1 java programming- introduction

What is Java
• A general purpose Programming Language
• Most Famous Object Oriented Programming
Language.
• Suited for the web and networked services,
applications, platform independent desktops,
robotics, embedded devices.

Java No Longer a
Programming Language
but a Platform

History of Java
• Developed by James Gosling, Mike Sheridan,
Patrick Naughton et. al;
• When working on green project;
• The goal of this project was to control
microprocessor embedded in electronic
devices.
• To perform this task, a platform independent,
reliable, and compact language was needed

History of Java
• Gosling was given task to identify the proper
programming language for the project.
• Gosling decided to develop a new language to
avoid the problems of current programming
languages(C++ especially)
• The new language developed for this purpose
is called “Oak”

History of Java
It consist of –
• Oak Programming Language
• An Operating System
• An User Interface
• A Hardware

Java Versions
• JDK 1.0 1996
• JDK 1.1 1997
• JDk 1.2 1998 (Java 2)
• JDK 1.3 2000 (Java 2)
• JDK 1.4 2002 (Java 2)
• JDK 5.0 2004 (J2SE 5.0)
• JDK 6.0 2006 (Java SE 6)
• JDK 7.0 2010

With the advancement of Java and its
widespread popularity, multiple
configurations were built to suite various
types of platforms.
Ex: J2EE for Enterprise Applications
J2ME for Mobile Applications.

• Sun Microsystems has renamed the new J2
versions as Java SE, Java EE and Java ME
respectively.
• Java is guaranteed to be Write Once, Run
Anywhere.

1 java programming- introduction

Object Oriented
• Java, everything is an Object.
• Java can be easily extended since it is based
on the Object model.

Platform independent
• Unlike many other programming languages
including C and C++, when Java is compiled, it
is not compiled into platform specific
machine, rather into platform independent
byte code. This byte code is distributed over
the web and interpreted by virtual Machine
(JVM) on whichever platform it is being run.

Simple
• Java is designed to be easy to learn. If you
understand the basic concept of OOP Java
would be easy to master.

Secure
• With Java's secure feature it enables to
develop virus-free, tamper-free systems.
Authentication techniques are based on
public-key encryption.

Architectural-neutral
• Java compiler generates an architecture-
neutral object file format which makes the
compiled code to be executable on many
processors, with the presence of Java runtime
system.

Portable
• Being architectural-neutral and having no
implementation dependent aspects of the
specification makes Java portable.
• Compiler in Java is written in ANSI C

Robust
• Java makes an effort to eliminate error prone
situations by emphasizing mainly on compile
time error checking and runtime checking.

Multithreaded
• With Java's multithreaded feature it is
possible to write programs that can do many
tasks simultaneously. This design feature
allows developers to construct smoothly
running interactive applications.

Interpreted
• Java byte code is translated on the fly to
native machine instructions and is not stored
anywhere. The development process is more
rapid and analytical since the linking is an
incremental and light weight process.

High Performance
• With the use of Just-In-Time compilers, Java
enables high performance

Distributed
• Java is designed for the distributed
environment of the internet

Dynamic
• Java is considered to be more dynamic than C
or C++ since it is designed to adapt to an
evolving environment. Java programs can
carry extensive amount of run-time
information that can be used to verify and
resolve accesses to objects on run-time.

Java Environment
Java Programming Language
JDK JSL
Java Programming Environment

Java Environment
(Java Development Environment)
A set of Programming Tools
(Java Standard Library)
(A collection of Classes and Methods)
It is also called Java API

Java Development Kit
(JDK)
• A Software Package
• Sun Microsystems made it available free on
the website
• Includes all basic components of Java
Environment

(JDK) - Tools

File Structure(C:jdk)
jdk
include
bin
lib
jre demo
Src.zip

• A collection of utilities that help u to develop,
execute, debug and document java programs
• Include-
– javac.exe
– java.exe
– javadoc.exe
– appletviewer.exe
bin

• Contains files that support native code
programming
• integrate java with other programming
languages.
• Include-
– jawet.h
– jni.h
include

Un T
• Root directory of Java Run Time Environment
• Includes
– JVM(Java Virtual Machine)
– runtime class libraries
– java application launcher
jre

• Includes additional class libraries and support
files required by the development tools
– htmlconvertor
– jconsole
lib

• Consist of sample programs with source code
demo

• Compressed files contain source code for java
API classes(that are included in JDK)
• We can unpack these files.
• The source code in these files helps
developers to learn and use the java
programming language
src.zip

• A command line tool that reads java source
code files and compiles it into executable byte
code classes.
Hello.java Hello.class
javac [Options] [File Name]
Example- Javac Hello.java
Syntax

• Java programs are compiled to byte code and
not in native code.
• Hence it can not be run on the machine
directly.
• Java Runtime Interpreter implements Java
Virtual Machine(JVM) and runs java
applications.

• It is used to run java .class file by doing
following two things-
– Starting Java Runtime Environment(JRE)
– Loading the specified class
– Invoking the main() method

Hello.class
java [ Options ] [ Class Name ] [ Arguments ]
Example- Java Hello
Syntax
OUTPUT

• A command line tool
• Used to run applet without the need of a web
browser

• Command line tool helps
• Used to find out and fixes bugs in java
programs
jdb [Options] [Class Name] [Arguments]

• Reads java bytecode class files
• Print human readable version of API defined
by those classes.

• Produces two c files
– C Header file
– C stub file
• These C files are used to implement native
methods

• The header file contains the declaration for
the C implementation function and structure
definition
• Stub file provide the glue that binds the java
method invocations and object references to
the C code
• The header file name is appended with .h
• The stub file name is appended with .c

• Used to generate documentation in the form
of HTML pages

txt • Text Editor
.java • Java Source Code
Javac • Java Compiler
.class • Java Class File
Java • Java Interpreter
Java • Java Program Output
javadoc HTML Files
javah Header and Stub Files
jdb

• A Component of Java System
• It interprets and executes the instructions in
our class files.

Figure 1: Memory configuration by the JVM.
ea
Class Files
Execution
Engine
Native Method
Interface
Native Method
Libraries

• Each instance of the JVM has one method area,
one heap, and one or more stacks - one for each
thread
• When JVM loads a class file, it puts its information
in the method area
• As the program runs, all objects instantiated are
stored in the heap
• The stack area is used to store activation records as
a program runs

Stack
Frame
Stack
Frame
Stack
Frame
Stack
Frame
Stack
Frame
Stack
Frame
Object Object
Object Object
Object Object
Class
Data
Class
Data
Class
Data
Class
Data
Class
Data
Class
Data
Method Area Heap Stack Area
Thread 1 Thread 2

• The class loader performs three main functions of
JVM, namely:
• The linking process consists of three sub-tasks,
namely,

Load
Link
Initialize
Verify
Prepare
Resolve

• Loading means reading the class file for a type, parsing it to get its
information, and storing the information in the method area.
• For each type it loads, the JVM must store the following information
in the method area:
– The fully qualified name of the type
– The fully qualified name of the type's direct superclass or if the type is an
interface, a list of its direct super interfaces .
– Whether the type is a class or an interface
– The type's modifiers ( public, abstract, final, etc)
– Constant pool for the type: constants and symbolic references.
– Field info : name, type and modifiers of variables (not constants)
– Method info: name, return type, number & types of parameters, modifiers,
bytecodes, size of stack frame and exception table.

• The end of the loading process is the creation of an instance of java.lang.Class for the loaded
type.
• The purpose is to give access to some of the information captured in the method area for
the type, to the programmer.
• Some of the methods of the class java.lang.Class are:
• Note that for any loaded type T, only one instance of java.lang.Class is created even if T is
used several times in an application.
• To use the above methods, we need to first call the getClass() method on any instance of T
to get the reference to the Class instance for T.
public String getName()
public Class getSupClass()
public boolean isInterface()
public Class[] getInterfaces()
public Method[] getMethods()
public Fields[] getFields()
public Constructor[] getConstructors()

Instances of Class objects created in the heap at runtime

import java.lang.reflect.Method; // Required!
//you must import your Circle class
public class TestClassClass{
public static void main(String[] args) {
String name = new String(“Ahmed”);
Class nameClassInfo = name.getClass();
System.out.println("Class name is : “ + nameClassInfo.getName());
System.out.println("Parent is : “ + nameClassInfo.getSuperclass());
Method[] methods = nameClassInfo.getMethods();
System.out.println("nMethods are: ");
for(int i = 0; i < methods.length; i++)
System.out.println(methods[i]);
}
}

• The next process handled by the class loader is Linking. This involves three sub-
processes: Verification, Preparation and Resolution
• Verification is the process of ensuring that binary representation of a class is
structurally correct
• The JVM has to make sure that a file it is asked to load was generated by a valid
compiler and it is well formed
• Class B may be a valid sub-class of A at the time A and B were compiled, but class A
may have been changed and re-compiled
• Example of some of the things that are checked at verification are:
– Every method is provided with a structurally correct signature
– Every instruction obeys the type discipline of the Java language
– Every branch instruction branches to the start not middle of another
instruction

• In this phase, the JVM allocates memory for the class (i.e static) variables
and sets them to default initial values.
• Note that class variables are not initialized to their proper initial values until the
initialization phase - no java code is executed until initialization.
• The default values for the various types are shown below:

• Resolution is the process of replacing symbolic names for types, fields and methods used
by a loaded type with their actual references.
• Symbolic references are resolved into a direct references by searching through the method
area to locate the referenced entity.
• For the class below, at the loading phase, the class loader would have loaded the classes:
TestClassClass, String, System and Object.
• The names of these classes would have been stored in the constant pool for TestClassClass.
• In this phase, the names are replaced with their actual references.
public class TestClassClass{
public static void main(String[] args){
String name = new String(“Ahmed”);
Class nameClassInfo = name.getClass();
System.out.println("Parent is: “ + nameClassInfo.getSuperclass());
}
}

• This is the process of setting class variables to their proper initial values - initial values desired by
the programmer.
• Initialization of a class consists of two steps:
– Initializing its direct superclass (if any and if not already initialized)
– Executing its own initialization statements
• The above imply that, the first class that gets initialized is Object.
• Note that static final variables are not treated as class variables but as constants and are
assigned their values at compilation.
class Example1 {
static double rate = 3.5;
static int size = 3*(int)(Math.random()*5);
...
}
class Example2 {
static final int angle = 35;
static final int length = angle * 2;
...
}

• After a class is loaded, linked, and initialized, it is ready for use. Its
static fields and static methods can be used and it can be instantiated.
• When a new class instance is created, memory is allocated for all its instance
variables in the heap.
• Memory is also allocated recursively for all the instance variables declared in its
super class and all classes up is inheritance hierarchy.
• All instance variables in the new object and those of its superclasses are then
initialized to their default values.
• The constructor invoked in the instantiation is then processed according to the
rules shown on the next page.
• Finally, the reference to the newly created object is returned as the result.

Rules for processing a constructor:
1. Assign the arguments for the constructor to its parameter variables.
2. If this constructor begins with an explicit invocation of another constructor in the
same class (using this), then evaluate the arguments and process that constructor
invocation recursively.
3. If this constructor is for a class other than Object, then it will begin with an explicit
or implicit invocation of a superclass constructor (using super). Evaluate the
arguments and process that superclass constructor invocation recursively.
4. Initialize the instance variables for this class with their proper values.
5. Execute the rest of the body of this constructor.

class GrandFather {
int grandy = 70;
public GrandFather(int grandy) {
this.grandy = grandy;
System.out.println("Grandy: "+grandy);
}
}
class Father extends GrandFather {
int father = 40;
public Father(int grandy, int father) {
super(grandy);
this.father = father;
System.out.println("Grandy: "+grandy+" Father: "+father);
}
}
class Son extends Father {
int son = 10;
public Son(int grandy, int father, int son) {
super(grandy, father);
this.son = son;
System.out.println("Grandy: "+grandy+" Father: "+father+" Son: "+son);
}
}
public class Instantiation {
public static void main(String[] args) {
Son s = new Son(65, 35, 5);
}
}

1 java programming- introduction

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