When each instance of a class needs access to a value that can vary from one run of the applet or application to the next, but will be constant within a run, it’s time to use a class variable. Such a variable will be established when the class is loaded, and will not be replicated in each object of the class. For example, a class may need to categorize the current platform’s operating system. This code, outside any of the constructors or methods, is all that you need:
private static final boolean wind = System.getProperty("os.name"). toLowerCase().startsWith("win");
Perhaps surprisingly, you are not limited to assigning constants to these “object-less” variables.