Java: History and Introduction

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Java History and Introduction

• (Lecture 1)

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History

• Java
• Based on C and C
• Developed in 1991 for intelligent consumer
  electronic devices
• Green Project (According to Gosling, "the goal
  was ... to build a system that would let us do a
  large, distributed, heterogeneous network of
  consumer electronic devices all talking to each
  other." )
• James Gosling Modified C
• Named Oak then Java

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History

• Platform independent ("Write Once, Run Anywhere")
  
• Interpreted Language
• Intermediate Code (Byte Code)
• Reliable
• Multiple inheritance and Operator overloading
  removed
• No Pointers because of security reasons

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History

• Internet exploded in 1993, saved project
• Reliability
• Security
• Platform Independence
• Java formally announced in 1995
• Now used to create web pages with interactive
  content and applications for consumer devices
  (pagers, cell phones)...

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Why Java??
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Motivation ?

• Simple
• Pure Object Oriented Language
• Allows Multi-threading
• Automatic Garbage Collection
• More Robust (Run time exceptions)
• (Anyone heard of smart cards?)

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Disadvantages ?

• SpeedSlower than C
• To the rescue
• Just-in-time (JIT) compilation
• Inability to access platform-specific code
• To the rescue
• Java Native Interface (JNI) (can also speed up
  code)

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Difference Between C and Java

• Java is C minus
• Pointers
• Destructors
• Structs and unions
• Templates
• Multiple Inheritance
• Operator Overloading

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Types Of Java Programs

• Applications Stand-alone java programs runnable
  from command prompt
• Applets Run from inside a web browser or the
  appletviewer

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Basics of a Typical Java Environment

• Java Systems
• Consist of environment, language, Java
  Applications Programming Interface (API)
• Java programs have five phases
• 1. Edit
• .java extension
• 2. Compile
• javac command javac MyProgram.java
• Creates .class file containing bytecodes

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Basics of a Typical Java Environment

• 3. Loading
• Class loader transfers .class file into memory
• Classes loaded and executed by interpreter with
  java command
• To load,
• appletviewer Welcome.html
• Or java MyFile

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Basics of a Typical Java Environment

• 4. Verify
• Bytecode verifier makes sure bytecodes are valid
  and do not violate security
• 5. Execute
• Computer interprets program one bytecode at a
  time
• Performs actions specified in program

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Program is created in the editor and stored on
disk.
Phase 1
Compiler creates bytecodes and stores them on
disk.
Phase 2
Primary Memory
Phase 3
Class loader puts bytecodes in memory.
Disk
Primary Memory
Phase 4
Bytecode verifier confirms that all bytecodes
are valid and do not violate Javas security
restrictions.
Primary Memory
Interpreter reads bytecodes and translates them
into machine language for that computer.
Phase 5

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Java Virtual Machine (JVM)

• Hypothetical computer that executes a Java
  program the interpreter
• Each computer architecture (Intel x86, Macintosh,
  etc.) has its own JVM implemented in software
• Sun Microsystems also has a JVM processor
  (hardware chip)
• Web browsers have built-in interpreter

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Intro to Some Java Technologies

• Servlets Small server-side programs
• JDBC Links Java programs to databases
• RMI Helps build distributed applications
• EJBs Reusable software components

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Thank you!!