AbstractMethodError in Java: Causes & Fixes

The AbstractMethodError in Java is a runtime exception that occurs in the following situations:

1. When a class inherits or implements an abstract class or interface, and does not implement or override its abstract methods, an AbstractMethodError is thrown. This typically occurs when there is a mismatch between the class used during compilation and the one used during runtime, possibly due to inconsistencies in class library versions or a corrupted class library.
2. An AbstractMethodError may occur when a class being compiled depends on a library that has been modified or had an abstract method removed. This issue can arise if the library is updated without recompiling the code that depends on it.
3. When a old version of a library is used at runtime, and the library references an abstract method that has been deleted or modified in the new version, an AbstractMethodError will also be thrown.
4. When a class loader loads two classes with the same name from different class loaders, and one of the classes inherits or implements an abstract method from an abstract class or interface while the other class does not implement or override that abstract method, an AbstractMethodError will also be thrown.

In conclusion, AbstractMethodError is usually caused by inconsistencies in library versions, modifications or deletions in libraries, or problems with class loaders. Solutions include updating the library, recompiling the code, and checking the class loader.