Shulou(Shulou.com)06/02 Report--

Today, I will talk to you about how to understand java polymorphism and abstract classes, which may not be understood by many people. In order to make you understand better, the editor has summarized the following content for you. I hope you can get something according to this article.

Preface

Today is the 2021LOL global finals, has not been favored by everyone EDG rushed to the final against South Korea's DK, it can be said that EDG faced such a strong opponent, it is difficult to win, in order to cheer the Chinese team I set up flag, if EDG wins then I will reward myself to study until 6 o'clock the next day, and I came, congratulations EDG!

For the next study of the interface, let's first learn some knowledge about polymorphism and abstraction, which will help us to better learn the focus of the interface.

Polymorphism first acquainted with polymorphism

Polymorphism, which literally means "multiple states", is the core feature of object-oriented programming language. From a certain point of view, encapsulation and inheritance are almost always prepared for polymorphism.

What is polymorphism?

Polymorphism is the ability of the same behavior to have many different forms or forms of expression. Polymorphism is the same interface, using different instances to perform different operations, for example

Both color printers and black-and-white printers are printers, but you can print color files with color printers, and you can print black-and-white documents with black-and-white printers, using different examples to perform different operations.

Classification of polymorphisms

Compile-time polymorphism (design-time polymorphism): overloading of methods

Run-time polymorphism: the JAVA runtime system decides which method to call based on the type of instance that calls the method. This is what we usually call polymorphism.

A reference variable points to an instance object of which class, and the method emitted by the reference variable

The method implemented in which class is called must be decided during the run of the program.

Necessary conditions for polymorphism

Inherit

The parent class reference points to the subclass object: Parent p = new Child ()

Rewrite

Let's first explain the third one:

Method rewriting and overloading are different manifestations of java polymorphism. Rewriting is a manifestation of polymorphism between parent and child classes, and overloading can be understood as a specific form of polymorphism.

Realize polymorphism

If we want to achieve polymorphism, we need to understand the upward and downward transitions.

Upward transformation

Upward type conversion (Upcast): converts a child type to a parent type. Implicit / automatic type conversion, which is the conversion from small type to large type

For upward type conversion, you do not need to display the specification, that is, you do not need to add the preceding parentheses and the parent class type name

For example:

Parent p = new Child ()

This is a very typical one, provided that Parent is the parent class of Child, that is, the parent class reference points to the subclass instantiation. This is done automatically and implicitly.

Downward transformation

Downward type conversion (Downcast): converts a parent type to a child type. Assign a parent class reference to a subclass object to a subclass reference

Cast, which is from large type to small type

The reference to the parent type must point to the object of the child class of the transformation, that is, who can be converted to whom. Otherwise, there will be compilation errors.

For example:

Parent p = new Child (); Child c = (Child) p

As you should have noticed, there is a downward transition before the downward transition, which is the premise of the downward transition, that is, assigning a parent class reference to a subclass object to a subclass reference.

In that case, the question arises, how do we know if the reference to this parent class points to an object of this subclass type? Isn't it very error-prone when there are many subclasses of the parent class?

You don't need to worry too much about this, because we have an operator instanceof.

Instanceof operator

The instanceof operator is used to determine whether the object pointed to by the reference type variable before the operator is the latter class, or the object created by its subclass or interface implementation class. If yes, return true, otherwise return false

Its use format is as follows: reference type variable instanceof (class, abstract class, or interface)

For example:

P instanceof Child;//true

The instanceof operator is used to check the real type of an object before casting to avoid type conversion exceptions, thereby improving code robustness.

Concrete realization

When the parent class reference points to the subclass instance, you can call the methods that the subclass overrides the parent class and inherit the parent class directly, but cannot call the methods specific to the subclass.

If you want to call a specific method of a subclass, you have to transform it down to a subclass reference.

Static static methods are special cases. Static methods can only be inherited and cannot be overridden. Whose reference is used when the call is made, the version will be called.

When calling a method in a polymorphic way, first check whether the method exists in the parent class, if not, compile the error; if so, call the method of the child class with the same name.

Let's give a simple example to demonstrate it.

Class Father {int age=35; public void high () {System.out.println ("Dad's height 175cm");} public void hobby () {System.out.println ("Dad likes to watch the news");} public static void sta () {System.out.println ("I am the static method of the parent class") }} class Son extends Father {public void high () {System.out.println ("son's height 180cm this year");} public void hobby () {System.out.println ("son likes to hit League of Legends");} public static void sta () {System.out.println ("I am the static method of subclass") } public void self () {System.out.println ("I am a son's way");}} public class Text_3 {public static void main (String [] args) {Father f = new Son (); f.hobby (); f.high (); f.sta (); / / f.self (); / / error Son s = (Son) f S.hobby (); s.self (); s.sta ();}}

The output is:

My son likes to beat League of Legends.

My son is tall 180cm this year.

I am the static method of the parent class

My son likes to beat League of Legends.

The way I'm a son.

I am the static method of the subclass

Notice that both the subclass and the parent class have sta () methods, which are not overrides, they are methods of their own classes, and they don't matter.

Why should abstract classes and abstract methods use abstraction?

When some methods of the parent class are uncertain, you can modify the method with the abstract keyword, that is, the abstract method, and the abstract to modify the class, that is, the abstract class.

As we all know, the parent class is to extract the properties and methods shared by the subclasses. some of these attributes and methods have been explicitly implemented or not determined, so we can define them as abstract. later classes are reused and materialized. In this way, abstract classes are born.

Classes modified with abstract, that is, abstract classes; methods modified with abstract, that is, abstract methods.

Abstract method

An abstract method cannot have a method body. The format is as follows:

/ / the animal will be called public abstract void cry (); abstract class

Abstract classes are designed to extract the same but uncertain things for future reuse. The purpose of defining an abstract class is to implement an abstract class in a subclass. In fact, abstract classes appear for inheritance.

Abstract defines abstract classes. Abstract classes can have no abstract methods.

Abstract classes cannot be instantiated directly, they can only be inherited, and object instances can be completed through upward transformation.

Abstract defines abstract methods and does not require concrete implementation

A class that contains abstract methods is an abstract class

When the inherited parent class is an abstract class, you need to implement all the abstract methods in the abstract class. If it is not fully implemented, then this subclass must be defined as an abstract class, otherwise an error will be reported.

Abstract cannot coexist with static, final and private

When an abstract method is implemented in a subclass, the access permission must be greater than or equal to the parent method.

The format is as follows:

Abstract class Animal {String name; int age; / / abstract method public abstract void cry ();}

I hope everyone can prove themselves like EDG, so that those who don't like you can have a good look.

Come on, boy!

After reading the above, do you have any further understanding of how to understand java polymorphism and abstract classes? If you want to know more knowledge or related content, please follow the industry information channel, thank you for your support.

Welcome to subscribe "Shulou Technology Information " to get latest news, interesting things and hot topics in the IT industry, and controls the hottest and latest Internet news, technology news and IT industry trends.