Download The Java Language Topics of this Course

The Java Language
Topics of this Course
Introduction to Java
The Java Language
Object Oriented Programming in Java
Exceptions Handling
Threads
Applet
GUI
Networking
Chapter 1
Introduction to Java
 Bibliography
 History
 What is Java?
 Environments
 Security
 Memory Managment
 Java Syntax
Bibliography
 The Java Programming Language –
Ken Arnold,James Gosling
 The Java Tutorial - Third Edition
Mary Campione & Kathy Walrath
Addison Wesley
 Online :
http://java.sun.com/docs/books/tutorial/index.html
History
1991 - language for consumer devices
– small, tight,efficient
– OOP
– C++ ruled out
– named Oak, name collision, became
Java
History
 1994 Browsers - static pages of html.
 Gosling realized that Java could provide
an architecture neutral browser with
downloadable and executable content.
 J. Gosling, J. Payne, P. Naughton build
HOTJAVA
 Demonstration of technology at
SunWorld May 23, 1995 was “Hit” of
show.
 Netscape releases Java interpreter in
2.0
Versions
 1.0 initial release
 1.1
– major changes to fix event handling
problems in 1.0
– support new and needed
methodologies (read that as beans)
 1.2 , 1.3(AKA Java 2)
mostly expanding capabilities.
What is Java ?
 An Object-oriented Programming
Language
 A Virtual Machine Architecture (JVM)
 Platform independent and secure
(portable bytecodes)
 A feature for dynamic WEB pages
 A program library
 Development tools
Virtual Machine
 Java is both compiled and interpreted
language
 Java source turned into simple binary
instructions
 C/C++ source is refined to native
instructions for a particular processor
 Java compiled into a universal format instructions for a virtual machine.
 Compiled Java byte-code (J-code) is
executed by Java run-time interpreter in
a safe, virtual environment .
 Executes stack-based instruction set,
manages a storage heap, creates and
manipulates primitive data types.
 Executes in accordance with a strictly
defined open specification that can be
implemented by anyone on any
platform.
Java Interpreter
 Java interpreter is lightweight and
small.
 Written in C/C++
 Interpreter can be run as
– separate, self-standing, application
– embedded in another piece of
software, such as Web browser
 Java code is implicitly portable
Source
code
Java Runtime
Byte
Code
Java
Runtime
UNIX
PC
Macintosh
...
Environments
 javac SomeName.java
– a class named SomeName is defined in this
file.
 java SomeName
– instantiates the SomeName.class and starts
the static public void main(String[] x) {}
method.
 appletviewer File.html
– file contains an applet. The browser loads
the applet, calls the init(), then start()
methods. If applet becomes invisible the
stop() method is called. After a time the
destroy() method is called.
Environments
 package means directory.
 package mechanism is a compile time
(javac) visibility issue, not a #include,
and does not mean the interpreter
(java) will find the classes it needs.
 work in a directory and be sure your
interpreter looks for classes in your
current directory. CLASSPATH and
PATH variables are sometimes required
to fix visibility issues.
Applet vs. Application
 Application - normal Java program
 Applet - program to be executed by a
WEB browser
 Applets
• typically loaded from some URL on
the WEB before execution
• new thing which makes Java
popular (dynamic pages)
“Just in Time” Compilation
 Interpreted languages slow
 Java is considered to be a fast
interpreted language - interpreter
executes compiled byte-code.
 Software implementations of the runtime system can optimize performance
by compiling byte-code to native
machine code on the fly.
Security
 Java provides several layers of
protection from dangerously flawed
code, viruses and Trojan horses.
 Java virtual machine architecture
assesses the safety of the code before
it’s run.
 These features provide foundation for
high-level security policies.
Security
 is essential when running applets over
the net
 three layers of security
–
language
– bytecode (verified before execution)
– runtime (security manager to prevent
unallowed IO and net operations)
Language Security
 No direct memory addressing allowed
 No pointer arithmetic
 Automatic storage release with a
garbage collector.
 All casts checked at runtime.
 All array accesses checked at
runtime.
Memory Managment
 The most important differences
between Java and C/C++ involve how
Java manages memory.
 Java
– eliminates add hoc pointers
– adds garbage collection
– adds true arrays
 These features eliminate many
problems related to safety, portability
and optimization.
Memory Managment
 Explicit memory allocation and
deallocation are considered the largest
source of programming errors in C/C++
 Java maintains objects in memory as
well as tracks all references to those
objects. When an object is no longer in
use, Java removes it from memory.
 Garbage collector runs in the
background. This means that most
garbage collection is done between
events (mouse clicks, keyboard hits,…)
Memory Managment
 Java does not have pointers as we know
them in C/C++. However, Java provides
references - safe pointer.
 A reference is a strongly-typed handle
for an object. All objects, except
primitive numeric types, are accessed
through references.
 References can be used for building
linked lists, trees and other data
structures.
Memory Managment
 Pointer arithmetic can’t be done using
references.
 References are passed by value.
 References can’t reference an object
through more than a single level of
indirection.
 Protection of references is one of the
fundamental aspects of Java security.
This means that references can’t
examine memory locations that should
not be examined.
Java Syntax
 Java does not allow programmerdefined operator overloading as we
know it in C++, for instance.
 The string concatenation operator + is
the only system-defined, overloaded
operator in Java.
 All methods in Java are like C++ virtual
methods, so overridden methods are
dynamically selected at run-time.
Java Syntax
 Java does not have a preprocessor, so it
doesn’t have
– macros
– #define statements
– conditional source compilation
 Other languages need these constructs
in order to address various systemdependent issues.
 Another use for conditional compilation
is for debugging purposes.
Java Syntax
 Debugging code in Java can be included
directly in Java source code by making it
conditional on a constant (static vs.
final) variable. Java compiler takes care
of this at compile time - it simply
removes it when it determines that it will
not be called.
 Java also provides a well-defined
package structure for organizing class
files. The compiler works with compiled
Java classes (all the info is there).
Arrays
 Arrays in Java are first-class citizens.
Arrays...
– can be dynamically allocated and
assigned like other objects
– know their own size and type
 True arrays eliminate the need for
pointer arithmetic.
Classes
 Fundamental unit of Java
 Class is an application component that
holds executable code and data
 Java classes are distributed in a
universal binary format that contains
Java byte-code and other class
information.
 Classes can be maintained discretely
and stored in files or archives (local or
net)
 Classes are located and loaded
Error Handling
 Java was first written for the networked
devices and embedded systems. This is
why Java provides robustness and
intelligent error management.
 Java has powerful exception-handling
mechanism. Allows us to separate errorhandling code from normal code cleaner&readable code.
MultiThreading
 Today’s apps require parallelism
(unrelated tasks must execute, if possible,
at the same time)
 Threads provide efficient multiprocessing
and distribution of tasks.
 Java makes threads easy to use - support
built into the language
 Synchronization of threads
 Java’s support of synchronization is
based “lock and key system for accessing
resources”.