Question

Please give me a real time example for singleton pattern .
Different threads accessing a shared file is singleton or not ? Since each thread access the same instance of the file not individual instances of their own .

Answer 1

Yes, but only if all threads access the same file, and you are using a custom implementation (not java.io.File, perhaps a wrapper)

the singleton pattern is a design pattern that is used to restrict instantiation of a class to one object

Singletons (being often a bad choice) are classes that can have only one instance throughout the whole program.

For example a SingletonConfigFile might look like this. Have in mind that:

• it is for reading one file only. It would make sense for this to be a config file.
• if your class can be instantiated more than once, for different files, it is not singleton.
• don’t use this code – it doesn’t take into account concurrency problems which are a whole different area of discussion.

.

public SingletonConfigFile {
   private static String filename = "config.xml";
   private File file;
   private static SingletonConfigFile instance;

   private SingletonConfigFile() {
       if (instance != null) {
           throw new Error();
       }
       file = new File(filename);
   }

   public synchronized SingletonConfigFile getInstance() {
      if (instance == null) {
          instance = new SignletonConfigFile();
      }
      return instance
   }

   public String read() {
       // delegate to the underlying java.io.File
   }
}

But this example is on the edge of sense. Singletons are used in cases when there is only one object (as I stated above). For example it would make sense to have:

• RingOfPower.getInstance() – there is only one ring of power (Sauron’s), and there can’t exist more.
• Sun.getInstance() – only one star called “sun”.
• all objects in the withing of your application that logically should exist only once – a registry, an application context, etc.