java notes, object oriented programming using java, java tutorial, lecture notes, java programming notes

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Class Notes on Basic concepts of OOP (Part - 2)
Contents:-Class & Object Overview Basic concepts of Java programming-advantages of Java, byte-code & JVM,
data types, Basic idea of inheritance, encapsulation, polymorphism, How to compile and execute Java
Programs
Class
In object-oriented programming, a class is a construct that is used to create instances of itself – referred to as
class instances, class objects, instance objects or simply objects. A class defines constituent members which
enable its instances to have state and behavior. Data field members (member variables or instance variables)
enable a class instance to maintain state. Other kinds of members, especially methods, enable the behavior of
class instances. Classes define the type of their instances.
Objects
An object is a location in memory having a value and referenced by an identifier. An object can be a variable,
function, or data structure. With the introduction of object-oriented programming the same word, "object,"
refers to a particular instance of a class.
Basic concepts of Java Programming
Java platform overview
Java technology is used to develop applications for a wide range of environments, from consumer devices to
heterogeneous enterprise systems.
Like any programming language, the Java language has its own structure, syntax rules, and programming
paradigm. The Java language's programming paradigm is based on the concept of object-oriented
programming (OOP), which the language's features support.
The Java language is a C-language derivative, so its syntax rules look much like C's: for example, code blocks
are modularized into methods and delimited by braces ({and }), and variables are declared before they are
used.
The java Compiler
When you program for the Java platform, you write source code in .java files and then compile them. The
compiler checks your code against the language's syntax rules, then writes out bytecodes in .class files.
Bytecodes are standard instructions targeted to run on a Java virtual machine (JVM). In adding this level of
abstraction, the Java compiler differs from other language compilers, which write out instructions suitable for
the CPU chipset the program will run on.

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The JVM
At run time, the JVM reads and interprets .class files and executes the program's instructions on the native
hardware platform for which the JVM was written. The JVM interprets the bytecodes just as a CPU would
interpret assembly-language instructions. The difference is that the JVM is a piece of software written
specifically for a particular platform. The JVM is the heart of the Java language's "write-once, run-anywhere"
principle. Your code can run on any chipset for which a suitable JVM implementation is available. JVMs are
available for major platforms like Linux and Windows, and subsets of the Java language have been
implemented in JVMs for mobile phones and hobbyist chips.
The garbage collector
Rather than forcing you to keep up with memory allocation the Java platform provides memory management
out of the box. When your Java application creates an object instance at run time, the JVM automatically
allocates memory space for that object from the heap, which is a pool of memory set aside for your program
to use. The Java garbage collector runs in the background, keeping track of which objects the application no
longer needs and reclaiming memory from them. This approach to memory handling is called implicit memory
management because it doesn't require you to write any memory-handling code. Garbage collection is one of
the essential features of Java platform performance.
The Java Development Kit
When you download a Java Development Kit (JDK), you get — in addition to the compiler and other tools — a
complete class library of prebuilt utilities that help you accomplish just about any task common to application
development.
The Java Runtime Environment
The Java Runtime Environment (JRE; also known as the Java runtime) includes the JVM, code libraries, and
components that are necessary for running programs written in the Java language. It is available for multiple
platforms. You can freely redistribute the JRE with your applications, according to the terms of the JRE license,
to give the application's users a platform on which to run your software. The JRE is included in the JDK.
Object-oriented programming concepts
The Java language is object-oriented. If you haven't used an object-oriented language before, its concepts
might seem strange at first. This section is a short introduction to OOP language concepts, using structured
programming as a point of contrast.
What is an object?
Structured programming languages like C and COBOL follow a very different programming paradigm from
object-oriented ones. The structured-programming paradigm is highly data-oriented, which means that you

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have data structures on one hand, and then program instructions that act on that data. Object-oriented
languages like the Java language combine data and program instructions into objects.
An object is a self-contained entity that contains attributes and behavior, and nothing more. Rather than
having a data structure with fields (attributes) and passing that structure around to all of the program logic
that acts on it (behavior), in an object-oriented language, data and program logic are combined. This
combination can occur at vastly different levels of granularity, from fine-grained objects like a Number, to
coarse-grained objects such as a Funds Transfer service in a large banking application.
Parent and child objects
A parent object is one that serves as the structural basis for deriving more-complex child objects. A child object
looks like its parent but is more specialized. The object-oriented paradigm allows you to reuse the common
attributes and behavior of the parent object, adding to its child objects attributes and behavior that differ.
Object communication and coordination
Objects talk to other objects by sending messages (method calls in the Java language). Furthermore, in an
object-oriented application, program code coordinates the activities among objects to perform tasks within
the context of the given application domain.
Object summary
A well-written object:
 Has crisp boundaries
 Does a finite set of activities
 Knows only about its data and any other objects that it needs to accomplish its activities
In essence, an object is a discrete entity that has only the necessary dependencies on other objects to perform
its tasks. Now you'll see what an object looks like.
The Person object
I'll start with an example that is based on a common application-development scenario: an individual being
represented by a Person object.
Going back to the definition of an object, you know that an object has two primary elements: attributes and
behavior. You'll see how these apply to the Person object.
Attributes
What attributes can a person have? Some common ones include:
 Name
 Age
 Height
 Weight
 Eye color
 Gender

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You can probably think of more (and you can always add more attributes later), but this list is a good start.
Behavior
An actual person can do all sorts of things, but object behaviors usually relate to some kind of application
context. In a business-application context, for instance, you might want to ask your Person object, "What is
your age?" In response, Person would tell you the value of its Age attribute.
More-complex logic could be hidden inside of the Person object, but for now suppose that Person has the
behavior of answering these questions:
 What is your name?
 What is your age?
 What is your height?
 What is your weight?
 What is your eye color?
 What is your gender?
State and string
State is an important concept in OOP. An object's state is represented at any moment in time by the value of
its attributes.
In the case of Person, its state is defined by attributes such as name, age, height, and weight. If you wanted to
present a list of several of those attributes, you might do sousing a String class, which I'll talk more about later
in the class.
Together, the concepts of state and string allow you to say to Person: tell me who you are by giving me a
listing (or String) of your attributes.
Principles of OOP using JAVA
If you come from a structured-programming background, the OOP value proposition might not be clear yet.
After all, the attributes of a person and any logic to retrieve (and convert) their values could be written in C.
This section clarifies the benefits of the OOP paradigm by explaining its defining principles: encapsulation,
inheritance, and polymorphism.
Encapsulation
Recall that an object is above all discrete, or self-contained. This is the principle of encapsulation at work.
Hiding is another term that is sometimes used to express the self-contained, protected nature of objects.
Regardless of terminology, what's important is that the object maintains a boundary between its state and
behavior, and the outside world. Like objects in the real world, objects used in computer programming have
various types of relationships with different categories of objects in the applications that use them.
On the Java platform, you can use access specifiers (which I'll introduce later in the class) to vary the nature of
object relationships from public to private. Public access is wide open, whereas private access means the
object's attributes are accessible only within the object itself.
The public/private boundary enforces the object-oriented principle of encapsulation. On the Java platform,
you can vary the strength of that boundary on an object-by-object basis, depending on a system of trust.
Encapsulation is a powerful feature of the Java language.
Inheritance

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In structured programming, it is common to copy a structure, give it a new name, and add or modify the attributes
that make the new entity (such as an Account record) different from its original source. Over time, this approach
generates a great deal of duplicated code, which can create maintenance issues.
OOP introduces the concept of inheritance, whereby specialized objects — without additional code — can "copy"
the attributes and behavior of the source objects they specialize. If some of those attributes or behaviors need to
change, then you simply override them. You only change what you need to change in order to create specialized
objects. As you know from the Object-oriented programming concepts section, the source object is called the
parent, and the new specialization is called the child.
Inheritance at work
Suppose you are writing a human-resources application and want to use the Person object as the basis for a
new object called Employee. Being the child of Person, Employee would have all of the attributes of a Person
object, along with additional ones, such as:
 Taxpayer identification number
 Hire date
 Salary
Inheritance makes it easy to create the new Employee class of the object without needing to copy all of the
Person code manually or maintain it.
You'll see plenty of examples of inheritance in Java programming later in the class and in my class notes.
Polymorphism
Polymorphism is a harder concept to grasp than encapsulation and inheritance. In essence, it means that
objects that belong to the same branch of a hierarchy, when sent the same message (that is, when told to do
the same thing), can manifest that behavior differently.
To understand how polymorphism applies to a business-application context, return to the Person example.
Remember telling Person to format its attributes into a String? Polymorphism makes it possible for Person to
represent its attributes in a variety of ways depending on the type of Person it is.
Polymorphism is one of the more complex concepts you'll encounter in OOP on the Java platform and not
within the scope of an introductory tutorial.
Advantages of JAVA
JAVA offers a number of advantages to developers.
Java is simple: Java was designed to be easy to use and is therefore easy to write, compile, debug, and learn
than other programming languages. The reason that why Java is much simpler than C++ is because Java uses
automatic memory allocation and garbage collection where else C++ requires the programmer to allocate
memory and to collect garbage.
Java is object-oriented: Java is object-oriented because programming in Java is centered on creating objects,
manipulating objects, and making objects work together. This allows you to create modular programs and
reusable code.

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Java is platform-independent: One of the most significant advantages of Java is its ability to move easily from
one computer system to another.
The ability to run the same program on many different systems is crucial to World Wide Web software, and
Java succeeds at this by being platform-independent at both the source and binary levels.
Java is distributed: Distributed computing involves several computers on a network working together. Java is
designed to make distributed computing easy with the networking capability that is inherently integrated into
it.
Writing network programs in Java is like sending and receiving data to and from a file. For example, the
diagram below shows three programs running on three different systems, communicating with each other to
perform a joint task.
Java is interpreted: An interpreter is needed in order to run Java programs. The programs are compiled into
Java Virtual Machine code called bytecode.
The bytecode is machine independent and is able to run on any machine that has a Java interpreter. With
Java, the program need only be compiled once, and the bytecode generated by the Java compiler can run on
any platform.
Java is secure: Java is one of the first programming languages to consider security as part of its design. The
Java language, compiler, interpreter, and runtime environment were each developed with security in mind.
Java is robust: Robust means reliable and no programming language can really assure reliability. Java puts a
lot of emphasis on early checking for possible errors, as Java compilers are able to detect many problems that
would first show up during execution time in other languages.
Java is multithreaded: Multithreaded is the capability for a program to perform several tasks simultaneously
within a program. In Java, multithreaded programming has been smoothly integrated into it, while in other
languages, operating system-specific procedures have to be called in order to enable multithreading.
Multithreading is a necessity in visual and network programming.
Disadvantages of JAVA
Performance: Java can be perceived as significantly slower and more memory-consuming than natively
compiled languages such as C or C++.
Look and feel: The default look and feel of GUI applications written in Java using the Swing toolk it is very
different from native applications. It is possible to specify a different look and feel through the pluggable look
and feel system of Swing.
Java bytecode
Java bytecode is the form of instructions that the Java virtual machine executes. Each bytecode opcode is one
byte in length, although some require parameters, resulting in some multi-byte instructions. A Java
programmer does not need to be aware of or understand Java bytecode at all.

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Java virtual machine
A Java virtual machine (JVM) is a virtual machine that can execute Java bytecode. It is the code execution
component of the Java platform. Sun Microsystems has stated that there are over 5.5 billion JVM-enabled
devices.
A Java virtual machine is a program which executes certain other programs, namely those containing Java
bytecode instructions. JVMs are most often implemented to run on an existing operating system, but can also
be implemented to run directly on hardware. A JVM provides a run-time environment in which Java bytecode
can be executed, enabling features such as automated exception handling, which provides root-cause
debugging information for every software error (exception). A JVM is distributed along with Java Class Library,
a set of standard class libraries (in Java bytecode) that implement the Java application programming interface
(API). These libraries, bundled together with the JVM, form the Java Runtime Environment (JRE).
JVMs are available for many hardware and software platforms. The use of the same bytecode for all JVMs on
all platforms allows Java to be described as a write once, run anywhere programming language, versus write
once, compile anywhere, which describes cross-platform compiled languages. Thus, the JVM is a crucial
component of the Java platform.
Java bytecode is an intermediate language which is typically compiled from Java, but it can also be compiled
from other programming languages. For example, Ada source code can be compiled to Java bytecode and
executed on a JVM.
Java Basic Data Types
Variables are nothing but reserved memory locations to store values. This means that when you create a
variable you reserve some space in memory.
Based on the data type of a variable, the operating system allocates memory and decides what can be stored
in the reserved memory. Therefore, by assigning different data types to variables, you can store integers,
decimals, or characters in these variables.
There are two data types available in Java:
 Primitive Data Types
 Reference/Object Data Types
Primitive Data Types
There are eight primitive data types supported by Java. Primitive data types are predefined by the language
and named by a key word. Let us now look into detail about the eight primitive data types.
byte:
 Byte data type is a 8-bit signed two's complement integer.
 Minimum value is -128 (-2^7)
 Maximum value is 127 (inclusive)(2^7 -1)
 Default value is 0

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 Byte data type is used to save space in large arrays, mainly in place of integers, since a byte is four
times smaller than an int.
 Example : byte a = 100 , byte b = -50
short:
 Short data type is a 16-bit signed two's complement integer.
 Minimum value is -32,768 (-2^15)
 Maximum value is 32,767(inclusive) (2^15 -1)
 Short data type can also be used to save memory as byte data type. A short is 2 times smaller than an
int
 Default value is 0.
 Example : short s= 10000 , short r = -20000
int:
 Int data type is a 32-bit signed two's complement integer.
 Minimum value is - 2,147,483,648.(-2^31)
 Maximum value is 2,147,483,647(inclusive).(2^31 -1)
 Int is generally used as the default data type for integral values unless there is a concern about
memory.
 The default value is 0.
 Example : int a = 100000, int b = -200000
long:
 Long data type is a 64-bit signed two's complement integer.
 Minimum value is -9,223,372,036,854,775,808.(-2^63)
 Maximum value is 9,223,372,036,854,775,807 (inclusive). (2^63 -1)
 This type is used when a wider range than int is needed.
 Default value is 0L.
 Example : int a = 100000L, int b = -200000L
float:
 Float data type is a single-precision 32-bit IEEE 754 floating point.
 Float is mainly used to save memory in large arrays of floating point numbers.
 Default value is 0.0f.
 Float data type is never used for precise values such as currency.
 Example : float f1 = 234.5f
double:
 double data type is a double-precision 64-bit IEEE 754 floating point.
 This data type is generally used as the default data type for decimal values. generally the default
choice.
 Double data type should never be used for precise values such as currency.
 Default value is 0.0d.

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 Example : double d1 = 123.4
boolean:
 boolean data type represents one bit of information.
 There are only two possible values : true and false.
 This data type is used for simple flags that track true/false conditions.
 Default value is false.
 Example : boolean one = true
char:
 char data type is a single 16-bit Unicode character.
 Minimum value is 'u0000' (or 0).
 Maximum value is 'uffff' (or 65,535 inclusive).
 Char data type is used to store any character.
 Example . char letterA ='A'
Reference Data Types:
 Reference variables are created using defined constructors of the classes. They are used to access
objects. These variables are declared to be of a specific type that cannot be changed. For example,
Employee, Puppy etc.
 Class objects, and various type of array variables come under reference data type.
 Default value of any reference variable is null.
 A reference variable can be used to refer to any object of the declared type or any compatible type.
 Example : Animal animal = new Animal("giraffe");
Java Literals:
A literal is a source code representation of a fixed value. They are represented directly in the code without any
computation.
Literals can be assigned to any primitive type variable. For example:
byte a = 68;
char a = 'A'
byte, int, long, and short can be expressed in decimal(base 10),hexadecimal(base 16) or octal(base 8) number
systems as well.
Prefix 0 is used to indicates octal and prefix 0x indicates hexadecimal when using these number systems for
literals. For example:
int decimal = 100;
int octal = 0144;
int hexa = 0x64;
String literals in Java are specified like they are in most other languages by enclosing a sequence of characters
between a pair of double quotes. Examples of string literals are:

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"Hello World"
"twonlines"
""This is in quotes""
String and char types of literals can contain any Unicode characters. For example:
char a = 'u0001';
String a = "u0001";
Java language supports few special escape sequences for String and char literals as well. They are:
Notation Character represented
n Newline (0x0a)
r Carriage return (0x0d)
f Formfeed (0x0c)
b Backspace (0x08)
s Space (0x20)
t tab
" Double quote
' Single quote
backslash
ddd Octal character (ddd)
uxxxx Hexadecimal UNICODE character (xxxx)
A Simple Java Program
Now that the basic object-oriented underpinning of Java has been discussed, let’s look at some actual Java
programs. Let’s start by compiling and running the short sample program shown here. As you will see, this
involves a little more work than you might imagine.
/*
This is a simple Java program.
Call this file "Example.java".
*/
class Example {
// Your program begins with a call to main().
public static void main(String args[]) {
System.out.println("This is a simple Java program.");
}
}
About the Program
For most computer languages, the name of the file that holds the source code to a program is arbitrary.
However, this is not the case with Java. The first thing that you must learn about Java is that the name you
give to a source file is very important. For this example, the name of the source file should be Example.java.
Let’s see why? In Java, a source file is officially called a compilation unit. It is a text file that contains one or
more class definitions. The Java compiler requires that a source file use the .java filename extension. Notice
that the file extension is four characters long. As you might guess, your operating system must be capable of
supporting long filenames. This means that DOS and Windows 3.1 are not capable of supporting Java.
However, Windows 95/98 and Windows NT/2000/XP work just fine. As you can see by looking at the program,

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the name of the class defined by the program is also Example. This is not a coincidence. In Java, all code must
reside inside a class. By convention, the name of that class should match the name of the file that holds the
program. You should also make sure that the capitalization of the filename matches the class name. The
reason for this is that Java is case-sensitive. At this point, the convention that filenames correspond to class
names may seem arbitrary. However, this convention makes it easier to maintain and organize your programs.
Compiling the Program
To compile the Example program, execute the compiler, javac, specifying the name of the source file on the
command line, as shown here:
C:>javac Example.java
The javac compiler creates a file called Example.class that contains the bytecode version of the program. As
discussed earlier, the Java bytecode is the intermediate representation of your program that contains
instructions the Java interpreter will execute. Thus, the output of javac is not code that can be directly
executed. To actually run the program, you must use the Java interpreter, called java. To do so, pass the class
name Example as a command-line argument, as shown here:
C:>java Example
When the program is run, the following output is displayed:
This is a simple Java program.
When Java source code is compiled, each individual class is put into its own output file named after the class
and using the .class extension. This is why it is a good idea to give your Java source files the same name as the
class they contain—the name of the source file will match the name of the .class file. When you execute the
Java interpreter as just shown, you are actually specifying the name of the class that you want the interpreter
to execute. It will automatically search for a file by that name that has the .class extension. If it finds the file, it
will execute the code contained in the specified class.