SINGLETON CLASSES QUESTIONS

Question

Hello,
Could someone please explain what the following code does? How does it work? How can I use it in the real program?
More importantly, if you are kind enough, please give me some examples how to use singleton class? Why do we have to use this class?
I spent two days on this topic, Singleton, but my mind still goes blank.....
thank you.
public class Singleton
static public Singleton getInstance()
if(theInstance == null)
theInstance = new Singleton();
return theInstance;
protected Singleton()
//initializing instance fields
//instance fields and methods
private static Singleton theInstance = null;

Answer 1

Hello,
Could someone please explain what the following code
does? The code defines a class called Singleton. :)
How does it work? The static method "getInstance()" returns an object of type Singleton. Static methods can be called without having an instance of a class, so you can type:
Singleton.getInstance();
How can I use it in the
real program?This class is pretty useless in a real program. The point of this class is to show you how the Singleton design pattern can be implemented in Java. Any useful Singleton object needs to have additional properties and methods. (Oh I know someone will say that you can still use this class for something, but I'm trying to explain something here...)
More importantly, if you are kind enough, please give
me some examples how to use singleton class? Sure, here's an example of a singleton: Suppose you want a debugging class that will simply write text to a file. You want to only have one instance of this class (i.e. you don't want to have multiple log files, you want to provide one single interface point for this):
class LoggerSingleton
  private static final String LOGFILENAME = "log.txt";
  private BufferedWriter write = null;
  private LoggerSingleton instance;
  synchronized static public LoggerSingleton getInstance()
  { if(instance == null)  instance = new LoggerSingleton(LOGFILENAME);
    return instance;
  protected LoggerSingleton(String filename)
    try {
      write = new BufferedWriter(new FileWriter(filename));
      write.write("Log File Opened");
      write.newLine();
      write.flush();
    catch(IOException ioe)
      System.err.println("Error!  LoggerSingleton could not be initialized");
      ioe.printStackTrace(System.err);
      System.exit(1);
  public log(String text)
    write.write(text);
    write.newLine();
    write.flush();
}This class will make sure you only have one log file and ensures that all classes are using it:
class FirstClass {
  SecondClass second = new SecondClass();
  public FirstClass() {
    LoggerSingleton.getInstance().log("Inside FirstClass' constructor");
  public void func1() {
    LoggerSingleton.getInstance().log("Inside FirstClass.func1()");
class SecondClass {
  public SecondClass() {
    LoggerSingleton.getInstance().log("Inside SecondClass' constructor");
Why do we have to use this class?You don't have to use this class, but it is one technique to ensure that multiple instances of an object are not instantiated. It is also a technique to ensure that there is only one interface point across the application (you can also use a Singleton object to implement the Bridge and Factory design patterns). The Singleton technique is achieved by making the constructor protected, which means that only methods inside the class (and package) can construct instances of Singleton. The only time an instance of Singleton is constructed is the first time that getInstance() is called.
I spent two days on this topic, Singleton, but my mind
still goes blank.....
Still blank?