The Java Language

Topics of this Course

Introduction to JavaThe Java LanguageObject Oriented Programming in JavaExceptions Handling

ThreadsAppletGUINetworking

Bibliography

History

What is Java?

Environments

Security

Memory Managment

Java Syntax

Chapter 1Introduction to

Java

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

S ourc ec ode

Java Runtime

U N IX P C Mac intosh

J ava R untim e

B yte C ode

. . .

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.

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.

Environments

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 run-time 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 programmer-defined 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 system-dependent 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 dynamically at run-time

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 error-handling 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”.