What is Singleton Pattern?

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.

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

   }

}