What Is Inheritance?

A class inherits state and behavior from its superclass. Inheritance provides a powerful and natural mechanism for organizing and structuring software programs.

For example, mountain bikes, racing bikes, and tandems are all kinds of bicycles. In object-oriented terminology, mountain bikes, racing bikes, and tandems are all subclasses of the bicycle class. Similarly, the bicycle class is the superclass of mountain bikes, racing bikes, and tandems. This relationship is shown in the following figure.

What Is a Message?

Software objects interact and communicate with each other using messages. This may be termed as argument or a parameter

A single object alone is generally not very useful. Instead, an object usually appears as a component of a larger program or application that contains many other objects. Through the interaction of these objects, programmers achieve higher-order functionality and more complex behavior. Your bicycle hanging from a hook in the garage is just a bunch of titanium alloy and rubber; by itself, the bicycle is incapable of any activity. The bicycle is useful only when another object (you) interacts with it (pedal).

Software objects interact and communicate with each other by sending messages to each other. When object A wants object B to perform one of B's methods, object A sends a message to object B

Sometimes, the receiving object needs more information so that it knows exactly what to do; for example, when you want to change gears on your bicycle, you have to indicate which gear you want. This information is passed along with the message as parameters.

The next figure shows the three components that comprise a message:
The object to which the message is addressed (YourBicycle)
The name of the method to perform (changeGears)
Any parameters needed by the method (lowerGear)

What Is an Object?

An object is a software bundle of related variables and methods. Software objects are often used to model real-world objects you find in everyday life.

They all have state and behavior. For example, dogs have state (name, color, breed, hungry) and behavior (barking, fetching, and wagging tail). Bicycles have state (current gear, current pedal cadence, two wheels, number of gears) and behavior (braking, accelerating, slowing down, changing gears).

Software objects are modeled after real-world objects in that they too have state and behavior. A software object maintains its state in one or more variables. A variable is an item of data named by an identifier. A software object implements its behavior with methods. A method is a function (subroutine) associated with an object.

Definition: An object is a software bundle of variables and related methods.

In addition to its variables, an object, say a bicycle would also have methods to brake, change the pedal cadence, and change gears. (The bike would not have a method for changing the speed of the bicycle, as the bike's speed is just a side effect of what gear it's in, how fast the rider is pedaling, whether the brakes are on, and how steep the hill is.) These methods are formally known as instance methods because they inspect or change the state of a particular bicycle instance.

The object diagrams show that the object's variables make up the center, or nucleus, of the object. Methods surround and hide the object's nucleus from other objects in the program. Packaging an object's variables within the protective custody of its methods is called encapsulation This conceptual picture of an object-a nucleus of variables packaged within a protective membrane of methods-is an ideal representation of an object and is the ideal that designers of object-oriented systems strive for. However, it's not the whole story. Often, for practical reasons, an object may wish to expose some of its variables or hide some of its methods.
• Modularity: The source code for an object can be written and maintained independently of the source code for other objects. Also, an object can be easily passed around in the system. You can give your bicycle to someone else, and it will still work.
• Information hiding: An object has a public interface that other objects can use to communicate with it. The object can maintain private information and methods that can be changed at any time without affecting the other objects that depend on it. You don't need to understand the gear mechanism on your bike to use it.

Java Basics

History
James Gosling was not satisfied with C++. There were several insufficiencies which hindered his plans. So, he created his own. Initially called Oak by James Gosling for the Green Project. However, since that name was already taken, it was later named Java.

Language Features
Platform Independent – can run on any environment
Garbage Collection – objects are automatically destroyed from memory
Object Oriented
Multithreaded – can perform several tasks at the same time
Dynamically Linked - classes are loaded as needed

OOP Concepts
If you've never used an object-oriented language before, you need to understand the underlying concepts before you begin writing code. You need to understand what an object is, what a class is, how objects and classes are related, and how objects communicate by using messages. The first few sections of this trail describe the concepts behind object-oriented programming. The last section shows how these concepts translate into code.