12 - Abstract Classes
In Java, an abstract class is a class that cannot be instantiated on its own and is meant to be subclassed by other classes. Abstract classes can have both abstract and non-abstract (concrete) methods, as well as fields. Abstract methods declared in an abstract class are meant to be implemented by its subclasses.
Abstract classes are useful when you want to provide a common base with some shared functionality for subclasses while leaving certain methods to be implemented by those subclasses. They are often used in situations where you have a common structure among multiple classes.