes as you want for your program, and as long as (1) they are in the same directory, (2) listed in your CLASSPATH, or (3) explicitly declare using java command option –classpath, Java will be able to find them when your program runs.
7.2 Java Applications and Command-Line Arguments
• Passing Arguments to Java Programs: § append them to the command line! § They are separated by spaces.
(example)
java Myprogram argumentOne 2 three “fourth argument”
• Handling Arguments in Your Java program
§ The arguments are stored in an array of strings.
Example:
class EchoArgs {
public static void main(String args[]) { for (int i = 0; i < args.length; i++) {
System.out.println(“Argument ” + i + “: “ + args[i]); } } // end of main() } // end of EchoArgs Example:
class SumAverage {
public static void main(String args[]) { int sum = 0;
for (int i = 0; i < args.length; i++) { sum += Integer.parseInt(args[i]);
// à convert string arguments to integer values } System.out.println(“Sum = ” + sum); System.out.println(“Average = ” + (float)sum / args.length); } // end of main() } // end of SumAverage
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8 What you need for (serious) Java Programming
8.1 Buy a Java book (or use online tutorials)
• Java Tutorial from Sun: http://java.sun.com/docs/books/tutorial/
8.2 Use (hyperlinked) Java APT specification
• Java 2 (v1.4) API Specification: http://java.sun.com/j2se/1.4/docs/api/
8.3 Use RAD Tools (especially for GUI programming) if possible
• Full fledged RAD tools: Vis ual Café, JBuilder, etc.
• Or, Java editor for writing, compiling, executing, and locating syntax errors: JPad, Editplus, etc.
8.4 * Some (possibly) important, but not covered topics
• Graphic / Window User Interface : JFC/Swing (javax.swing.*), 2D Graphics (java.awt.Graphics2D, java.awt.geom.*), etc. • Multiple tasks at once: Multithreading (java.lang.thread) • Database Access: JDBC Database Access (java.sql.*) • Integrating Native Codes: Java Native Interface (JNI) • And, Networking, Remote Method Invoking, Security, etc