What is meant by Singleton Pattern?

SINGLETON:
                       One instance of a class or one value accessible globally in an application. 

Where to use & benefits

• Ensure unique instance by defining class final to prevent cloning.
• May be extensible by the subclass by defining subclass final.
• Make a method or a variable public or/and static.
• Access to the instance by the way you provided.
• Well control the instantiation of a class.
• Define one value shared by all instances by making it static.
• Related patterns include 

For example, to use a static variable to control the instance;

class Connection {
    public static boolean haveOne = false;
    public Connection() throws Exception{
        if (!haveOne) {
           doSomething();
           haveOne = true;
        }else {
          throw new Exception("You cannot have a second instance");
        }
    }
    public static Connection getConnection() throws Exception{
        return new Connection();
    }
    void doSomething() {}
    //...
    public static void main(String [] args) {
        try {
            Connection con = new Connection(); //ok
        }catch(Exception e) {
            System.out.println("first: " +e.getMessage());
        }
        try {
            Connection con2 = Connection.getConnection(); //failed.
        }catch(Exception e) {
            System.out.println("second: " +e.getMessage());
        }
    }
}

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