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Java Chapter 3

Q. What is an Object?

Ans:- Objects are software bundles of data and related procedures.

Software objects are often used to model real-world objects you find in everyday life. As the name implies, objects are the key concept in understanding object-oriented technology.

You can look around you now and see many examples of real-world objects: your dog, your desk, your television set or your bicycle.

These real-world objects share two characteristics: they all have state and they all have behavior. For example, dogs have state (name, color, breed, hunger) and, behavior (barking and  fetching). Bicycles
have state (current gear, current pedal cadence, two wheels, number of gears) and behavior (braking, accelerating, slowing down and 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 variables and implements its behavior with methods.

You can represent real-world objects in programs using software objects, consider an example, you may represent dogs or bicycle as software objects in an animation program.

Q. Explain the Term Encapsulation.

Ans:- Everything that the software object knows (state) and can do (behavior) is expressed by the variables and methods within that object. A software object that model your real-world bicycle will have variables that indicates the bicycle's current state: its speed is 25 kmph, its pedal cadence is 900 rpm, and its current gear is the 3rd gear. These variables and methods are formally known as instance variables and instance methods, to distinguish them from class variables and class methods. The software bicycle will also have methods to brake, change the pedal cadence and change gear(The bike would not have a method for changing the speed of the bicycle as the bike’s speed is really just a side-effect of what gear it’s in, how fast the rider is pedaling and how steep the hill is.)

Anything that an object does not know or cannot do is excluded from the object. For example, your bicycle(probably) doesn’t have a name, and it can’t run, bark of fetch. Thus there are no variables or methods for those states and behaviors. object's variables make up the center or nucleus of the object and the 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.  Typically, encapsulation is used to hide unimportant implementation details from other objects. When you want to change gears on your bicycle, you don't need to know how the gear mechanism works, you just need to know which lever to move. Thus, the implementation can change at any time without changing other parts of the program.

The Benefits of Encapsulation

Encapsulating related variables and methods into a neat software bundle is a simple yet powerful idea that provides two primary benefits to software developers:

·         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—Through information hiding, the complicity of the system can be reduced by not exposing unnecessary data. For example, you don’t need to understand the gear mechanism on your bike in order to use it.

Q. What Are Messages?

Ans:- Software objects interact and communicate with each other via messages. A single object alone is not very useful and usually appears as a single component of a larger program or application that contains many other objects. It is through the interaction of these objects that 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) interact with it (starts pedaling).

Software objects interact and communicate with each other by sending messages to each other. When object A wants object B to perform one of its 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.
Three components comprise a message:
·         the object to whom the message is addressed (bicycle)
·         the name of the method to be performed (changing gears) any parameters needed by the method (to a higher gear)
These three components provide enough information for the receiving object to perform the desired method. No other information or context is required.

The Benefits of Messages

·         Everything an object can do is expressed through its methods, so message passing supports all possible interactions between objects.
·         Objects don't need to be in the same process or even on the same machine to send and receive messages back and forth to each other.

What are Classes?
A class is a blueprint or prototype that defines the variables and the methods common to all objects of a certain kind.
In the real world, you often have many objects of the same kind. For example, your bicycle is really just one of the many bicycles in the world. Using object-oriented terminology, we say that your bicycle object is an instance of the class of objects known as bicycles. All bicycles have some state (current gear, current cadence, two wheels) and behavior (changing gears, braking) in common. However, each bicycle's state is independent of and can be different from other bicycles.  When producing bicycles, manufacturers take advantage of the fact that bicycles share characteristics and they build many bicycles from the same blueprint--it would be very inefficient to produce a new blueprint for every individual bicycle manufactured.

Like the bicycle manufacturers, you can take advantage of the fact that objects of the same kind are similar and you can create a blueprint for those objects. Software "blueprints" for objects are called classes. For example, you could create the bicycle class that would declare several variables to contain the current gear, the current cadence, etc. It would also declare and provide implementations for the methods that allow the rider to change gears, brake and change the pedaling cadence.
The values for the variables are provided by each instance of the class. So, after you've created the bicycle class, you must instantiate it (create an instance of it) before you can use it. When you create an instance of a class, the variables declared by the class are allocated space in the memory. You can use the instance's methods to assign values to the variables. Instances of the same class share method implementations.

Q. What do you mean by the Term "Object"?

Ans:- You have probably noticed that the illustrations of objects and classes look very similar to one another. And indeed, the differentiation between classes and objects is often the source of some confusion. In the real world it’s obvious that classes are not themselves the objects that they describe. A blueprint of a bicycle is not a bicycle. However, it’s a little difficult to differentiate classes and objects in software. This is partially because software objects are merely electronic models of real-world objects or abstract concepts in the first place. Then also occurs because many people use the term “object” inconsistently and use it to refer to both classes and instances.

The main difference between a class and an object is that objects are tangible, but a class is always intangible. You can’t see a class but you can always see an object. In Figure 1.5 and Figure 1.6, a class’s methods and variables are not shaded because they don’t exist yet. You must create an instance from the class before you can call the methods and before the variables can have any values. In comparison, an object’s methods and variables are shaded indicating that the object actually exists and you can use it. You can send the object a message and it will respond by performing the method and perhaps modifying the values of the variables.

The Benefits of Classes

Objects provide the benefit of modularity and information hiding. Classes provide the benefit of reusability. Bicycle manufacturers reuse the same blueprint over and over again to build lots of bicycles. Software programmers use the same class over and over again to create many objects.

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