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Q: I still miss global variables. What can I do instead?

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Q: I still miss global variables. What can I do instead?

Answer: That depends on what you want to do. In each case, you need to decide two things: how many copies of this so-called global variable do I need? And where would be a convenient place to put it? Here are some common solutions:

If you really want only one copy per each time a user invokes Java by starting up a Java virtual machine, then you probably want a static instance variable. For example, you have a MainWindow class in your application, and you want to count the number of windows that the user has opened, and initiate the "Really quit?" dialog when the user has closed the last one. For that, you want:
// One variable per class (per JVM)
public Class MainWindow {
  static int numWindows = 0;
  // when opening: MainWindow.numWindows++;  
  // when closing: MainWindow.numWindows--;
In many cases, you really want a class instance variable. For example, suppose you wrote a web browser and wanted to have the history list as a global variable. In Java, it would make more sense to have the history list be an instance variable in the Browser class. Then a user could run two copies of the browser at once, in the same JVM, without having them step on each other.
// One variable per instance
public class Browser {
  HistoryList history = new HistoryList();
  // Make entries in this.history
Now suppose that you have completed the design and most of the implementation of your browser, and you discover that, deep down in the details of, say, the Cookie class, inside the Http class, you want to display an error message. But you don't know where to display the message. You could easily add an instance variable to the Browser class to hold the display stream or frame, but you haven't passed the current instance of the browser down into the methods in the Cookie class. You don't want to change the signatures of many methods to pass the browser along. You can't use a static variable, because there might be multiple browsers running. However, if you can guarantee that there will be only one browser running per thread (even if each browser may have multiple threads) then there is a good solution: store a table of thread-to-browser mappings as a static variable in the Browser class, and look up the right browser (and hence display) to use via the current thread:
// One "variable" per thread
public class Browser {
  static Hashtable browsers = new Hashtable();
  public Browser() { // Constructor
    browsers.put(Thread.currentThread(), this);
  public void reportError(String message) {
    Thread t = Thread.currentThread();
Finally, suppose you want the value of a global variable to persist between invocations of the JVM, or to be shared among multiple JVMs in a network of machines. Then you probably should use a database which you access through JDBC, or you should serialize data and write it to a file.

This tip is reprinted on JavaFAQ.nu by by courtesy of Peter Norvig I am thankful for his important contributions to my site - 21 Infrequently Answered Java Questions. Alexandre Patchine

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