Multi-State and interface

One, polymorphic

1, what is polymorphic?

Parsing: Different objects respond differently to the same operation

Features that have the ability to express multiple forms

2. Advantages of using polymorphism

Parsing: In order to achieve unified invocation

A small example:< the parent class type as a parameter >

Parent class (Pet) subclass (Gog,penguin) master Human (master) test Class (test)

Pet

 Public Abstract class Pet {    publicabstractvoid  eat ();}

Dog

 Public class extends pet{    @Override    publicvoid  eat () {        System.out.println ( "Enough to eat Bones");            }

Penguin

 Public class extends Pet {    @Override    publicvoid  eat () {        System.out.println ("Penguin Eats Fish" );            }

Master

 Public class Master {    // parent class type as parameter publicvoid  feed (Pet pet) {    pet.eat ();    }}

Test

   Public Static void Main (string[] args) {        Master master=new  master ();                Pet Pet=new  Dog ();                Master.feed (PET);    }

Transformation of subclasses to parent class (upward transformation)

Rules:

① a reference to a parent class to a subclass object, which becomes an upward transformation, automatic type conversion

② a method that is called by a parent class reference variable is a child class that overrides or inherits a method of the parent class, not the parent class's method

③ a method that is unique to a subclass cannot be called through a parent class reference variable

Third, two ways to implement and use polymorphism in Java

① using a parent class as a method parameter

② using the parent class as a method return value

Iv. conversion of parent class to subclass (downward transition)

Assigns a reference to a subclass object to a parent class reference, called a downward transformation. Forced type conversions are required at this time

Five, instanceof operators

Grammar:

Object instanceof class or interface

This operator is used to determine whether an object belongs to a class or implements an interface with the result of true or false

Note:

　　① when using the instanceof operator, the object must be of the same type as the class or interface specified by the second parameter of Instanceof, or a compilation error will occur

②instanceof commonly used in conjunction with forced type conversions

Vi.. Interface

In the Java language, interfaces have two meanings:

① conceptual interface, i.e. all services provided externally by the system

② refers to an interface defined with the interface keyword, also known as an interface type

Definition: An interface is a specification and a standard that they can constrain the behavior of a class, which is a collection of some method features

Grammar:

"Modifier" Interface interface name extends parent interface 1, connector 2 ....

Class Name extends parent class name implements Interface 1, interface 2 ....

Vii. characteristics

The member variables in the ① interface are public, static, and final type by default and must be displayed for initialization.

The methods in the ② interface are default to public, abstract type

③ interface has no constructor method and cannot be instantiated

④ an interface cannot implement another interface, but it can inherit multiple other interfaces

The ⑤ interface must implement its abstract method through a class

⑥ A class can inherit only one direct parent class, but can implement multiple interfaces

Viii. Grammar

　① modifier Interface Interface name extends parent interface 1, parent interface 2 .... {

Constant definition

Method definition

}

②class class Name extends parent class name implements Interface 1, interface 2 .... {

Class Member

}

A small example:

Using the Java interface to simulate the USB interface in life

1, first define the USB interface, through the service () method to provide services

 Public Interface usbinterface{void  service ();

2. Define USB drive class, realize the service () method in USB interface, carry out data transmission

 Public class udisk implementd usbinterface{publicvoid  service () {System.out.println ("Connect USB port , Start data Transfer! ");}}

3, defines the USB fan class, realizes the service () method in the USB interface, obtains the current causes the fan to turn

 Public class usbfan implementd usbinterface{publicvoid  service () {System.out.println ( "Connect the USB port to get the current, the fan starts to turn!" ");}}

4. Write Test class

 Public class test{publicstaticvoid  Main (String[]args) {//U-Disk Usbinterface udisk=New  udisk (); Udisk.service (); // USB Fan Usbinterface usbfan=New  Usbfan (); Usbfan.service ();}}

Multi-State and interface