Your first Java program

1. A Simple App:

Source code: HelloWorld.java

Type in and save the source code in a file named HelloWorld.java.

Compile: javac HelloWorld.java

Compile the source code, assuming that the PATH is set correctely. If the compilation is successful, a file named HelloWorld.class will be generated. This is the Java byte-code file.

Run: java HelloWorld

Execute the app by invoking the Java byte-code interpreter (resides in c:\jdk1.3\bin\).

Notes:
• the argument of java is the class name not the file name (i.e. no extension).
• the output will be visible directly under the line at which you type this.
• If you need to capture the output for printing purposes, add   >   filename.out to the end of this command line, (where filename is a name that you choose for the output file).

Example:
java HelloWorld > hello.out
The output will now not appear on the screen, but will be placed into a file named hello.out in your working folder.

2. A Simple Applet:

Graphics and Coordinate Systems

Steps to build and run an applet

Type in the source code:
HelloWorldApplet.java

Compile the source code:
javac HelloWorldApplet.java

Type in and save the following HTML source in a file named HelloDemo.html.

One way to view th eapplet is to use a Java-enable browser such as IE or Netscape

Another way to view th eapplet is to use the applet viewer in JDK
appletviewer HelloDemo.html

Basic Applet Structure

• In general an applet must extends the Applet class.
• It does not need a main() method, but should at least implement the paint() method (or other methods that will be discussed later on).
• It is embeded as follows:
      <applet   code=class_filename   width=pixels   height = pixels> < /applet>
• Each applet is assigned a rectangular region on the web page. The dimensions are set in the applet tag in the HTML file.
• The paint() method is invoked when the applet is initially loaded in a browser
• The origin (0,0) is loacted at the upper left corner of the rectangular region

Java basics

• Program structure, naming convention,

All Java applications have a similar basic structure which require:
• A program -app (or applet) comprises one or more classes. In the above eg. the program has a single class HelloWorld.
• An app must contain a class that implements the main method. The main() method is similar to the main function of C and C++ programs.
Each class can have a special static method main(), which must be declared as follows:

public static void main(String[] arg)
{
........

}


The main method serves as the entry point for the Java Virtual Machine (JVM)(i.e, JVM is an abstraction of the CPU of a computer). Recall that the machine code of the JVM is the Java byte-code, to invoke the Java app. See Java Execution Model When the app starts, no instance of the class has been created, so only a static method can be invoked. Thus main() must be static.

System.out object refers to the standart output.
The println() method prints a string and append a newline character at the end.
The print() method prints a string but does not advance to the next line.


Source Files:

Java programs are stored in files with the following considerations:
• All Java source files must have the extension .java
• Usually, each file contains a single class.
• File name should match the class name (public). Java is case sentsitive.

Comments:

• // This is a comment.
• /*
  This another comment that
  takes several lines.
  */


• Packages
• Java provides mechanisms to organize large-scale programs in a logical and maintainable fashion.
• Class:
  Classes are the basic building blocks of Java programs. They should be realatively small and comprise highly cohesive functionalities. Java provides two mechanizmes for logically organizing large programs:
• File:
  It may contain a main public class and possibly few more closely related classes.
• Package:
  It comprises many related classes, interfaces, or other packages.

Style Convention:
Package names are all in lower case letters such as: java.awt. Each class belongs to a package. Package declaration is file based; all classes in same source file belong to the same package.


• The Java class library is organized into a number of packages:
• java.lang --- general, basic language and run-time support
• java.awt --- AWT (Abstract Window Toolkit)for GUI
• java.io --- I/O
• java.util --- utilities including collections, time management
• java.applet --- applet
• java.sql --- java database connectivity (JDBC)
• java.net --- networking
• Import
• Example Time.java
• Use import
• Example Time1.java
• Use fully qualified class name



• Identifiers
  for instance, Hello, args An identifier can be any combination of letters, digits, the underscore character (_), and the dollar sign ($). But it cannot start by a digit.


• Reserved words
  See the list of Java reserved words, on p.30


• Data types
• Primitive types:
  See also:   Internal representation of primitive data


Datatype	Meaning	Size in Bytes	Min Value	Max Value	Sig. Digits
byte	Integer	1	-128	127
short	Integer	2	-32,768	32,767
int	Integer	4	-2 billion	2 billion
long	Integer	8	-9.2x1018	9.2x1018
float	Floating Point	4	-3.2x1038	3.2x1038	7
double	Floating Point	8	-1.8x10308	1.8x10308	15
char	Character	2	Any Unicode character
boolean	boolean	1	false	true

• Boolean type: boolean
• Boolean constants:

true
false
• Integer types: int byte short long
• Examples of integer constants:

354
-37
0
246543
• Floating point type: double float
• Examples of floating point constants:

4.642
-0.034
5.98e24
9.11e-31
• Character type: char
• 16-bit Unicode character.
• ASCII is a subset of Unicode --- ISO-8859 (Latin-1)
• Examples of character constants:

'A'
'y'
'8'
'*'
' ' (space)
'\n' (new line).
• Escape sequences:

 

\b	backspace
\n	new line (line feed)
\r	carriage return
\t	tab
\"	double quote
\'	single quote
\\	backslash

• Class types:
• String:
  A character string is an object in Java, defined by the class String.
• Examples of string literals:

"watermelon"
"fig"
"$%&*^%!!"
"354"
" " (space)
"" (empty string)
• constructor
• operator + (Concatenation)  and +=
• methods:
• charAt(int i)
• length()


• Primitive type conversion:
• widening:   safest, it does not cause los of information. Eg. convert byte (8 bits) to short (16 bits)
• narrowing:   should be avoided. Eg. short to byte
• Three kinds of conversion:
  • assignment conversion
    eg.,   float f;   int n;  f = n;
  • arithmetic promotion
    float result;  result = f / n; 
  • casting
    eg., avoid integer didvision when a float result is needed (with decimal part), eg.:   int m;   result = (float) n / m;


• Arithmetic operators and expressions
• +   addition
• -  subtraction
• *  multiplication
• /  division
• %  remainder



• Precedence and association: see appendix D, p.549



• Example: Time2.java
• Use Calendar class. Mixed-type string concatenation.


• Variable declaration and initialization
byte maxAge;
short timeInSeconds;
int x, y, z;
long count;
char a, b;
boolean flag;
float massInKilos;
double maxVal;

short timeInSeconds = 245;
char a = 'K', b = '$';
boolean flag = true;
double maxVal = 35.875;



• Canstants
Constants are identifiers similar to variables but hold a value at initialization that cannot be changed in the program. We use the keyword final. The naming convension is to use all capital letters for the identifier.


• Assignments
total = quantity * unitPrice;
count = count + 1;
count += 1;
count++;


• Creating objects

A class is used to define an object. The class is then like the data type of the object. For example to create a reference to a String object we use the following declation:

String str;               //no string object actually exist yet
To create an object, we use the new operator:
str = new String ("Hello, class!");     // This is an instantiation.
                    // The new operator call a constructor

We can do the above statements in one single statement as follows:

String str = new String ("Hi, Class");
After the object is instantiated, we use the dot operator to access its methods:

str.length();