One of the three characteristics — encapsulation

Package literally means package , Specialty is information hiding , Encapsulating data and data-based operations with abstract data types , Make it an indivisible and independent entity , Data is protected inside abstract data types , Hide interior details as much as possible , Only some external interfaces are reserved for external connection . Other objects of the system can only communicate and interact with the encapsulated object through authorized operations wrapped outside the data . In other words, the user does not need to know the details inside the object （ Of course, I don't know ）, But the object can be accessed through the external interface provided by the object .

For packaging , An object encapsulates its own properties and methods , So it does not need to rely on other objects to complete its own operations .

There are three benefits to using encapsulation ：

1、 Good packaging reduces coupling .

2、 The structure inside the class can be freely modified .

3、 More precise control over members .

4、 Hidden information , Implementation details .

First, let's look at two classes ：Husband.java、Wife.java

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public class Husband {
 
     /*
      * Encapsulation of attributes
      * A person's name 、 Gender 、 Age 、 The wife is the private property of this person
      */
     private String name ;
     private String sex ;
     private int age ;
     private Wife wife;
 
     /*
      * setter()、getter() Is the interface for external development of the object
      */
     public String getName() {
         return name;
     }
 
     public void setName(String name) {
         this .name = name;
     }
 
     public String getSex() {
         return sex;
     }
 
     public void setSex(String sex) {
         this .sex = sex;
     }
 
     public int getAge() {
         return age;
     }
 
     public void setAge( int age) {
         this .age = age;
     }
 
     public void setWife(Wife wife) {
         this .wife = wife;
     }
}
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public class Wife {
     private String name;
     private int age;
     private String sex;
     private Husband husband;
 
     public String getName() {
         return name;
     }
 
     public void setName(String name) {
         this .name = name;
     }
 
     public String getSex() {
         return sex;
     }
 
     public void setSex(String sex) {
         this .sex = sex;
     }
 
     public void setAge( int age) {
         this .age = age;
     }
 
     public void setHusband(Husband husband) {
         this .husband = husband;
     }
 
     public Husband getHusband() {
         return husband;
     }
 
}

From the above two examples, we can see that Husband Inside wife Reference is not getter() Of , meanwhile wife Of age Also no getter() Methodical . As for the reason, I think you all know it , A man, a wife in a deep room , No woman wants to be known her age .

So encapsulation privatizes the properties of an object , At the same time, it provides some methods of properties that can be accessed by the outside world , If you don't want to be outside , We don't have to provide access . But if a class does not provide methods for external access , So this class doesn't make any sense . For example, we regard a house as an object , Beautiful decoration inside , Like sofa 、 TV play 、 Air conditioner 、 Tea table and so on are all private properties of the house , But if we don't have those walls , Is it possible for others to have a clear view ？ No privacy ！ There's the wall that's blocking it , We can have our own privacy, and we can change the decorations at will without affecting others . But if there are no doors and windows , A packed black box , What is the significance of existence ？ So people can see the scenery through the doors and windows . So doors and windows are the interfaces for the external access of the house objects .

We can't really appreciate the benefits of encapsulation through this . Now let's analyze the benefits of encapsulation from a program perspective . If we don't use encapsulation , Then the object has no setter() and getter(), that Husband Class should write like this ：

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public class Husband {
     public String name ;
     public String sex ;
     public int age ;
     public Wife wife;
}

We should use it like this ：

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Husband husband = new Husband();
         husband.age = 30 ;
         husband.name = " Zhang San " ;
         husband.sex = " male " ;    // It seems a little redundant

But that day if we need to change Husband, For example age It is amended as follows String What about the type? ？ You only have one place to use this class. OK , If you have dozens or even hundreds of such places , Are you going to break down . If encapsulation is used , We don't need to make any changes , Just a little change Husband Class setAge() The method can .

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public class Husband {
 
     /*
      * Encapsulation of attributes
      * A person's name 、 Gender 、 Age 、 The wife is the private property of this person
      */
     private String name ;
     private String sex ;
     private String age ;    /* Change to String Type of */
     private Wife wife;
 
     public String getAge() {
         return age;
     }
 
     public void setAge(int age) {
         // Conversion can be
         this.age = String.valueOf(age);
     }
 
     /** Omitting other attributes setter、getter **/
 
}

Other places still quote that (husband.setAge(22)) remain unchanged .

Here we can see , Encapsulation does make it easy to modify the internal implementation of a class , Without modifying the customer code that uses this class .

We're looking at the benefits ： More precise control of member variables .

Or that one? Husband, Generally speaking, when we refer to this object, it is not easy to make mistakes , But sometimes you're confused , It's written like this ：

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Husband husband = new Husband();
         husband.age = 300 ;

Maybe you wrote it carelessly , You found out it's okay , If we don't find out, it's a big problem , Approaching who has seen 300 Age old monster ！

But with encapsulation, we can avoid this problem , We are right. age Control the access of (setter) Such as ：

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public class Husband {
 
     /*
      * Encapsulation of attributes
      * A person's name 、 Gender 、 Age 、 The wife is the private property of this person
      */
     private String name ;
     private String sex ;
     private int age ;    /* Change to String Type of */
     private Wife wife;
 
     public int getAge() {
         return age;
     }
 
     public void setAge(int age) {
         if(age > 120){
             System.out.println("ERROR：error age input....");    // Prompt error message
         }else{
             this.age = age;
         }
 
     }
 
     /** Omitting other attributes setter、getter **/
 
}

It's all right setter Method control , In fact, through the use of encapsulation, we can also control the export of objects very well . For example, gender in the database is generally already 1、0 Way to store , But we can't show it at the front desk 1、0, Here we just need to getter() Method, just do some conversion .

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public String getSexName() {
         if ( "0" .equals(sex)){
             sexName = " Woman " ;
         }
         else if ( "1" .equals(sex)){
             sexName = " male " ;
         }
         else {
             sexName = " Simon? ???" ;
         }
         return sexName;
     }

When we use it, we only need to use it sexName To achieve the right gender display . The same can also be used to make different operations for different states .

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public String getCzHTML(){
         if ( "1" .equals(zt)){
             czHTML = "<a href='javascript:void(0)' onclick='qy(" +id+ ")'> Enable </a>" ;
         }
         else {
             czHTML = "<a href='javascript:void(0)' onclick='jy(" +id+ ")'> Ban </a>" ;
         }
         return czHTML;
     }

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