An overview of java

An Overview of Java

• As in all other computer languages, the elements of Java do not exist in isolation.

Rather, they work together to form the language as a whole. However, this
interrelatedness can make it difficult to describe one aspect of Java without
involving several others. Often a discussion of one feature implies prior knowledge of
another.

•  The material described here will give you a foothold that will allow you to write

and understand simple programs. Most of the topics discussed will be examined in greater
detail in the remaining

Object-Oriented Programming

• Object-oriented programming (OOP) is at the core of Java. In fact, all Java programs are to at

least some extent object-oriented. OOP is so integral to Java that it is best to understand its
basic principles before you begin writing even simple Java programs. Therefore, this chapter
begins with a discussion of the theoretical aspects of OOP.

Two Paradigms

• All computer programs consist of two elements: code and data. Furthermore, a program can

be conceptually organized around its code or around its data. That is, some programs are
written around “what is happening” and others are written around “who is being affected.”
These are the two paradigms that govern how a program is constructed. The first way is
called the process-oriented model. This approach characterizes a program as a series of linear
steps (that is, code). The process-oriented model can be thought of as code acting on data.
Procedural languages such as C employ this model to considerable success. problems with this approach appear as programs grow larger and
more complex.

• To manage increasing complexity, the second approach, called object-oriented programming,

was conceived. Object-oriented programming organizes a program around its data (that is,
objects) and a set of well-defined interfaces to that data. An object-oriented program can be
characterized as data controlling access to code. As you will see, by switching the controlling
entity to data, you can achieve several organizational benefits.

Abstraction

• An essential element of object-oriented programming is abstraction. Humans manage

complexity through abstraction. For example, people do not think of a car as a set of tens

of thousands of individual parts. They think of it as a well-defined object with its own

unique behavior. This abstraction allows people to use a car to drive to the grocery store

without being overwhelmed by the complexity of the parts that form the car. They can

ignore the details of how the engine, transmission, and braking systems work. Instead,

they are free to utilize the object as a whole.

• A powerful way to manage abstraction is through the use of hierarchical classifications.

This allows you to layer the semantics of complex systems, breaking them into more

manageable pieces. From the outside, the car is a single object. Once inside, you see that

the car consists of several subsystems: steering, brakes, sound system, seat belts, heating,

cellular phone, and so on. In turn, each of these subsystems is made up of more specialized

units. For instance, the sound system consists of a radio, a CD player, and/or a tape player.

The point is that you manage the complexity of the car (or any other complex system)

through the use of hierarchical abstractions.

• Hierarchical abstractions of complex systems can also be applied to computer programs.

The data from a traditional process-oriented program can be transformed by abstraction

into its component objects. A sequence of process steps can become a collection of messages

between these objects. Thus, each of these objects describes its own unique behavior. You

can treat these objects as concrete entities that respond to messages telling them to do something.

This is the essence of object-oriented programming.

• Object-oriented concepts form the heart of Java just as they form the basis for human

understanding. It is important that you understand how these concepts translate into

programs. As you will see, object-oriented programming is a powerful and natural paradigm

for creating programs that survive the inevitable changes accompanying the life cycle of any

major software project, including conception, growth, and aging. For example, once you

have well-defined objects and clean, reliable interfaces to those objects, you can gracefully

decommission or replace parts of an older system without fear.

The Three OOP Principles

All object-oriented programming languages provide mechanisms that help you implement

the object-oriented model. They are encapsulation, inheritance, and polymorphism. Let’s take

a look at these concepts now.

Encapsulation

Encapsulation is the mechanism that binds together code and the data it manipulates, and

keeps both safe from outside interference and misuse. One way to think about encapsulation

is as a protective wrapper that prevents the code and data from being arbitrarily accessed

by other code defined outside the wrapper. Access to the code and data inside the wrapper

is tightly controlled through a well-defined interface. To relate this to the real world, consider

the automatic transmission on an automobile. It encapsulates hundreds of bits of information

about your engine, such as how much you are accelerating, the pitch of the surface you are

on, and the position of the shift lever. You, as the user, have only one method of affecting

Figure  1:Encapsulation:

public methods

can be used to

protect private

data

       

                           Author : 

                                             Narayana KNR.