Singleton class (single-State class, only one instance exists)

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• Not long ago, I met an interview question, which gave me only one class object for the new instance. I didn't turn around for a moment. I came back to Google to check and realize that this could be answered in a disguised form in single-State mode.

Not long ago, I met an interview question, which gave me only one class object for the new instance. I didn't turn around for a moment. I came back to Google to check and realize that this could be answered in a disguised form in single-State mode.

Keyword: Singleton

The Singleton mode ensures that only one instance of a class exists in a Java application.
The Singleton mode generally has several forms:
The first form: defines a class. Its constructor is private. It has a static private class variable. When the class is initialized, use a public getinstance method to obtain its reference, and then call the method. Java code

1. Public
   
   
    
   Class
   
   
   Singleton {
   
2. 
   Private
   
   
   Singleton (){}
3. 
   // Define your own instance internally
   
   
4. 
   // Note that this is private for internal calls only
   
   
5. 
   Private
   
   
    
   Static
   
   
   Singleton instance =
   New
   
   
   Singleton ();
6. 
   // Here is a static method for external access to this class, which can be accessed directly.
   
   
7. 
   Public
   
   
    
   Static
   
   
   Singleton getinstance (){
8. 
   Return
   
   
   Instance;
9. }
10. }

Public class Singleton {private Singleton () {}// define your own instance internally // note that this is private for internal calls only Private Static Singleton instance = new Singleton (); // here is a static method for external access to this class. You can directly access public static Singleton getinstance () {return instance ;}}

Form 2: Java code

1. Public
   
   
    
   Class
   
   
   Singleton {
   
2. 
   Private
   
   
    
   Static
   
   
   Singleton instance =
   Null
   
   
   ;
3. 
   Public
   
   
    
   Static
   
   
    
   Synchronized
   
   
   Singleton getinstance (){
4. 
   // This method is better than above. You don't need to generate objects every time. It's just the first time.
   
   
5. 
   // Generate instances during use, improving efficiency!
   
   
6. 
   If
   
   
   (Instance =
   Null
   
   
   )
7. Instance =
   New
   
   
   Singleton ();
8. 
   Return
   
   
   Instance;
9. }
10. }

Public class Singleton {Private Static Singleton instance = NULL; public static synchronized Singleton getinstance () {// This method is better than above, so you do not need to generate objects every time, it is only the first time that an instance is generated/used, improving the efficiency! If (instance = NULL) instance = new Singleton (); Return instance ;}}

Other forms:
Defines a class. Its constructor is private and all methods are static.
It is generally considered that the first form is more secure.