The Java(TM) Language - An Overview

1. The JavaTM
Language
- An Overview
2000. 3. 13.
Kwang Shin Oh
Dept. of Research & Development
http://raytrust.pe.kr
raytrust@raytrust.pe.kr
Copyright 2000 © Kwang Shin Oh

2. 2JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Introduction
 Java Programming language
 designed to solve a number of problems
in modern programming practice
 started as a part of a larger project to
develop advanced software for consumer
electronics
 to devices are small, reliable, portable,
distributed, real-time embedded systems

3. 3JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java
 Java is a simple, object-oriented,
distributed, interpreted, robust, secure,
architecture neutral, portable, high-
performance, multithreaded, and
dynamic language.

4. 4JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java – Simple
 without a lot of esoteric training and
which leveraged today’s standard
practice
 designed as closely to C++ as
possible in order to make the system
more comprehensible
 omits many rarely used, poorly
understood, confusing features of C++

5. 5JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java – Simple
 omitted features
 operator overloading(although the Java
language does have method overloading)
 multiple inheritance
 extensive automatic coercions
 added auto garbage collection
 simplifying the task of Java Programming
 making the system somewhat more
complicated

6. 6JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java – Simple
 complexity in many C and C++
applications
 storage management : the allocation and
freeing of memory
 automatic garbage collection benefits
 makes the programming task easier
 dramatically cuts down on bugs

7. 7JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java – Simple
 small
 enable the construction of software that
can run stand-alone in small machines
 the size of the basic interpreter and
class support
 about 40K bytes
 basic standard libraries and thread
support
 additional 175K bytes

8. 8JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java – Object
Oriented
 Object Oriented design
 technique that focuses design on the
data(=objects) and on the interfaces to it
 OO design is very powerful
 facilitates the clean definition of interface
 makes it possible to provide reusable
software

9. 9JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java – Object
Oriented
 analogy with carpentry
 “object-oriented” carpenter

mostly concerned with the chair he was
building

secondarily with the tools used to make it
 “non-object-oriented” carpenter

think primarily of his tools
 mechanism for defining how modules
“plug and play”

10. 10JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java – Distributed
 extensive library of routines
 coping easily with TCP/IP protocols like
HTTP and FTP
 open and access objects across the
net via URLs
 same when accessing a local file
system

11. 11JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java – Robust
 must be reliable in a variety of ways
 a lot of emphasis
 early checking for possible problems
 later dynamic(runtime) checking
 eliminating situations that are error prone
 advantages of a strongly typed
language(like C++)
 allows extensive compile-time checking
so bugs can be found early

12. 12JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java – Robust
 C++ inherits a number of loopholes in
compile-time checking from C
 relatively lax(particularly method /
procedure declarations)
 in Java
 require declarations and do not support
C-style implicit declarations

13. 13JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java – Robust
 single biggest difference between
Java and C/C++
 java has a pointer model that eliminates
the possibility of overwriting memory and
corrupting data
 not possible to turn an arbitrary integer
into a pointer by casting
 you don’t have to worry about freeing
or corrupting memory

14. 14JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java – Secure
 java enables the construction of virus-free,
tamper-free systems
 authentication technique are based on
public-key encryption
 changes to the semantics of pointers make
it impossible for applications
 to forge access to data structures
 to access private data in objects that they do
have access to
 closes the door on most activities of viruses

15. 15JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java – Architecture
Neutral
 designed to support applications on
networks
 variety of systems with a variety of CPU and
operating system architectures
 to enable a Java application to execute
anywhere on the network
 the compiler generates an architecture neutral
object file format
 the compiled code is executable on many
processors, given the presence of the Java
runtime system

16. 16JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java – Architecture
Neutral
 useful not only for networks but also
for single system software distribution
 makes the production of software that
runs on all platforms
 Java compiler generating bytecode
instructions
 easy to interpret on any machine
 easily translated into native machine
code on the fly

17. 17JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java – Portable
 being architecture neutral is a big
chunk of being portable
 but there’s more to it than that
 unlike C and C++, there are no
“implementation dependent” aspects of
the specification
 sizes of the primitive data types are
specified, as is the behavior of arithmetic
on them(examples are int, float)

18. 18JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java – Interpreted
 Java interpreter
 execute Java bytecodes directly on any
machine to which the interpreter has
been ported
 linking is a more incremental and
lightweight process
 so, the development process can be
much more rapid and exploratory

19. 19JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java – High
Performance
 bytecode format was designed with
generating machine codes in mind
 so, the actual process of generating
machine code is generally simple
 Reasonably good code is produced
 does automatic register allocation and
the compiler does some optimization
when it produces the bytecodes

20. 20JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java – High
Performance
 in interpreted code
 about 300,00 method calls per second
 on an Sun Microsystems SPARCStation
10
 performance of bytecodes converted
to machine code
 almost indistinguishable from native C or
C++

21. 21JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java –
Multithreaded
 there are many things going on at the same
time in the world around us
 multithreading
 a way of building applications with multiple
threads
 Java has a sophisticated set of
synchronization primitives that are based on
the widely used monitor and condition
variable paradigm that was introduced by
C.A.R.Hoare

22. 22JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java – Dynamic
 In a number of ways, Java is a more
dynamic language than C or C++
 for example, one major problem with
using C++ in a production environment is
a side-effect of the way that code is
always implemented

company A
 produces a class library(a library of plug and play
components)

23. 23JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java – Dynamic

company B
 buys it and uses it tin their product

company A
 changes its library and distributes a new release

company B
 will almost certainly have to recompile and
redistribute their own software
 problems can result when the end user
gets A and B’s software independently
(say A is an OS vendor and B is an
application vendor)

24. 24JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java – Dynamic
 A distributes an upgrade to its libraries
 then all of the software from B will break
 it is possible to avoid this problem in
C++, but
 it is extraordinarily difficult
 it effectively means not using any of the
language’s OO features directly

25. 25JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java – Dynamic
 by making these interconnections
between modules later
 Java completely avoids these problems
 makes the use of the object-oriented
paradigm much more straightforward
 libraries can freely add new methods
and instance variables without any
effect on their clients

26. 26JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java – Dynamic
 Java uses interfaces
 a concept borrowed from Objective C which is
similar to a class
 simply a specification of a set of methods that an
object responds to
 not include any instance variables or
implementations
 can be multiply-inherited(unlike classes) and
they can be used in a more flexible way than the
usual rigid class inheritance structure

27. 27JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java – Dynamic
 classes have a runtime representation
 there is a class named Class, instances
of which contain runtime class definitions
 in a C or C++ program
 have a pointer to an object
 don’t know that type of object it is
 there is no way to find out

28. 28JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java – Dynamic
 in Java
 finding out based on the runtime type
information is straightforward
 because casts are checked at both
compile-time and runtime
 trust a cast in Java on the other hand in
C and C++
 compiler just trusts that you’re doing the
right thing

29. 29JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Java – Dynamic
 possible to look up the definition of a
class given a string containing its
name
 this means that you can compute a
data type name and have it easily
dynamically-linked into the running
system

30. 30JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Summary
 The Java language provides a
powerful addition to the tools that
programmers have at their disposal
 Java makes programming easier
 it is object-oriented
 has automatic garbage collection
 compiled Java code is architecture-
neutral
 Java applications are ideal for a diverse
environment like the Internet

31. 31JavaTM
Overview
©2000KwangShinOh
Http://raytrust.pe.kr
Reference
 Sun Microsystems
 The JAVATM
Language:An Overview