Download Figure 2: The Java "J2SE 1.5.0 API specification" home page

BIT 1204/CSC 1101
INTRODUCTION TO
PROGRAMMING IN JAVA: THE
JAVA PROGRAMMING
LANGUAGE
LESSON 2
CONTENTS
1. Object oriented programming languages
2. Origins of Java
2.1. "Java enabled"
2.2. Programming in Java
3. The mechanics of creating an executable
Java "application" program
3.1. Source code
3.2. Compilation
3.3. Summary
4. Program execution
5. Application Programming Interface (API)
6. Java Environments
1. OBJECT ORIENTED PROGRAMMING
LANGUAGES
Java is an object oriented language, i.e. it
follows the object-oriented paradigm.
Object oriented programming is currently
extremely popular although the concept has
been around since the early 1970s. A brief
overview of well known object oriented
languages other than Java is given below
(listed in chronological order):
SMALLTALK'80. The archetypal objectoriented language/environment.
Everything in Smalltalk is an object.
SIMULA. The first language to
introduce the concept of objects.
Essentially ALGOL'60 with objects.
(Not usually considered to be a "real"
OO language but significant within the
context given here).
Eiffel. Designed in 1980s as a simpler and
more efficient implementation of the
object-oriented paradigm than that
espoused by Smalltalk.
C++. Essentially C with object-oriented
features. Best be described as a hybrid
between a conventional imperative
language and an object-oriented
programming language.
C#. New OO lnaguage based on C++ and
Java.
There are many more, but the above are the
most significant.
2. ORIGINS OF JAVA
The Java language started life in 1990 when
a team at Sun, headed by James Gosling,
designed a new programming language,
known as Oak, for the development of
consumer electronics software (the Green
Project). It was not till the introduction of
the WWW that the language Java came into
being. With the introduction of the WWW
the Oak development team quickly realised
that Oak style programs could be invoked
over the Internet using what has become
known as a Java Applet. The significance of
this is that the language would be
completely "platform" independent ---remember that previously to run (say) a C
program on a PC platform you needed a PC
compiler and to run the same program on a
UNIX platform you needed a UNIX
compiler, Etc. To demonstrate this Sun
developed the worlds first Java enabled
Web browser and called it HotJava.
One way of running Java is to use the Java 2
SDK --- Software Development Kit (previously
known, with respect to Java Version 1.1, as the
JDK --- Java Development Kit). There are three
editions of Version 1.4 of the Java 2 Platform
and the Java 2 SDK:
1. J2SE (Java 2 Standard Edition): The
edition we will be using.
2. J2EE (Java 2 Enterprise Edition): An
extended version of the J2SE which has
additional facilities to enable Java to be
used with distributed systems.
3. J2ME (Java 2 Micro Edition): A
slimmed down version of the J2SE
which enables Java to be used in
embedded systems such as smart cards,
pagers, mobile phones, etc.
2.1. Java "Enabled"
This platform independence sets Java apart
from any other language. However, Java
programs do not have to be run from a Java
enabled WWW browser, they can be run
independently as applications in the
traditional manner.
The term Java enabled means that inside the
browser there is a hypothetical machine known
as a Java Virtual Machine (JVM) which can run
Java for that particular computer. The Java that
comes over the net when an Applet is invoked
is encoded in something known as Java Byte
As a relatively young language (compared Code (JBC). A JVM resident on a particular
to languages such as Ada or C) Java is still computer can interpret JBC and consequently
evolving. The core of the language is small "run" the Applet on that computer.
(with respect to other "OO" languages such
as C++), however more and more packages
(libraries) are constantly being added.
2.2. Programming in Java
Version 1.0 of the language was launched,
by Sun, in 1995. Version 1.1 was produced From Section 2 we have seen that we can write
in 1997, and included minor modifications two different kinds of Java program:
to the core language and major additions to
the libraries. Version 1.2 came out in
1. Applications: Stand alone programs
December 1998 and included many
significant changes that resulted in this
version being referred to as "Java 2" and
then as the "Java 2 Platform". Version 1.2
was followed, in 2000, by version 1.3. The
current version (summer 2003) is 1.4, and
this is the version we will be using.
similar in nature to the kinds of
programs that might be written in any
other high level language (C, C++, Ada,
etc.)
2. Applets (small applications): Programs
that are executed from a Web browser
such as Netscape Communicator.
Applets typically allow the user to
control execution through a Graphical
User Interface (GUI) --- a collection of
components such as buttons and scroll
bars within a window. An Applet can be
executed from any "Java enabled"
browser.
We will be focusing on the first of these
approaches.
3. THE MECHANICS OF CREATING AN
EXECUTABLE JAVA "APPLICATION" PROGRAM
3.1. Source Code
As noted above JBC comprises a set of
instructions written for a hypothetical machine
known as the Java Virtual Machine (JVM). In
this manner the Java language achieves its
platform independence. We say that Java code
is portable in that a Java program will run
(without modification) on many types of
machine. Portability is an important issue in
software development and an often quoted
advantage of the Java high level languages.
Thus, given a Java source code file called (say)
Mypro.java the Java compiler is invoked as
follows:
Using a high level language, such as Java, a
program is written as ordinary text stored in
a file. This text is referred to as the source
code. A file containing text is usually
referred to as a text file. Text files are
created using a program called a text editor.
This is a program the same as any other,
input is usually from the keyboard and the
output is a text file. Text editors allow users
to create and edit text files. There are many
examples of such programs (PFE,
javac Mypro.java
KWRITE), the more sophisticated are
referred to as word processors and are
This will produce a Java byte code file called
designed to produce documents rather than Mypro.class.
programs. Text editors can be viewed as
simple word processing programs.
It is good practice to give meaningful
3.3. Summary
suffixes to source code files, in some cases
(and this includes Java) this is a requirement
To summarise the above, the stages in the
of the "compiler" (see below). Thus Java
production of a working Java program are as
source code text files must have the suffix
similarly C source code would have follows:
the suffix .c and Ada source code the suffix
1. Create the program text (source code)
.ada (although this is not a requirement of
using a text editor.
either C or Ada compilers). A plain text file,
2.
Compile the source code to create an
containing (say) some notes, might then
executable file (the load module).
have the suffix .txt. In this manner you can
identify the different files contained in a
This process is illustrated in Figure 1. Note that
directory at a glance.
in the figure the Java compiler also has input
from packages, these are files containing
preprogrammed Java classes which come with
the Java language and which facilitate common
3.2. Compilation
operations such as input and output. Because
Once a program exists as a text file the next these operations are so common there seems
stage is to compile (translate) this text into little point in writing them over and over again
for every Java program that we write, and
an executable form. Generally most
compilers (e.g. C, C++ and Ada compilers) therefore they are provided as part of the
translate source code into machine code. As language.
noted previously the disadvantage of this is
that different compilers will be required for (The "packages concept" is common to many
different languages and machines (machine programming languages.)
code is machine specific), i.e. languages
such as C and Ada are not platform
independent. Java, on the other hand, is
platform independent because it compiles
the source code into Java Byte Code (JBC).
.java;
Figure 1: The mechanics of creating an executable Java program.
4. PROGRAM EXECUTION
In the case of languages such as C, C++ and
Ada, once code has been compiled the result
may be run (executed) through a command
to the operating system comprising the
name of the program (without any suffixes).
However, in the case of Java --- to achieve
the desired platform independence --- the
Java byte code must be interpreted using an
interpreter. Assuming the existence of a
Java Byte Code (JBC) file called
Mypro.class this can be achieved as
follows:
java Mypro
Java can thus be viewed as a cross between a
compiled high level language and an interpreted
one.
Note: Remeber that the alternative way of
invoking a Java program is by writing it in the
form of an Applet and invoking it from a (Java
enabled) WWW page --- however this is not of
interest to us at present.
5. APPLICATION PROGRAMMING INTERFACE
(API)
In Sub-section 3.3 we noted the
existence and usage of the
"packages" concept. Many
programming languages provide a
set of packages to support common
operations such as Input and Output
(I/O). Collectively the
preprogrammed classes contained in
the Java packages which are
provided with the language are
called the Application Programming
Interface or API (a term also
common to many programming
language and not just Java). The
Java API consists of a large
collection of classes (hundreds)
organised into different package
(each package thus contains a
number of classes). Currently (April
2005) the Java API comprises some
30 packages (some of which
comprise several sub-packages).
net
--- for networking.
applet
--- for creating applets. i.e.java programs
that tun on the WWW.
We will only be considering the first three of these.
We can inspect individual packages and their constants
through Sun's Java WWW page at
http://java.sun.com. If you go to this page and
select "API Specifications" (listed near the top on the
menu on the left under "reference") this will take you to
another page (
http://java.sun.com/reference/api/index.html).
If, under "Standard Edition", you now select "J2SE
1.5.0" this takes you to an extremely useful page (
http://java.sun.com/j2se/1.5.0/ docs/api/)
shown in Figure 2. It is a good idea to add this URL to
your bookmarks/ favourites. All the packages currently
available are listed here (and all the classes). If you
select a package this will bring up a new page listing all
the classes in that package. By selecting an individual
class you can then inspect the features of that class, and
A package can therefore best be
described as a collection of classes so on. For example if we select the package java.lang
(which provides classes that are fundamental to the Java
which logically fit together. The
most commonly used packages are: programming language) and then "scroll" down the
class summary and select the class system you will see
a summary of all the fields and methods contained in
lang --- for routine operations
this class.
(this package is always
automatically compiled into
every Java program).
util
--- for additional useful
utilities.
io
--- for input and output
(I/O).
text
--- for specialized
formatting.
awt
--- for graphics and
graphical user interfacing.
Figure 2: The Java "J2SE 1.5.0 API specification" home page
(Remeber that the java.lang package is always automatically incorporated into every Java
program.)
6. JAVA INTEGRATED DEVELOPMENT
ENVIRONMENTS
There are a great many Java integrated
development environments (IDEs) available
(Sun's J2SE is not an environment).
All these are available for PC platforms using
Examples include:
Windows 2000 or NT or later. There is also a
version of Code Warrior that will run on
1. BlueJ: Developed as part of a
Macintoshes.
university research project about
teaching object-orientation to
An additional advantage offered by some of
beginners and therefore probably
these environments is that they can outperform
well suited to COMP101.
the J2SE in speed. A word of warning however,
2. Studio Standard 5: Sun's own Java
most environments come with their own built-in
IDE.
Java compiler which may not be the latest
3. Microsoft's Visual J++.
version (Java is upgraded by Sun at intervals) -4. JBuilder: Produced by Borland.
- to upgrade such environments a new version
5. CodeWarrior: Produced by
must be obtained --- at cost!.
Metrowerks
6. JCreator.
Created and maintained by Pius Nyaanga Momanyi. Last updated 27 April 2013