NoClassDefFoundError

This is a very common error which causes headache for most of the beginners when they try to execute their first java program,it compiles without any error, class file is also generated but when you try to run the program 
the most frustrating error pop up,that is:

Exception in thread "main" java.lang.NoClassDefFoundError

Note: don't misunderstand this with ClassNotFoundException,i'll mention the difference between the two similar looking exceptions at the end of this post.


I have faced it a lot of times in the beginning,so i want to share it here
what is the reason behind this error and how to rectify 
here is the reason:
you should know that when we try to run a program like this 
command prompt:/java classname
the JVM searches the classpath to locate the class name you provide to it.
If JVM doesn't find definition of a class in the CLASSPATH which was available at the time of compilation it generates the error 

Exception in thread "main" java.lang.NoClassDefFoundError
So you can easily understand that the problem is not with the code but it is because the JVM is unable to locate the class file,so to rectify the problem you need to :

 >> identify the class which is creating the problem, you can easily find out it from the error message

take an example 

//hello.java
class SayHello
{
 public static void main(String[] args) 
 {
  System.out.println("Hello World!");
 }
}

in the above program there is only one class i.e SayHello after compiling this program 
promt:/javac hello.java
a class "SayHello.class" will be automatically generated. and when you try to run your program from the same directory like this 
prompt:/java SayHello
you get the error Exception in thread "main" java.lang.NoClassDefFoundError
the only reason behind this is you haven't provided the classpath for you class. Provide the class path for you class file and the error will go away automatically.
there are two options 
1> declare the CLASSPATH environment variable 
"Control Panel\System --> advanced system settings -->environment variables"   which sets the classpath permanently. or
2> run your program like this 
prompt:/java -cp . SayHello.java
the -cp argument stands for classpath and "." for the current directory,it tells JVM to look in current directory for the class file. 
this is all you need to do,to get rid of this error.

difference between

ClassNotFoundException	NoClassDefFoundError
Occurs when a class is not found which is referenced in the program you are trying to execute	Occurs when a class is unavailable to JVM at the time of execution which was there at the time of compilation
Reason may be a missing package or jar file which contains the class	only possible reason is problem with CLASSPATH which is explained above
Problem may be in manifest file which may not have mentioned some jar file which is the source of the class

Interface

An Interface is collection of methods with empty bodies.
Interface contains only

• static and final fields.
• abstract methods.

Interface is declared as

interface abc
{
     //static & final fields
     //empty body methods are declared here.
}

Constraints:

• An interface can contain only abstract methods and any class implementing the interface thereby inherits the abstract methods of the interface.
• Unless the class that implements the interface is abstract,all the methods of the interface must be defined in the class. 

Comparison between a normal class and interface:

class	interface
can have constructors	don't have any constructor
can be instantiated	can't be instantiated
is extended	is implemented

how to implement an interface ?

here is a sample code

interface Vehicle
{
         public static final int speed;
         public void displayInfo();
}
class Car implements Vehicle
{
         speed=150;
         public void displayInfo()
         {
               System.out.println("it's a car");
          }
}

Abstract Class

Abstract class is a class which contains methods without implementation.
If a class contains even a single abstract method you need to declare your class as abstract
like this

abstract class <classname>                          
{                                                                  
       //abstract methods                                
}                                                                  

Abstract class can't be instantiated because an abstract class contains abstract methods without definitions so creating object for such a class is meaningless.
Any subclass of an abstract class must provide the definition for the all the abstract methods of it's super class or declare itself also as abstract.

Now the question is
Q) Why we need an Abstract method  ?
Answer:
If you are unable to decide the exact definition of a method,lets take an example like if there is a method area() in your class,i bet you will get confused while providing definition for this method.you will definitely think whose area you are going to compute in this method either rectangle,triangle or circle ?
and finally you will come up with abstract area() so that any subclass of your abstract class will provide the definition of area() according to the requirement.

i am taking an example of an Automobile class

abstract class Car
{
int price;
String manufacturer;
public void display()
{
        System.out.println("its a car");
}
abstract public String getPrice();

}

class Bmw extends Car
{
        public String getPrice()
        {
                return "40 lacs";
         }

}
class Hyundai extends Car
{

        public String getPrice()
        {
                return "10 lacs";
         }
}

public class mainclass
{
         public static void main(String args[])
         {
                  Bmw car1=new Bmw();
                  Hyundai car2=new Hyundai();
                  car1.display();
                  car1.getPrice();
                car2.display();
                  car2.getPrice();
          }
}

in this example getPrice() method is abstract method in the Car class which is abstract itself.
getPrice() is overrided in the subclasses Bmw and Hyundai.