Introduction to Java

1
Introduction to Java

• Huseyin Ergun
• CS204 Advanced Programming

2
What do we do with Java?
3
OBSS

• The company i work is an Enterprise Java Company
• Provides solutions and services to banks, large
  insurance companies, gsm operators ...
• What i will present is the core technology of all
  projects
• We have an open source framework? A-ha ?

4
History of Java

• Founded by Michael Java at 1945
• ? joking
• Just to know, it is an object oriented easy to
  use language
• On the market for around 10 years

5
Pay attention (the thing to remember)
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Architecture of Java (Client) Applications

• Java applications are compiled and run on a
  machine just like any other general programming
  language such as C/C. No web server or network
  are required although Java applications may also
  use network connections for distributed
  computing.
• One can download dynamically for applets or
  servlets
• There are also native (conventional) Java
  compilers

Java code is compiled to produce byte
code run by Java Virtual Machine (JVM) to
produce results
7
Java Applications in a Nutshell

• Java programs written in a file with extension
  .java.
• Applications are .java files with a main()
  method. This is called by the Java system.
• Compile and run a Java application (using
  bytecodes)
• Run the compiler on a .java file
• javac MyProgram.java
• producing a file of Java byte code,
  MyProgram.class
• Run the interpreter on a .class file
• java MyProgram
• which executes the byte code
• The tools javac and java are part of JDK.

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The Simplest Java Application Hello,World!

• Since Java is object-oriented, programs are
  organized into modules called classes, which may
  have data in variables called fields, and
  subroutines called methods.

Each program is enclosed in a class definition.
main() is the first method that is run.
class HelloWorld public static void main
(String args) System.out.println(Hello
World!)
The notation class.method or package.class.method
is how to refer to a public method (with some
exceptions).
Syntax is similar to C - braces for blocks,
semicolon after each statement.
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Hands on Experience

• Now will write the same in Eclipse

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Java vs. JavaScript

• Despite the name, JavaScript is a different
  language from Java, albeit with some
  similarities.
• A JavaScript program is written directly in the
  HTML page, and executed by the JavaScript
  interpreter, so also allows dynamic web page
  content in the browser window.
• JavaScript is special purpose - it is an
  object-based language that deals directly with
  browser entities like windows, text fields,
  forms, frames and documents.
• JavaScript can respond to browser events like
  mouse clicks and user-typed text.
• JavaScript is fast to write, but not as powerful
  as Java.

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Multi-tier Architecture

• Distributed applications on the web naturally
  have a multi-tier architecture.
• Java plays a role at all three levels
• Graphical User Interface and client side analysis
  systems, including visualization
• Middle layer servers and software integration,
  including web servers, distributed object servers
  and other application servers.
• Less important for back end client software,
  which may be legacy code.

Internet
Client user interface running through browser
Internet or proprietary network
Middle level servers
Backend computing or databases
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Java FeaturesSimple and Familiar

• Familiar as it looks like C, but simpler to
  program.
• omits several confusing features of C including
  operator overloading, multiple inheritance,
  pointers and automatic type coercions
• Adds automatic garbage collection to make dynamic
  memory management much easier than in C or C.
• No more frees or deletes. No more memory leaks.
• Adds Interface construct, similar to Objective C
  concept, to compensate for the lack of multiple
  inheritance.
• Small kernel is suitable for Java ports to
  consumer electronic devices.
• But need customization J2ME to be effective
  (remove unnecessary features and support special
  capabilities of PDAs etc.)

13
Java FeaturesArchitecture-Neutral

• C/C programming in a heterogeneous network
  environment demands compatibility across several
  vendor platforms and their compilers.
• Solved in Java by designing platform-independent
  binary representation called Java
  bytecodecomparable to P-code in UCSD Pascal.
• Java compiler reads Java source and generates
  Java bytecode, which is shipped to user.
• Each client must have a Java Virtual Machine
  program, which interprets (runs) Java
  bytecodes.

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Java FeaturesRobust

• Java enforces compile-time type checking and this
  eliminates some error prone constructs of C/C.
• Pointer arithmetic is eliminated which allows
  for, e.g., runtime checking of array subscripts,
  and enforces security of the Java model.
• Explicit declarations are always required
  argument types of methods are always checked
  (unlike C). This allows the Java complier to
  perform early error detection.
• Java is most secure of popular languages because
  it is strict and security was built in

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The Java 2 Platform

• Sun are now offering 3 editions
• Java 2 platform, Standard Edition (J2SE)
• Refines earlier JDKs
• Available in version 1.4.
• Java 2 platform, Enterprise Edition (J2EE)
• Incorporates multiple technologies for
  server-side and multi-tier applications.
• Java 2 platform, Micro Edition (J2ME)
• Optimized run-time environment for consumer
  products.

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Installing Java on Your Comp

• Become administrator (for Windows) or root (for
  Linux, etc.)
• This is recommended though probably not
  essential.
• Go to http//java.sun.com/j2se/1.4
• Select Download Java 2 SDK, standard edition,
  v1.4.x, for Windows or Linux, etc.
• Download the software, then read
  http//java.sun.com/j2se/1.4/install-windows.html
  or http//java.sun.com/j2se/1.4/install-linux.html
  , etc.
• Pay attention to instructions for setting PATH.
• These give a command line interface for the Java
  compiler javac and the Java interpreter (JVM
  driver) java, etc.
• If you are familiar with UNIX environments, but
  want to use these commands from Windows, consider
  installing Cygwin www.cygwin.com.

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Java Tools
18
Books (From Bilmer)

• Complete Java 2 certification study guide
• Java how to program
• Ivor Horton's beginning Java 2, JDK
• Introduction to Java programming comprehensive
  version
• Murach's beginning Java 2, JDK 5 Java
  developer's guide

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Development Studios

• Eclipse http//www.eclipse.org/
• Netbeans http//www.netbeans.org/
• Jbuilder http//www.borland.com/de/products/jbuild
  er/
• Notepad http//www.microsoft.com

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Java Language Basics
21
Obvious similarities to C, C

• Java syntax has many similarities to C, C.
• All variables must be declared
• Syntax of expressions and control structures
  almost identical to C, C
• C or C style comments allowed.

22
Obvious differences from C, C

• No low-level pointers or pointer arithmetic.
• Instead have variables and expressions of
  reference type.
• No malloc() or free() to allocate more memory for
  dynamically created data structures instead have
  a new operator for creating objects, plus
  automatic garbage collection.
• Can declare variables almost anywhere (like C).
• No struct, union, enum, typedefclasses and
  objects are used uniformly instead.

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Documentation Comments

• Used by documentation-generating tools like
  javadoc to produce documentation, typically in
  HTML form.
• Optionally include formatting tags like _at_param,
  which flags a description of a method parameter
• / This method does what it feels like.
• _at_param bar This is a pointless
  argument. /
• void foo (int bar) . . .
• Other formatting tags include _at_returns which
  flags a description of a method result value, or
  _at_see name, which creates a hypertext link to name.

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Crash Course

• Now it is time to gain some sound knowledge ...

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The Java Object Model Classes, Instances and
Methods
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You should know ?

• This is you expected to know from previous
  classes ?

27
The Java Object Model Inheritance and the Class
Hierarchy
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Some Dependencies between Classes

• Use
• A uses B the most informal and general
  relation. A might, for example, call a method
  from class B, or have a method with argument type
  B or return type B.
• Containment
• A has a B an important special case of
  useclass A has a field of type B.
• Inheritance
• B is an A class B has all the properties of
  class A. The compiler treats B as a special case
  of A, and allows an instance of B to be used in
  any place where an instance of A could appear.
  In general the class B will extend A with some
  extra properties of its own.

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Inheritance

• The inheritance relation is (unexpectedly?)
  powerful it is built into all fully
  object-oriented languages.
• In Java, if some class A has been defined, we can
  subsequently declare a new class, B, and specify
  that it extends A.
• Class A is called the superclass of B. Class B
  is a subclass of A.
• The class B is automatically given (inherits) all
  the fields and method definitions of A. Further
  fields and methods can be added that are specific
  to B.
• In particular, for every method signature in
  class A, class B will have a method with
  identical signature.
• Crucially, though, the class B may define a
  different implementation for some of those
  methods.

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Trivial use of Inheritance

• class Shape
• void setColor(Color color)
  this.color color
• Color color
• int x, y //
  position of center, say
• class Circle extends Shape
• void drawCircle() . . .
• double radius
• class Rectangle extends Shape
• void drawRectangle() . .
  .
• double height, width
• Subclasses automatically inherit color, x, y
  fields of Shape, and setColor() method.

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Polymorphism?

• Polymorphism?

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Exceptions
33
Exceptions are Pervasive

• Java has a concept of exceptions similar to C.
• Unlike C, Java exceptions are strictly checked.
• Most classes in the standard Java library throw
  some exceptions. We will see, these must be
  caught or thrown.
• This means that it is almost impossible to write
  useful Java code without some knowledge of the
  exception mechanism!

34
Exception Objects, and throw

• Any kind of exception that can be thrown by Java
  code is described by an exception object. Its
  class must be a subclass of Throwable.
• If e is a Throwable object, the statement
• throw e
• behaves something like a break statement it
  causes the enclosing block of code to end
  abruptly.
• If the throw statement appears inside a try
  statement whos catch clause matches the class of
  e, control is passed to the catch clause.
• Otherwise the whole method (or constructor) ends
  abruptly. The exception e is thrown again at the
  point of invocation (in the calling code).

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throw compared with break

• myBlock
• . . .
• break myBlock
• . . .
• . . .
• Control jumps to end of matching block

• try
• . . .
• throw new MyException()
• . . .
• catch (MyException e)
• . . .
• . . .
• Control jumps to start of matching catch clause

36
Interfaces
37
Abstract Classes Revisited

• Recall an abstract class is a class that contains
  some abstract method declarations, with no
  implementation.
• An abstract class can only be instantiated
  indirectly, as a superclass of a class that
  overrides all the abstract methods, and gives
  them an implementation. You cannot directly
  create an instance of an abstract class.
• Constructors, static methods, private methods
  cannot be abstract.
• A subclass that does not override all abstract
  methods is still abstract.
• A method that overrides a superclass method
  cannot be abstract
• But an abstract class will generally also contain
  non-abstract membersmethod implementations,
  instance variables, etcand constructors.

38
Interfaces

• An interface is something like an abstract class
  where every method is required to be abstract.
• An interface specifies a collection of instance
  methods (behaviors) without giving the
  implementation of their bodies akin to giving an
  API
• public interface Storable
• public abstract void
  store(Stream s)
• public abstract void
  retrieve(Stream s)
• Interfaces cannot include instance variables,
  constructors, or static methods.
• They can include class variables, but only if
  they are declared finalessentially constant
  definitions.

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Implementing an interface

• As for an abstract class, one cannot directly
  create an instance of an interface.
• Unlike an abstract class, one cannot even extend
  an interface to create a class. An interface is
  not a class, and it cannot have subclasses.
• Instead, a class must implement an interface
• public class Picture implements Storable
• public void store(Stream s)
• // JPEG compress image
  before storing
• . . .
• public void retrieve(Stream s)
• // JPEG decompress image
  after retrieving
• . . .

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Packages
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Packages

• One file can contain several related classes, but
  only one of them can be public. If the public
  class is called Wheat, then the file must be
  called Wheat.java.
• A set of classes in different files can be
  grouped together in a package. Each file must
  start with a package declaration, eg
• package mill

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Import statements

• The import declaration allows you to avoid giving
  fully qualified names, eg
• import java.util.Vector //
  import declaration
• public class VectorTest
• public static void main (String
  args)
• Vector bag new Vector()
• bag.addElement(new
  String(item))
• Can also import all classes in, eg, java.util by
• import java.util.
• (but note wildcard can only appear in last
  position).
• Note classes (like String) in java.lang are
  automatically imported.

43
Some data Types?

• HashMap ?
• ListMap ?
• Array List ?

44
Questions?

• Questions?
• Now it is time to open Eclipse, and do some
  programming