Static class declarations

Can a class -- inner or outer -- be declared static?

Q: Can a class (whether an inner or outer class) be declared static?

A: In order to understand the use of the static keyword in class declaration, we need to understand the class declaration itself. You can declare two kinds of classes: top-level classes and inner classes.

Top-level classes

You declare a top-level class at the top level as a member of a package. Each top-level class corresponds to its own java file sporting the same name as the class name.

A top-level class is by definition already top-level, so there is no point in declaring it static; it is an error to do so. The compiler will detect and report this error.

Inner classes

You define an inner class within a top-level class. Depending on how it is defined, an inner class can be one of the following four types:

1. Anonymous. Anonymous classes are declared and instantiated within the same statement. They do not have names, and they can be instantiated only once.

The following is an example of an anonymous class:

okButton.addActionListener( new ActionListener(){
   public void actionPerformed(ActionEvent e){
      dispose();
   }
});

Because an anonymous class doesn't have a normal class declaration where it's possible to use static, it cannot be declared static.

2. Local. Local classes are the same as local variables, in the sense that they're created and used inside a block. Once you declare a class within a block, it can be instantiated as many times as you wish within that block. Like local variables, local classes aren't allowed to be declared public, protected, private, or static.

Here's a code example:

//some code block .......{
   class ListListener implements ItemListener {
      List list;
      public ListListener(List l) {
         list = l;
      }
      public void itemStateChanged(ItemEvent e) {
         String s = l.getItemSelected();
         doSomething(s);
      }
   }
   List list1 = new List();
   list list2 = new List();
   list1.addItemListener(new ListListener(list1));
   list2.addItemListener(new ListListener(list2));
}

3. Member. Member classes are defined within the body of a class. You can use member classes anywhere within the body of the containing class. You declare member classes when you want to use variables and methods of the containing class without explicit delegation.

The member class is the only class that you can declare static. When you declare a member class, you can instantiate that member class only within the context of an object of the outer class in which this member class is declared. If you want to remove this restriction, you declare the member class a static class.

When you declare a member class with a static modifier, it becomes a nested top-level class and can be used as a normal top-level class as explained above.

4. Nested top-level. A nested top-level class is a member classes with a static modifier. A nested top-level class is just like any other top-level class except that it is declared within another class or interface. Nested top-level classes are typically used as a convenient way to group related classes without creating a new package.

If your main class has a few smaller helper classes that can be used outside the class and make sense only with your main class, it's a good idea to make them nested top-level classes. To use the nested top-level class, write: TopLevelClass.NestedClass.

See the following example:

public class Filter {
   Vector criteria = new Vector();
   public addCriterion(Criterion c) {
      criteria.addElement(c);
   }
   public boolean isTrue(Record rec) {
      for(Enumeration e=criteria.elements();
      e.hasMoreElements();) {
         if(! ((Criterion)e.nextElement()).isTrue(rec))
             return false;
      }
      return true;
   }
   public static class Criterion {
      String colName, colValue;
      public Criterion(Stirng name, String val) {
         colName = name; colValue = val;
      }
      public boolean isTrue(Record rec) {
         String data = rec.getData(colName);
         if(data.equals(colValue)) return true;
         return false;
      }
   }
}

And when you want to use it:

Filter f = new Filter();
f.addCriterion(new Filter.Criterion("SYMBOL", "SUNW"));
f.addCriterion(new Filter.Criterion("SIDE", "BUY"));
.....
if(f.isTrue(someRec)) //do some thing .....

One important note: The static keyword does not do to a class declaration what it does to a variable or a method declaration.

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