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Tip of the Day
Language: Java
Expertise: Beginner
Sep 23, 1997

Create a Single Instance of an Object

There are situations when you need to insure that only a single instance of an object is created in a program. Examples of this include a connection to a database system, a lock manager in a server program, and so on. This code shows you how to implement the "Singleton" design pattern in Java. This particular implementation includes lazy instantiation:

 
public class SingleInstance
     {
     // Java guarantees that si is set to null
     private static SingleInstance si;


     // declare the constructor to be private so that no
     // one can create an instance of this class except the class itself
     private SingleInstance() {}


     // This static method will return a reference to
     // the same instance no matter how many times it is called.
     public static SingleInstance getSingleInstance()
         {
         // si will be null the first time this is called.
         if ( null == si )
             {
             si = new SingleInstance();
             }
         return si;
         }
     }
To get an instance of this class, use the static method of the class:
 SingleInstance i = SingleInstance.getSingleInstance();

Randy Kahle
 
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