Class

A class is nothing but a blueprint or a template for creating different objects which defines its properties and behaviors. Java class objects exhibit the properties and behaviors defined by its class. A class can contain fields and methods to describe the behavior of an object.

class Fabonic
{
 public static void main(String s[])
 {
   int a=0,fab=0,b=1,c,n=11,i;
   System.out.println("how much element do u want of fabocci series :");
   System.out.println("fabocci series : ");
   System.out.println(a);
   System.out.println(b);
   for(i=0;i<=n;i++)
  {
   fab=a+b;
   System.out.println(fab);
   c=a;
   a=b;
   b=fab;
  }
 }
}

What is the difference between abstract class and interface?

feature	interface	abstract class
multiple inheritance	A class may implement several interfaces.	A class may extend only one abstract class.
default implementation	An interface cannot provide any code at all, much less default code.	An abstract class can provide complete code, default code, and/or just stubs that have to be overridden.
constants	Static final constants only, can use them without qualification in classes that implement the interface. On the other paw, these unqualified names pollute the namespace. You can use them and it is not obvious where they are coming from since the qualification is optional.	Both instance and static constants are possible. Both static and instance intialiser code are also possible to compute the constants.
third party convenience	An interface implementation may be added to any existing third party class.	A third party class must be rewritten to extend only from the abstract class.
is-a vs -able or can-do	Interfaces are often used to describe the peripheral abilities of a class, not its central identity, e.g. An Automobile class might implement the Recyclable interface, which could apply to many otherwise totally unrelated objects.	An abstract class defines the core identity of its descendants. If you defined a Dog abstract class then Dalmatian descendants are Dogs, they are not merely dogable. Implemented interfaces enumerate the general things a class can do, not the things a class is. In a Java context, users should typically implement the Runnable interface rather than extending Thread, because they’re not really interested in providing some new Thread functionality, they normally just want some code to have the capability of running independently. They want to create something that can be run in a thread, not a new kind of thread.The similar is-a vs has-a debate comes up when you decide to inherit or delegate. multiple inheritance for further discussion of is-a vs has-a
plug-in	You can write a new replacement module for an interface that contains not one stick of code in common with the existing implementations. When you implement the interface, you start from scratch without any default implementation. You have to obtain your tools from other classes; nothing comes with the interface other than a few constants. This gives you freedom to implement a radically different internal design.	You must use the abstract class as-is for the code base, with all its attendant baggage, good or bad. The abstract class author has imposed structure on you. Depending on the cleverness of the author of the abstract class, this may be good or bad.
homogeneity	If all the various implementations share is the method signatures, then an interface works best.	If the various implementations are all of a kind and share a common status and behaviour, usually an abstract class works best. Another issue that’s important is what I call "heterogeneous vs. homogeneous." If implementors/subclasses are homogeneous, tend towards an abstract base class. If they are heterogeneous, use an interface. (Now all I have to do is come up with a good definition of hetero/homo-geneous in this context.) If the various objects are all of-a-kind, and share a common state and behavior, then tend towards a common base class. If all they share is a set of method signatures, then tend towards an interface.
maintenance	If your client code talks only in terms of an interface, you can easily change the concrete implementation behind it, using a factory method.	Just like an interface, if your client code talks only in terms of an abstract class, you can easily change the concrete implementation behind it, using a factory method.
speed	Slow, requires extra indirection to find the corresponding method in the actual class. Modern JVMs are discovering ways to reduce this speed penalty.	Fast
terseness	The constant declarations in an interface are all presumed public static final, so you may leave that part out. You can’t call any methods to compute the initial values of your constants. You need not declare individual methods of an interface abstract. They are all presumed so.	You can put shared code into an abstract class, where you cannot into an interface. If interfaces want to share code, you will have to write other bubblegum to arrange that. You may use methods to compute the initial values of your constants and variables, both instance and static. You must declare all the individual methods of an abstract class abstract.
adding functionality	If you add a new method to an interface, you must track down all implementations of that interface in the universe and provide them with a concrete implementation of that method.	If you add a new method to an abstract class, you have the option of providing a default implementation of it. Then all existing code will continue to work without change.

Swing is a light weight component, and awt is a heavy weight component, why?

A heavyweight component is one that is associated with its own  native screen resource (commonly known as a peer). A lightweight component is one that "borrows" the screen resource of an ancestor  (which means it has no native resource of its own -- so it's "lighter").

• A lightweight component can have transparent pixels; a heavyweight  is always opaque.
• A lightweight component can appear to be non-rectangular because of  its ability to set transparent areas; a heavyweight can only be rectangular.
• Mouse events on a lightweight component fall through to its parent;  mouse events on a heavyweight component do not fall through to its parent.
• When a lightweight component overlaps a heavyweight component, the  heavyweight component is always on top, regardless of the relative z-order  of the two components. 

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 Light weight Component takes the components from the jvm, means it does not request the operating System for getting the components. Swings are light weight components. thats why swing look and feel is different and same on every operating System. Where as, heavy weight components are depending on the O.S. if we request a button component, then JVM ask the button to the os and then it give to the JVM. it is very time consuming process. So awt look and feel is depending on the os. It changes from one os to another os.

Wrapper Class

Java is an object-oriented language and as said everything in java is an object. But what about the primitives? They are sort of left out in the world of objects, that is, they cannot participate in the object activities, such as being returned from a method as an object, and being added to a Collection of objects, etc. . As a solution to this problem, Java allows you to include the primitives in the family of objects by using what are called wrapper classes.

There is a wrapper class for every primitive date type in Java. This class encapsulates a single value for the primitive data type. For instance the wrapper class for int is Integer, for float is Float, and so on. Remember that the primitive name is simply the lowercase name of the wrapper except for char, which maps to Character, and int, which maps to Integer.

The wrapper classes in the Java API serve two primary purposes:

1.  To provide a mechanism to “wrap” primitive values in an object so that the primitives can be included in activities reserved for objects, like as being added to Collections, or returned from a method with an object return value.
2. To provide an assortment of utility functions for primitives. Most of these functions are related to various conversions: converting primitives to and from String objects, and converting primitives and String objects to and from different bases (or radix), such as binary, octal, and hexadecimal.

The wrapper object of a wrapper class can be created in one of two ways: by instantiating the wrapper class with the new operator or by invoking a static method on the wrapper class.