Polymorphism, final and interface

polymorphic ： Multiple states （ Different states at different times ）

Necessary condition ：

1. Have inherited , A parent class can represent a subclass .
2. There's rewriting （ Refers specifically to abstract methods ）, After rewriting , Call subclass overridden methods .
3. The parent class reference points to the subclass object .
4. In polymorphism , Compile to the left , Run to the right .

Be careful ：

• For static methods , Both compilation and operation are on the left .
• For member variables , Both compilation and operation are on the left . 

The transformation of ：

Upward transformation ：

Animal1 animal = new Bird1();
        animal.eat();

Move down ：

Animal1 animal = new Bird1();
Bird1 bird=(Bird1)animal;      

Polymorphic deficiency ： A parent type cannot access methods unique to a child class .

final：

final Keywords represent the final 、 Unalterable .
 There are four common uses ：
1.  Can be used to modify a class 
2.  Can be used to modify a method 
3.  It can also be used to modify a local variable 
4.  It can also be used to modify a member variable 

 

• final Used to declare properties , Methods and classes .
• attribute : The definition must be assigned directly or in the construction method , And later can't
  modify .
• Method : Subclasses cannot be overridden
• class : Cannot be defined as an abstract class or interface , Not to be inherited .

Interface ：

The interface is implicitly abstract , When declaring an interface , No need to use abstract keyword

• Methods in interfaces can be abstract , Static , default

• The properties declared in the interface default to public static final Of

• Interface is not inherited by class , But to be implemented by class

• Interface cannot instantiate object

• Similar to inheritance , There is polymorphism between the interface and the implementation class

• One interface can inherit multiple other interfaces

• When a class implements an interface , Class should implement all abstract methods in the interface . otherwise , Class must

Classes declared as abstract

  abstract 、 Common points of interfaces ：

1. It's all an abstraction
2. Can't be instantiated
3. Are used to be inherited and implemented by other classes

abstract 、 Interface differences ：

abstract class ： You can define member variables , Methods can be constructed directly , You can define non Abstract member methods （ Contains abstract methods , Cannot create object ）

Interface ： Define constants 、 Abstract method 、 static state 、 The default method 、 Cannot have constructor in interface , Interfaces can inherit multiple interfaces .