Shulou(Shulou.com)06/03 Report--

This article mainly explains "what are the common modifier grammars in Java". The explanation in the article is simple and clear, and it is easy to learn and understand. Please follow Xiaobian's train of thought to study and learn "what are the common modifier grammars in Java"?

1. 11 common Java modifiers 1, modifiers apply table modifiers class construction methods data block interpretation (default) ✔ classes, constructors, methods, or data fields are visible in the package in which the public ✔✔✔✔ class, constructor, method, or data field is visible in any package, private ✔✔✔ constructor, method, or data field is visible only in the class in which the protected ✔✔✔ constructor, The method or data field is visible in the package to which it belongs Or the static ✔✔✔ definition class method, class data field, or static initialization module final ✔✔✔ ultimate class cannot be extended as seen in any subclass of the class. The ultimate method cannot be modified in subclasses. The ultimate data field is a constant abstract ✔✔ abstract class that must be extended. The abstract method must be implemented in a concrete subclass native ✔ with native modification method shows that it is implemented in a language other than Java synchronized ✔✔ only one thread at a time can execute this method strictfp ✔✔ uses the exact floating point calculation model to ensure that the calculation results in all Java virtual machines are the same transient ✔ marks the instance data field without serialization 2, access rights comparison

Public > protected > default > private

Description: the following only introduces our commonly used 7 kinds of modifier syntax description and development application description, the other 4 uses to see the modifier application table.

II. Public modifier

Syntax description:

Modify classes, methods, and variables. Can be accessed anywhere, public.

Develop applications:

In development, classes and business methods are usually decorated with public.

3. Private modifier

Syntax description:

Modify methods, variables, and inner classes. Can only be accessed in this class, private.

Develop applications:

In development, member variables in an entity class (Entity,Vo) are usually decorated with private, providing a method to access these variables, getter or

Setter, in principle, requires that private methods are not defined.

The table of an entity data usually corresponds to an entity class.

4. Protected modifier

Syntax description:

Modify methods, variables, and inner classes.

The same package can be accessed and subclasses can be accessed (subclasses and parents can also be accessed if they are not in the same package).

Develop applications:

In development, usually methods or variables are inherited and are decorated with protected.

In inheritance, when a method is overridden, the access to the subclass method must be greater than or equal to that of the parent method.

Class Fu {public void play () {}} class Son extends Fu {void play () {}} V, default (without any access modifier)

Syntax description:

Modify classes, methods, and variables.

Can only be accessed in the same package, and subclasses must be in the same package.

Develop applications:

In a project, classes, methods, and variables usually specify access permissions according to requirements.

Static modifier 1, static modifier member variables (static variables / class variables)

Syntax description:

In Java, you can modify variables with the static keyword to achieve the effect of global variables.

Static-decorated variables (static variables) belong to the class and are assigned to the method area the first time the class passes through the class loader to jvm.

Variables that are not modified by static are called instance variables.

1.1. The difference between class variables and instance variables

The instance variable, which belongs to a specific object, is allocated to the heap memory after the object is created. When an object is created, the instance variable appears in the heap memory, and when the object is garbage collected, the instance variable immediately frees memory.

For example, we create two objects.

Cell C1 = new Cell (4); Cell c2 = new Cell (5)

When creating the object C1 ~ c2, the instance variables (4 ~ 5) and (5 ~ 5 ~ 6) appear in the heap memory. When the object C1 ~ ~ c2 is garbage collected, the instance variable immediately frees memory.

Class variables, stored in the method area, "only one copy", are shared by all objects. When the class is loaded, it is immediately stored in the method area, and when the class is unloaded, the class variable immediately frees memory.

Develop applications:

Class variables can be accessed directly by the class name, and the full name of the class name is recommended in development.

For example, we define an entity class.

Public class Student {/ / instance variable, after the object is created, is assigned to heap memory. The instance variable belongs to a specific object / / when the object is created, the instance variable appears in heap memory. When the object is garbage collected, the instance variable immediately releases the memory String name;// class variable, which is stored in the method area, "only one copy", and the static String jtName is shared by all objects. Public void print () {System.out.println (this.name); System.out.println (jtName);}}

The next time we use the static variable jtName, we can call it directly using the class name of Student. At the same time, we can understand that the property values of the following two objects are the same.

Public class Demo01 {public static void main (String [] args) {/ / TODO Auto-generated method stub Student stu1 = new Student (); stu1.name = "Zhang San"; stu1.jtName = "Miss Wang"; Student stu2 = new Student (); / / generally take the class name directly / / stu2.jtName = "Miss Lu" Student.jtName = "Miss Lu"; System.out.println (stu1.name); / / Zhang San System.out.println (stu2.name); / / null System.out.println (stu1.jtName); / / Miss Lu System.out.println (stu2.jtName) / / Mr. Lu}} 2. Static modifies member methods (static methods / class methods)

Static methods: static-decorated methods are class methods that can be called without creating an object. Keywords such as this and super cannot be used in static methods, non-static methods cannot be called, and only static member variables and static methods of the class to which they belong can be accessed. Static methods (class methods). When the class is loaded, the static method is immediately loaded into the method area, and the class method can be called directly by the class name.

Instance method: there is no static modified method, instance method, when the object is created, the instance method is immediately loaded into the method area, and multiple instances share a single instance method.

2.1. The difference and conclusion between static method and case method.

The this keyword cannot be used in a class method, and the parameters that call the method object are not implied in the class method.

Instance methods can directly call static methods, static methods can not directly access instance members, objects must be created and accessed by objects.

Conclusion: the data shared by all objects is defined as a static variable, otherwise it is defined as an instance variable method, and no instance member is accessed in the method, which can be defined as a static method.

Develop applications:

In a project, usually the methods in the tool class are static.

3. Static static code snippet

Syntax description:

JVM executes static static code snippets when the class is loaded, which is often used to initialize static variables.

The static code is executed only once when the class is loaded.

Static is better than object.

Develop applications:

In development, it is usually used to load static resources, read configuration files and other operations, implemented in static code snippets.

For example, we define a tool class as follows.

Public class SomeUtil {/ / default no-parameter construction public SomeUtil () {System.out.println ("create object!") ;} / / static is better than object / / static code snippet is executed immediately when a class is loaded, and a class is loaded only once static {System.out.println ("load static resources!") in the same process. ;} / / instance code segment {System.out.println ("example code snippet!") ;} public static void do1 () {System.out.println ("do1....");} public static void do2 () {System.out.println ("do2....");} public static void do3 () {System.out.println ("do3....");} public static void main (String [] args) {SomeUtil.do1 () SomeUtil.do2 (); SomeUtil.do3 (); SomeUtil S1 = new SomeUtil (); SomeUtil S2 = new SomeUtil ();}}

Execute the main method, and we can clearly see the order of execution based on the output. The static code snippet is executed only once, then the static method is executed, and finally the new object executes the no-parameter construction and the instance code snippet, and the new executes once at a time. At the same time, we can also draw our conclusion: static is better than object.

4. Static inner class

Syntax description:

Static inner classes can be instantiated without relying on external class instance objects, while inner classes can only be instantiated after the external class is instantiated.

Static inner classes cannot access ordinary variables of external classes, only static member variables and static methods of external classes.

7. Final modifier

Syntax description:

Final class, which cannot be inherited.

The final method cannot be overridden.

The variable modified by final is constant.

Develop applications:

In development, final is used to define the data dictionary.

For example, in the following Card class, we define a data dictionary for the output and query of the main function.

Note: data dictionary refers to the definition and description of data items, data structure, data flow, data storage, processing logic, etc. Its purpose is to make a detailed description of each element in the data flow chart. Use a data dictionary for a simple modeling project. In short, a data dictionary is a collection of information that describes data and a collection of definitions of all data elements used in the system.

Public class Card {/ / in development, use final to define data dictionaries. Public static final int SPADE = 1: public static final int HEART = 2: public static final int FLOWER = 6: public static final int THREE = 0: public static final int EIGHT = 5: public static final int JACK = 8: public static final int KING = 9: 10: public static final int ACE = 11: public static final int DUCE = 12: public static final int JOKER = 13: public int suit;private int num;public Card () {} public Card (int suit, int num) {this.suit = suit This.num = num;} public int getSuit () {return suit;} public void setSuit (int suit) {this.suit = suit;} public int getNum () {return num;} public void setNum (int num) {this.num = num;} public static void main (String [] args) {Card card = new Card (Card.HEART, Card.THREE); System.out.println (card.getNum ());}} VIII, abstract modifier

Syntax description:

Abstract, can modify classes, modify methods.

Abstract classes:

The meaning of the existence of abstract classes is to be inherited.

Abstract classes cannot create objects.

Abstract cannot be used with final.

Abstract method:

Abstract methods, only definitions are not implemented.

If there are abstract methods in a class, the class must be abstract.

In an abstract class, there can be no abstract methods.

A subclass of an abstract class that must implement all abstract methods in the parent class.

Develop applications:

Abstract classes can have either default implemented methods or unimplemented methods.

Interface adapter-use subclasses to implement methods in the interface.

Interfaces (special abstract classes)

Syntax description:

The methods in the special abstract class interface are all static constants in the abstract method interface.

Define the interface using the interface keyword.

The implementation interface uses the implements keyword.

Class implements the interface and must implement all the methods in the interface.

The public interface MyInterface {/ / interface is a higher level of abstraction. Can not be instantiated, so can only define constants / / define variables need to be instantiated before assignment can be used, contrary to the interface / / the methods in the special abstract class interface are all abstract method interface variables are static constants int I = 10bot / if all methods in the class are abstract methods, use the interface void method1 (); void method2 ();} import java.io.Serializable In fact, the adapter is just a class that implements some kind of interface and provides the method body. * in this way, when you use this interface, you can directly inherit the adapter, so you don't have to populate every method in the interface again. * you just need to copy the methods you need in this class. It's so simple and convenient. * * @ author bailu * * / public class MyImpClass implements MyInterface, Serializable {/ / Serializable serialization private static final long serialVersionUID = 1L System.out.println / rewrite interface method-Adapter, @ Overridepublic void method1 () {System.out.println (I);} @ Overridepublic void method2 () {}}

Develop applications:

If all the methods in the class are abstract, use the interface.

1. The difference between abstract classes and interfaces

In abstract classes, abstract and non-abstract methods can be defined. Interface, all methods are abstract methods.

A class can inherit only one abstract class. A class can implement multiple interfaces, which are separated by commas.

For example, the following class implements both interface MyInterface and Serializable serialization.

Import java.io.Serializable;/**-the adapter is really just a class that implements some kind of interface and provides the method body. In this way, when you use this interface, you can directly inherit the adapter, so you don't have to populate every method in the interface again. -you just need to overwrite the methods you need in this class. It's so simple and convenient. -- @ author bailu-* / public class MyImpClass implements MyInterface, Serializable {/ / Serializable Serialization private static final long serialVersionUID = 1L System.out.println / rewrite interface method-Adapter, @ Overridepublic void method1 () {System.out.println (I);} @ Overridepublic void method2 () {}}

You can also inherit between abstract classes, but only single inheritance is supported. Interfaces can also be inherited, and an interface can inherit multiple interfaces.

Develop applications:

Interfaces are used to develop standards or specifications.

It can reduce the coupling between components and expand the function of components.

It embodies the opening and closing principle in the design pattern.

2. What is an adapter?

The adapter is just a class that implements the interface and provides the method body. When you use this interface, you can inherit the adapter directly, so you don't need to populate every method in the interface again, just copy the methods you need in this class. The interface well embodies the opening and closing principle in the design pattern.

For example, when we do an online mall system, we need to call the third-party payment-the bank or Alipay's payment interface. We need the third party to provide us with the interface, which defines the abstract method and the method to implement the interface-the adapter, and we complete the payment by calling the method in the adapter.

ZsBankInter obj = get the implementation class object; obj.send () JD.com, Taobao Bank interface ZsBankInter send () class class DoSend implements ZsBankInter {send () specific method} Thank you for reading, these are the contents of "what are the common modifier grammars in Java". After the study of this article, I believe you have a deeper understanding of the common modifier syntax in Java. Here is, the editor will push for you more related knowledge points of the article, welcome to follow!

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.