Abstract Class.md

date updated	sr-due	sr-interval	sr-ease
2021-11-20 12:21	2021-12-06	11	263

Topic: #oop #java_programming Tags: #review #pn_2_1 Links: [[Class Visibility]] [[Class]] Date Created: 23-10-21

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Abstract Class

Abstract Class in few words

abstract-keyword class-keyword class-name {
	...
}

abstract class AnAbstractClass {
	...
}

Abstract Class in details

[[Abstract Class|Abstract Classes]] are another tool in the tool box to achieve [[Abstraction]].

It can have:

• [[Abstract Method]]
• Concrete [[Method]]

Like [[Interfaces in Java]] they cannot be [[Instantiation|instantiated]]. Contrary to [[Interfaces in Java]] they can contain [[Field|fields]], therefore can describe change of state and be used [[Constructor]] to be used for [[Subclass]]. Contrary to [[Interfaces in Java]] they cannot be [[Inheritance|inherrited]] multiple times meaning this is wrong

class E extends A,B,C,D { // this is impossible
	...
}

Like [[Interfaces in Java]] a [[Class]] that [[Inheritance|inherits]] from an [[Abstract Class]] must redefine [[Abstract Method]] in order to be a concrete [[Class]] and therefore be [[Instantiation|instantiable]]. An [[Abstract Class]] doesn't need to have [[Abstract Method]].

In Practice

If abstract is used then the [[Class]] cannot be [[Instantiation|instantiated]] but [[Subclass|Sub-Classes]] can be [[Instantiation|instantiated]].

jshell> abstract public class Fruit {
   ...>     public Fruit() {}}
|  modified class Fruit

jshell> Fruit banana = new Fruit();
|  Error:
|  Fruit is abstract; cannot be instantiated
|  Fruit banana = new Fruit();
|                 ^---------^

jshell> public class Banana extends Fruit {
   ...> }
|  created class Banana

jshell> Banana bigBanana = new Banana();
bigBanana ==> Banana@27bc2616

jshell> 

References

• [[Class#References]]