Abstract classes

• created using abstract modifier
• If a class defines one or more abstract methods, then the class itself must be declared abstract
• An abstract class can declare both abstract and concrete methods
• An abstract class can be subclassed, but it can’t be instantiated
• A subclass derived from an abstract class must either implement all the base class’s abstract methods or be abstract itself

When to use & prefer abstract classes over interfaces?

• code reuse is a very compelling reason to use abstract classes, as long as the “is-a” relationship within the class hierarchy is preserved
• We want to encapsulate some common functionality in one place (code reuse) that multiple, related subclasses will share
• We need to partially define an API that our subclasses can easily extend and refine
• The subclasses need to inherit one or more common methods or fields with protected access modifiers