Introduction to JAVA

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Introduction to JAVA

• Vijayan Sugumaran
• School of Business Administration
• Oakland University
• Rochester, MI 48309

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Introduction to JAVA

• Ancestor to JAVA - Oak
• small, robust, architecture independent,
  Object-Oriented, language to control interactive
  TV.
• didnt go anywhere
• refocused for Internet, Java was born.
• JAVA is both compiled and interpreted language.

UNIX
Compiled JAVA byte-code
JAVA Source Code
PC
MAC
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Java Introduction (contd.)

• JAVA byte-code (or J-code) is executed by JAVA
  run-time interpreter (JAVA compliant Virtual
  Machine)
• The interpreter can be run as a separate
  application or embedded in another software such
  as a browser.
• The fundamental unit of JAVA code is the class.
• Since JAVA interpreter runs compiled byte-code,
  JAVA is relatively a fast interpreted language.
• The run-time system can compile byte-code to
  native machine code on the fly to optimize
  performance. This is called Just in time. (one
  sticky problem is the array-bounds checking)

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Java and Javascript

• Dont confuse JAVA with Javascript
• Javascript is an object-based scripting language
  developed by Netscape
• Lets you create active user interface and capture
  and validate user inputs
• Client-side replacement for cgi-script, client
  side computing instead of server side
• Java is full-featured programming language
• Javascript can interact with Java applets

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Java Characteristics

• Unlike C, Java doesnt allow programmer defined
  operator overloading
• Java doesnt have a preprocessor, so it doesnt
  have macros, define statements, or conditional
  source compilation
• Java provides a well-defined package structure
  for organizing class files
• Supports only single inheritance class hierarchy,
  but allows multiple inheritance of Interfaces
• -Interface of a class--like an abstract class in
  C which specifies the behavior of an object
  without defining its implementation

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Java Characteristics (contd.)

• Statistically typed
• data types etched in stone for the compiler (C,
  C)
• Dynamically typed
• type checking performed during execution (lisp or
  smalltalk)
• Early binding
• binding method calls to definitions at compile
  time (C, C)
• Late binding
• locating definitions of methods dynamically at
  run time
• Java is statistically typed and late-binding
  language
• Java carries all data-type and method-signature
  information with it from its source code to its
  compiled byte-code form

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Java Characteristics (contd.)

• Base class in Java can evolve with out affecting
  derived classes as long as it maintains a valid
  form of its original structure. (in C, if the
  base class changes, the derived classes should be
  recompiled)
• Java handles garbage collection
• Java uses references instead of pointers.
  Cant do pointer arithmetic with references. A
  references is a strongly typed handle for an
  object. All objects are accessed through
  references
• Security manager controls access to system
  resources like the file system, network parts,
  windowing environment, etc.

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JAVA Security
System Resources
Security Manager
Class Loader
Verifier
Untrusted Source
JAVA Binary
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Safety Implementation

• Verifier guarantees the integrity of incoming
  classes
• Verifier
• reads byte-code makes sure it behaves properly
• it is a type of theorem prover - steps through
  the byte-code and determines the behavior
• Class Loader handles loading classes from the
  network and protects basic system classes
• Class Loaders and security managers are
  implemented by applications that load applets
  such as browser and applet viewer

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More about Class Loader

• Class Loader
• Java adds second layer of security with class
  loader
• It brings in Java binary classes that contain
  byte-code into the interpreter
• Classes loaded remain associated with class
  loader
• If a class references another class, the request
  is served by its original class loader
• Classes loaded from one location can be
  restricted to interact with only those classed
  loaded from that same location

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Security Manager

• Security Manager
• Application level security decisions
• Security manager is consulted every time the
  application tries to access system resources
• The security manager can be simple or complex as
  the application warrants
• The integrity of the security manager is based on
  the protection afforded by the lower levels of
  the Java security model

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Java Applets

• Applets
• Small embeddable application
• A Java applet is a compiled Java program,
  consisting of classes just like any other Java
  program
• Applets are autonomous programs confined within
  the walls of the browser or applet viewer. Can
  interact with user and communicate with the host
  over the network

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Error Handling

• The protection of references is one of the most
  fundamental aspects of Java Security
• Java code has to play by the rules--it cant peek
  into places it shouldnt
• Error Handling
• Exceptions allow separation of error-handling
  code from normal code
• Exception carries with it an object that contains
  information about the situation that caused its
  exception

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Multi Threading

• Thread
• flow of control within a program
• threads run in the same address space
• share instance variables, but not local variables
• Multi Threading
• Threads provide efficient multi processing and
  distribution of tasks
• Threads need to be synchronized--only one
  synchronized method within the object may run at
  a given time

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Java Packages

• Scalability
• Package--structure that groups classes into
  functional units
• Within a package, a class is publicly visible or
  protected from outside access
• Package promote reuse scalability
• Package names constructed in a hierarchical way
  using dot-separated naming convention
• e.g. Java.awt.event

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Sample Java Program
//Create a file called Welcome.java public class
Welcome public static void main(String
args) System.out.println(Hello
World, Welcome to Java!!) Compiling
and Executing (using JDK) compiling source code
gt javac Welcome.java (creates
Welcome.class - java bytecode) executing the
program gt java Welcome (JVM
executes the byte code)
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Sample Applet
//create a file called HelloWeb.java public class
HelloWeb extends java.applet.Applet public
void paint( java.awt.Graphics gc)
gc.drawString (Hello World, Welcome to Java,
125, 95) ? Compile the java source file
(using JDK) javac HelloWeb.java ? Create an
html file that embeds the applet using the
ltappletgt tag ? Load the html page into a java
supported browser
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Sample HTML Document
lthtmlgt ltheadgt lt/headgt ltbodygt ltapplet
codeHelloWeb width300 height200
lt/appletgt lt/bodygt lt/htmlgt ? Save it as
HelloWeb.html ? Open HelloWeb.html using the
browser