Objects

 Objects are key to understanding object-oriented technology. Look around right now and you'll find many examples of real-world objects: your dog, your desk, your television set, your bicycle.
Real-world objects share two characteristics: They all have state and behavior. Dogs have state (name, color, breed, hungry) and behavior (barking, fetching, wagging tail). Bicycles also have state (current gear, current pedal cadence, current speed) and behavior (changing gear, changing pedal cadence, applying brakes). Identifying the state and behavior for real-world objects is a great way to begin thinking in terms of object-oriented programming.
Take a minute right now to observe the real-world objects that are in your immediate area. For each object that you see, ask yourself two questions: "What possible states can this object be in?" and "What possible behavior can this object perform?". Make sure to write down your observations. As you do, you'll notice that real-world objects vary in complexity; your desktop lamp may have only two possible states (on and off) and two possible behaviors (turn on, turn off), but your desktop radio might have additional states (on, off, current volume, current station) and behavior (turn on, turn off, increase volume, decrease volume, seek, scan, and tune). You may also notice that some objects, in turn, will also contain other objects. These real-world observations all translate into the world of object-oriented programming.

 

                                  A software object.

Software objects are conceptually similar to real-world objects: they too consist of state and related behavior. An object stores its state in fields (variables in some programming languages) and exposes its behavior through methods (functions in some programming languages). Methods operate on an object's internal state and serve as the primary mechanism for object-to-object communication. Hiding internal state and requiring all interaction to be performed through an object's methods is known as data encapsulation — a fundamental principle of object-oriented programming.
Consider a bicycle, for example:

                  

            A bicycle modeled as a software object.

By attributing state (current speed, current pedal cadence, and current gear) and providing methods for changing that state, the object remains in control of how the outside world is allowed to use it. For example, if the bicycle only has 6 gears, a method to change gears could reject any value that is less than 1 or greater than 6.
Bundling code into individual software objects provides a number of benefits, including:

• Modularity: The source code for an object can be written and maintained independently of the source code for other objects. Once created, an object can be easily passed around inside the system.

• Information-hiding: By interacting only with an object's methods, the details of its internal implementation remain hidden from the outside world.

• Code re-use: If an object already exists (perhaps written by another software developer), you can use that object in your program. This allows specialists to implement/test/debug complex, task-specific objects, which you can then trust to run in your own code.

• Pluggability and debugging ease: If a particular object turns out to be problematic, you can simply remove it from your application and plug in a different object as its replacement. This is analogous to fixing mechanical problems in the real world. If a bolt breaks, you replace it, not the entire machine.

source:www.oracle.com

Different ways to create objects in Java
             There are four different ways (I really don’t know is there a fifth way to do this) to create objects in java:

1.Using new keyword
         This is the most common way to create an object in java. I read somewhere that almost 99% of objects are created in this way.

                         ClassName object = new ClassName();

2. Using Class.forName()
        If we know the name of the class & if it has a public default constructor we can create an object in this way.

                ClassName object = (ClassName) Class.forName("subin.rnd.ClassName").newInstance();

 3. Using clone() 

           The clone() can be used to create a copy of an existing object.

                          ClassName object= new ClassName();
                          ClassName object1= object.clone();

4. Using object deserialization
           Object deserialization is nothing but creating an object from its serialized form.

                         ObjectInputStream inStream = new ObjectInputStream(anInputStream );
                         ClassName object = (ClassName) inStream.readObject();

Now you know how to create an object. But its advised to create objects only when it is necessary to do so.

***Using new keyword is the most used way for creating an object.