Exception: java lang AbstractMethodError

This java.lang.AbstractMethodError is usually thrown when we try to invoke the abstract method.Generally this error is identified at the compile time itself,if this error is thrown at run-time then the class must be incompatibly(not compatible with preexisting classes) changed.Hence it is a sub class of IncompatibleClassChange Error.
We know that an abstract method cannot be invoked and if we try to do so then you will get a compile-time error.So you may think how this error is thrown at run-time?.
 
The reason is binary incompatibility- what does it mean?
Whenever a class is modified,other classes that refer to this (modified) class will not be aware of the changes made in it.So all the classes must be compiled as a whole. If not then you may encounter one of the subclasses of incompatible class change error.

This error indicates that the method that you invoke is converted into an abstract method now”.
 
see the following example to have an idea about this error
 class B
{
public void display()
{
System.out.println("I am inside B");
}
}


import java.util.*;
public class A extends B
{
public static void main(String args[])
{
A a=new A();
a.display();
}
}

Output:

 C:\blog>javac A.java

C:\blog>java A
I am inside B

Now i am going to convert the display() method as an abstract method and compile it alone.

abstract class B
{
public abstract void display();
}

Output:

 C:\blog>javac A.java

C:\blog>java A
I am inside B

C:\blog>javac B.java

C:\blog>java A
Exception in thread "main" java.lang.AbstractMethodError: B.display()V
        at A.display(A.java:3)
        at A.main(A.java:8)
As you see,the reason for this exception to be thrown at run-time is i have not compiled the classes as a whole.So whenever you make changes to the existing classes ensure that you have compiled the classes as a whole. 
Hence it is not a good practice to change a method into an abstract method in classes that are distributed.Mostly these kind of error occurs when you use third party library in your application.
If this error is not shown at compile time,even if you compile it as a whole package then you must have to check your library settings and class path settings. 
Because compiler usually searches for classes in system libraries like bootstrap libraries and extension libraries,also in the current directory,but JVM searches for classes in the specified classpath. 
If you accidentally placed your older version in the system libraries and the newer version in the class path then you will not be notified of this error even if you compile it as whole package.
So ensure that the settings relevant to older package has been removed.
Reference: java.lang.AbstractMethodError from our JCG partner Ganesh Bhuddhan at the java errors and exceptions blog.

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