SJAVA :: HTTP://WWW.SJAVA.NET

/**
 *
 * @author mcsong@gmail.com
 * @since 2009. 9. 17.
 */
public interface ILogger {
 
 /**
  *
  * @param log
  */
 public void log(String log);
}

/**
 *
 * @author mcsong@gmail.com
 * @since 2009. 9. 17.
 */
public class FileLogger implements ILogger {

 @Override
 public void log(String log) {
  System.out.println("file log - " + log);
 }
}

/**
 *
 * @author mcsong@gmail.com
 * @since 2009. 9. 17.
 */
public class NullLogger implements ILogger {
 
 /**
  * singleton instance
  */
 private static final NullLogger instance = new NullLogger();

 /**
  *
  */
 private NullLogger() {
 
 }

 /**
  * return singleton instance
  * @return
  */
 public static NullLogger getInstance() {
  return NullLogger.instance;
 }
 
 @Override
 public void log(String log) {
  // TODO Auto-generated method stub
  System.out.println("null log - " + log);
 }
}

/**
 *
 * @author mcsong@gmail.com
 * @since 2009. 9. 17.
 */
public class SocketLogger implements ILogger {

 @Override
 public void log(String log) {
  // TODO Auto-generated method stub
  System.out.println("socket log - " + log);
 }
}

/**
 *
 * @author mcsong@gmail.com
 * @since 2009. 9. 17.
 */
public class LoggerHandler {

 /**
  * type
  */
 private ILogger logger;
 
 /**
  * constructor
  * @param logger
  */
 public LoggerHandler(ILogger logger) {
  this.logger = logger;
 }
 
 /**
  * return logger
  * @return
  */
 public ILogger getLogger() {
  if(this.logger == null)
   return NullLogger.getInstance();
  
  return this.logger;
 }
 
 /**
  *
  * @param log
  */
 public void log(String log) {
  this.getLogger().log(log);
 }
 
 /**
  *
  * @param logger
  * @param log
  */
 public void log(ILogger logger, String log) {
  this.logger = logger;
  this.log(log);
 }
}

public class NullObjectTest {
 /**
  * handler
  */
 private static LoggerHandler handler;
  
 /**
  * @param args
  */
 public static void main(String[] args) {
  // TODO Auto-generated method stub
  
  handler = new LoggerHandler(null);
  handler.log("fileloggger");

  handler = new LoggerHandler(new FileLogger());
  handler.log("socketloggger");
  
  handler = new LoggerHandler(new SocketLogger());
  handler.log("socketloggger");
 }
}