The Abstract Modifier

Abstract Methods

An Abstract Method is one in which has a heading but no method body– this is to be defined in a derived classes.

• In order to postpone the definition of a method, Java allows an abstract method to be declared.
• Is like a placeholder for a method that will be fully defined in a descendent class.
• Has a complete method heading in addition to the abstract modifier.
• cannot be private.

Abstract Class

An Abstract Class is one which includes an abstract method.

• Must have the modifier abstract included in its class heading. -Can have any number of abstract and/or fully defined methods.
• If a derived class of an abstract class adds to or does not define all of the abstract methods, then it is abstract also, and must include the abstract modifier.
• Conversely, a class which includes no abstract class is known as a concrete class.
• Cannot be instantiated.
  • Constructor can be called by super and can call super from another abstract class itself though.
• Can be used as a type (for instance as a parameter).