A Little More Java Java 1st cup

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A Little More JavaJava -- 1st cup

• Based on lectures by Walter Makovoz, Ph.D.

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Java is a language which is

• simple
• object-oriented
• distributed
• interpreted
• robust
• secure
• architecture neutral
• portable
• high-performance
• multithreaded
• dynamic

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Java is ...

• ... high-level
• third generation programming
• language( like C, Fortran, Smalltalk, Perl, etc.)

• You can use Java to write computer applications
  that
• crunch numbers
• process words
• play games

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Java is distributed

• Java developer can create a class that is
  dependent on a class that resides on the Net,
  thus being assured of always picking up the
  latest copy of the remote class.
• The Net functionality makes Java a truly
  distributed environment.

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Java is interpreted

• Java uses an interpreted environment as its
  solution to the problem of functioning in a
  multi-platform environment.
• The Java compiler reads source files and from
  those files produces bytecode.

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Java is robust

• Java promotes the development of software robust
  enough to run in a multi-platform environment
• Takes over memory management responsibilities
• Eliminates pointers
• Makes arrays into true objects
• Tightens up type-checking
• Links dynamically

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Java is secure

• Java features assure system administrator that
  the Java environment is secure, and they can go
  ahead and install it on the their systems.

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Java is architecture-neutral

• No matter where you are located on the continuum
  of software development and use, Javas
  architecture-neutral interpreted bytecodes are a
  boon.

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Java is portable

• implementation according to the interpreter
• standardized data types
• windowing

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Java is multithreaded

• In the Java system, multiple processes (known as
  threads) execute simultaneously with Java itself
  apportioning system resources to each of the
  these threads.
• Java was written to take advantage of the
  pre-emptive, multithreaded capabilities of the
  modern operating systems on which its been
  implemented.

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Java is dynamic

• Java linking practices, combined with its
  flexible but strict class usage, make it dynamic
  enough to function reliably in an evolving
  environment.

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Java Tools

• Appletviewer
• Runs applets on your machine outside the context
  of a Web browser
• Java
• Java interpreted interprets and executes
  bytecodes
• Javac
• Java compiler compiles source files into
  bytecodes
• Javadoc
• Generates API documentation from Java source
  files
• Javah
• Generates C-language header and source files to
  connect Java programs with native compiled C code
• Javap
• Disassembles compiled byte code into its
  component variables and methods
• Jdb
• Java debugger an interactive debugger

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Java Terminology

• Java Applications
• Standalone Java programs that do not require a
  Web browser to run
• Java applications are more general-purpose
  programs like those you would find on any
  computer.
• Java Applets
• They run and are displayed inside a Web page with
  other page elements, and therefore have special
  rules for how they behave.
• Because of these special rules for applets,
  creating an applet may in many cases be more
  complex than creating an application.
• Java Bytecodes
• Special set of machine instructions that are not
  specific to any one processor or computer system
• The byte code interpreter is also called the Java
  virtual machine or the Java runtime interpreter.

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Java

• Java lets you write special programs called
  applets that can be downloaded from the Internet
  and run safely within a web browser.
• Traditional computer programs have far too much
  access to your system to be downloaded and
  executed willy-nilly.

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Java Features

• Java solves security problem by severely
  restricting what an applet can do
• A Java applet cannot write to your hard disk
  without your permission.
• It cannot write to arbitrary addresses in memory
  and thereby introduce a virus into your computer.
  
• It should not crash your system.

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Java is a Platform

• Java (with a capital J) is a platform for
  application development.
• A platform is a loosely defined computer industry
  buzzword that typically means some combination of
  hardware and system software that will mostly run
  all the same software.
• For instance PowerMacs running System 7.5 would
  be one platform.
• DEC Alphas running Windows NT would be another.

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Java is a Platform

• Computer programs are closely tied to the
  specific hardware and operating system.
• Eg., Windows program will not run on a computer
  that only runs DOS.
• A Mac application can't run on a Unix
  workstation.
• Major commercial applications like Microsoft Word
  or Netscape have to be written almost
  independently for all the different platforms
  they run on.

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Java is a Platform

• Java solves the problem of platform-independence
  by using byte code.
• The Java compiler does not produce native
  executable code for a particular machine like a C
  compiler would.
• Instead it produces a special format called byte
  code.
• Java byte code written in hexadecimal, byte by
  byte, looks like this
• CA FE BA BE 00 03 00 2D 00 3E 08 00 3B 08 00 01
  08 00 20 08
• This looks a lot like machine language, but
  unlike machine language Java byte code is exactly
  the same on every platform.

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Java is a Platform

• This byte code fragment means the same thing on a
  Solaris workstation as it does on a Macintosh
  PowerBook.
• Java programs that have been compiled into byte
  code still need an interpreter to execute them on
  any given platform.
• The interpreter reads the byte code and
  translates it into the native language of the
  host machine.
• Since the byte code is completely platform
  independent, only the interpreter and a few
  native libraries need to be ported to get Java to
  run on a new computer or operating system.
• The rest of the runtime environment including the
  compiler and most of the class libraries are
  written in Java.

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Java

• All these pieces, the javac compiler, the java
  interpreter, the Java programming language, and
  more are collectively referred to as Java. .