Abstract classes

An abstract class is an unfinished class: it can hold shared code and state, and declares abstract methods that every concrete subclass must fill in. You cannot create one with new.

A mental model

A form template with the header and the footer already printed, and blank boxes in the middle that each department must fill in.

How it works

Mark the class abstract. Methods marked abstract have no body (abstract double area();), and every non-abstract subclass has to implement them.

An abstract class may also have normal methods, fields and constructors. Subclass constructors call the abstract class constructor with super(...).

The template method pattern builds on this: the abstract class has a final method that runs the fixed steps and calls the abstract methods for the parts that vary.

Difference to an interface: a class extends only one abstract class but can implement many interfaces; an abstract class can hold state, an interface cannot (only constants).

Going deeper

Choose an interface for "can do" (Comparable, Runnable) and an abstract class for a shared skeleton with state (AbstractList). Often you offer both: the interface for callers and an abstract base for convenience.

An abstract class can implement an interface without implementing all its methods; the leaf classes do the rest.

Under the hood

The abstract flag on the class file makes new fail at link time with InstantiationError. Abstract methods get a vtable slot without code; the concrete subclass fills it, so calls through the parent type still dispatch dynamically.

Common mistakes

• Putting abstract on a class and then trying new Shape().
• Adding state to a base class that later subclasses cannot share cleanly.
• Calling an abstract method from the abstract class constructor, before the subclass has set its fields.