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• J2SE (Core Java) Quick Reference
• By A.R. KISHORE KUMAR

Topics Covered:

• Java Buzz Words
• Data Types
• Control Structures
• Arrays
• Strings
• OOPS in Java
• Static Methods
• Inner Class
• Abstract Class
• Interface
• Packages
• Exceptions
• Generic Types
• lang Package
• util Package
• io Package
• net Package
• Threads
• Abstract Window Toolkit
• Swings
• Applets

Introduction to Java

• In 1990, Sun Micro Systems Inc. conceived a project to develop software for consumer electronic devices controlled by a remote.
• The project initially named Stealth Project, was later changed to Green Project.
• In January 1991, James Gosling and his team met to discuss the project's development.
• Gosling considered using C and C++, but faced issues with their system-dependent nature.
• Gosling's team developed a language for system independence, initially named OAK, later changed to Java due to trademark issues.
• The language was named Java, inspired by the high-quality coffee consumed during the development.
• Good coffee was supplied from "Java Island".
• Java made its formal debut at the Sun World conference in 1995.
• JDK 1.0 was released January 23rd, 1996.

Features of Java (Buzzwords)

• Simple: Java is easy to learn and practice due to its resemblance to C and C++.
• Object-Oriented Programming (OOP): Java is purely OOP-based, unlike C++.
• Distributed: Designed for network use, Java has libraries that work with TCP/IP.
• Secure: Java is designed for Internet use and enables construction of virus and tamper-free systems.
• Robust: Java programs avoid crashing with exception handling and memory management.
• Interpreted: Java programs are compiled in bytecode, which is then downloaded and interpreted by the interpreter.
• Class files have bytecode instructions, and the JVM executes the bytecode.
• Portable: Java has no implementation-dependent aspects and yields the same result on any machine.
• Architectural Neutral: Java bytecode is machine-independent and can run on any machine with any OS.
• High Performance: Includes a Just In-Time (JIT) compiler along with an interpreter for faster performance.
• Multithreaded: Multithreading is executing different parts of a program simultaneously.
• An essential feature to design server-side programs.
• Dynamic: Java is used to develop programs that change dynamically on the internet like Applets.

Obtaining the Java Environment

• Java Development Kit (JDK), including the compiler and runtime, can be downloaded from http://java.sun.com/javase.
• After downloading, JDK installs into C:\Program Files\Java\jdk1.5.0_05 by default (jdk1.5.0_05 is JDK's version).

Setting up Java Environment

• Set an environment variable so Java programs can be compiled and run.
• A PATH variable enables the OS to locate JDK executables outside the JDK's binary directory
• When variables are set from a command prompt they hold only for that session.

Setting Environment Variables (command prompt)

• set PATH=C:\Program Files\Java\jdk1.5.0_05\bin;%PATH%

Setting Environment Variables (system variables)

• These persist continuously
• Right-click "My Computer," choose "Properties," then the "Advanced" tab.
• Click "Environment Variables" and select "Path" under "System variables." Click "Edit."
• In the edit window, append ;C:\Program Files\Java\jdk1.5.0_05\bin; without disturbing what's already there.
• Press OK.

Programming Structure

• Comments describe program aim and features, improving readability.
• Java has single-line, multi-line, and documentation comments.
• Single-line comments start with // .
• Multi-line comments begin with /* and end with */.
• Java documentation comments begin with /** and end with */ and help create HTML API files.

Structure of a Java Program

• Java requires importing packages, grouping related classes and interfaces like a directory.
• Packages contain classes and interfaces, which then contain methods.
• Java is purely object-oriented, so programs must have at least one class.
• Use the class keyword followed by the class name.
• Program execution begins from the main method, whose return type is void, as it does not return anything
• Sample Program: //A Simple Java Program import java.lang.System; import java.lang.String; class Sample { public static void main(String args[]) { System.out.print ("Hello world"); } }`
• Java necessitates calling even the main() method in a way that does not require object creation
• Static methods are methods which can, therefore, be called without creating objects.

How JVM calls methods

• JVM calls the main method via Classname.main() at runtime.
• JVM (Java Virtual Machine) is written by Java Soft (Java development team) and should be available to JVM, it should be declared public.
• Java code start with a { and ends with }
• Use objectname.methodname () to call a method through the object.
• The class or object contains variables and methods.
• System.out.print ("Hello world"); displays “Hello world”.
• System is the class name, and out is a static variable of the System class for calling object and method
• println() after displaying the result throws the cursor to the next line.

Escape Squences

• Java supports escape sequences, prefaced by a backslash (), for special character handling during print statements.
• \t - Inserts tab
• \b - Inserts backspace
• \n - Inserts newline
• \r - Inserts carriage return
• \f - Inserts form feed
• ' - Inserts single quote
• " - Inserts double quote
• \ - Inserts backslash

Creating a Source File

• Type in Notepad (Start menu, Programs, Accessories); save with the .java.
• Filename = Class_name. To do this in Notepad, first choose the File > Save menu item. Then, in the Save dialog box:
• Using the Save in combo box, specify the folder where you'll save your file.
• In the File name text field, type "Sample.java", including the quotation marks. Now click Save, and exit Notepad.

Compiling into a .class File:

• Get to DOS by choosing Run... from the Start menu and enter cmd
• To compile Sample.java program go to DOS prompt
• Change the directory to where the .java file is located if the current directory is wrong
• At the prompt, type javac Sample.java
• The compiler generates byte code and Sample.class will be created

Executing .class Files:

• To run, enter java followed by a class name.

Java Virtual Machine

• JVM converts .java into .class file, which contains byte code instructions .
• Next.class file is given to the JVM, following the JVM architecture.
• Java Virtual Machine (JVM) is the heart of entire Java program execution process.

JVM architecture.

• Loads into memory, verifies that the byte-code instructions are proper and allocates memory to run a program.
• The memory is divided into 5 called runtime data areas, that contains the data and result
• Memory includes Method area, Heap, Java Stacks, PC(Processor Counter) registers, and Native Method Stacks
• All native code is called thought the Native Method Interface

Run Time Data Area

• Stores class code, variables, and methods
• Heap Area. Objects are created in heap. The method and heap areas are created immediately the JVM reads the class
• Java stacks has methods that are stored on Method area. This area needs to store the data and results, where Java method are excuted
• PC (Program Counter) registers. Memory addresses of the instructions of the methods in the registers
• Native Method Stacks. Native Methods are excuted in here. Generally native method libaries are required here
• Excuetion Engine interprests and JIT compiler translates bytecode inton machine language
• JVM identifies hotspots in .class and hands to JIT, where .java instructions are run.

Naming conventions

• Set by Java programmer for packages, classes, and methods.
• Naming sets are
• Package names writting in lowercase
• Each work of class/inter face names are capitalized
• Method- lowercase beginnin, capitalized each word
• Variable names writting lowercase then capitalized letters
• Constants (PI, COUNT) are all caps
• Kwywords are always lower case

Data Types

• Java defines eight simple types of data including byte, short, int, long, char, float, double and boolean. These can be put in four groups.
• Integer data types store integer numbers and include Byte, Short, Int, and Long
• For example,
• Byte ranges from -128 to 127
• Short ranges from -32768 to 32768,
• Int(32-bit signed integer): -2,147,483,648 to 2,147,483,647
• Long(64-bit signed integer): -9,223,372,036,854,775,808 to 9,223,372,036,854,775,807
• Float data types handle floating point numbers like Float and Double.
• Character. Represents a single character with chars.
• Boolean values handle true or false

Operators

• Operators act of variables called operands.
• Arithmetic: Addition(+), subtraction(-), multiplication (*), Division(gives dividend) 14/7 = 2, modulus (% gives remainder when devided) = 20 % 7 = 6)
• Assign operators save to a value:
• Compound assignemnt, += and ++, and -+, *= /=, etc
• Unary operator acts on only one operand

Relational Operators

• Equal (==)
• Not Equal (!=)
• Less Than, (<), Greater Than (>), Less Than/Equal (<=)

Logical Operators

• Used to constructed compound conditions with and, or, not: &&, |, !
• Bitwise operatiors act on individual bits.
• Ternary Operator (? :): Has the following synax, Variable = Expression1? Expression2: Expression3;

Control Statements

• Statements are statements that alter the flow

Selection Statements

• Selection statements allow the selection of the program execution based on condition
• IF statements: if statement performs a task depending on whether a condtion is whether true of false.
• If (Statement)
• Else
• Switch statements are used when there is several options, and have choose from The avaiable one:

Iteration Statements:

• Java's iteration statements are for, while and do-while.
• The statements are used to repeat same set of instructions specified number
• loops. While loops loops are used to repeat a group of statements as long as a condition is true.
• Syntax:
• while (condition)
• {statements;}
• do while = Do.. while loop repeates a group as lonh as a condition is true.
• do { statments } while(condition)
• The For loop is the same as do while or while loop, but more compact syntcatally

Jump Statements:

• Java supports 3 jump statements: Break, Contune, and Return
• Breaks go to the end of nested block

break:

• can be used on a switch block or to come out the block
• continue statment is useful to continue the next repitition and statement inside and loop are excuted
• Can go to the end of a blocl.
• Note: goto statement is not available in java because it leads to confusion and forms infinite loops.

Acceptance Input from Keyboard

• Streams represents the flow of data.
• Can either be INPUT or OUTPUT data
• Streams are represented as classes in package Java.IO
• System.in represents InputSteam object, which can represent a standard input device with is the keyboard
• System.OUT presents PrintStrean object, with represents the standard output with is the monitor
• System.ERR this feild also represents PrintStream object, by default reprsents monitior.
• To accept Data form Keyboard need to connect the keyboard to an INPUT steam
• InputSteamReader obj = new InputStreamReader (System.in);
• Also need to Connet Intersteam REader to BufferReader
• BufferedReader br= new BufferedReader (obj);

Get Keyboard Method

• Reads method reads a keyboard but return a
• Single Charater = `(char)br.read(), intiger. IN order to stor a
• integer data type is converted to the read function.

Accepting a String from Keyboard:

• Create a BufferedReader class object (br). • Then read a string from the keyboard using readLine() method as: String str = br.readLine ();

• String Integer value, Boolean value, double, float

Array, String, and StringBuffer

• An array is a group of elements of the same type.
• Types categorized into single and multi dimensitional

Single Arrays

• A one D array represent a column of element.
• Creating 1 D array directly
  • Int marks [] = {50, 60, 55, 67,70);
  • Or declating the array first then allorcate memory for it
    • Int marks []; MARKS = new int [5];
• Two statemetn can also be written as
• INT marks [] < new int [5];`

Multi Deiminsional Arrays

• Combination of two or more 1 D ararys
• To represent the 2 D array, use 2 peairs of square braces[][] after the array name
• Or declare at same time like a SD array
• Knowing Array Size
• The property 'length' tells array size