Download Introduction to JAVA - School of Business Administration

Introduction to JAVA
Vijayan Sugumaran
School of Business Administration
Oakland University
Rochester, MI 48309
Introduction to JAVA
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Ancestor to JAVA - Oak
– small, robust, architecture independent, ObjectOriented, language to control interactive TV.
– didn’t go anywhere
– refocused for Internet, Java was born.
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JAVA is both compiled and interpreted
language.
JAVA
Source
Code
Compiled
JAVA
byte-code
UNIX
PC
MAC
Java Introduction (contd.)
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JAVA byte-code (or J-code) is executed by JAVA runtime 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 arraybounds checking)
Java and Javascript
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Don’t 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
Java Characteristics
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Unlike C++, Java doesn’t allow programmer
defined operator overloading
Java doesn’t have a preprocessor, so it doesn’t
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
Java Characteristics (contd.)
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Statistically typed
– data types etched in stone for the compiler (C, C++)
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Dynamically typed
– type checking performed during execution (lisp or smalltalk)
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Early binding
– binding method calls to definitions at compile time (C, C++)
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Late binding
– locating definitions of methods dynamically at run time
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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
Java Characteristics (contd.)
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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. Can’t
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.
JAVA Security
System Resources
Security Manager
Class Loader
Verifier
Untrusted
Source
JAVA Binary
Safety Implementation
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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
More about Class Loader
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Class Loader:
– Java adds second layer of security with class loader
– It brings in Java binary classes that contain bytecode 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
Security Manager
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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
Java Applets
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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
Error Handling
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The protection of references is one of the most
fundamental aspects of Java Security
Java code has to play by the rules--it can’t peek
into places it shouldn’t
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
Multi Threading
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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
Java Packages
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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
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:
> javac Welcome.java (creates Welcome.class - java bytecode)
executing the program:
> java Welcome
(JVM executes the byte code)
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 <applet> tag
 Load the html page into a java supported browser
Sample HTML Document
<html>
<head>
</head>
<body>
<applet code=HelloWeb width=300 height=200 </applet>
</body>
</html>
 Save it as HelloWeb.html
 Open HelloWeb.html using the browser