Name: ddT132432 Date: 08/15/2001


Java(TM) 2 Runtime Environment, Standard Edition (build 1.3.1-b24)

The following coding idiom, and variations of it, occur extremely frequently
in Java programs:

 ----------------------
 :
 private SomeType var;
 public SomeType getVar() {
   if (var == null) {
     var == new SomeType(...);
   }
   return var;
 }
 :
 ----------------------

A "once" keyword would reduce program size and improve readability
significantly. Example:

 ----------------------
 public once SomeType getVar() {
   return new SomeType(...); // Always return ref to same instance
 }
 ----------------------

Note that this is *not* the same as a static method declaration, since different
instances of class X which contains a non-static "once"-method with can deliver
different values (note this is in contrast to Eiffel's "once", which returns the
same value for all class instances.)

"once" can also be used in static method declarations:

 ----------------------
 public static once void initializeSystem() {
   // Do initialization
   :
 }
 ----------------------

This example also shows "once"'s use in a procedure. Currently the only way to
code this is by using tedious and error-prone code such as:

 ----------------------
 private static boolean alreadyInitalized = false;
 public static synchronized void initializeSystem() {
   if (!alreadyInitalized) {
      // Do initialization
      :
      alreadyInitalized = true;
   }
 }
 ----------------------

"once" methods are implicitly synchronized, since the "once" keyword garantees
that exactly one instance of the object is instantiated.

Primitive types work similarly, effectively caching the result in an unnamed
private instance or (in the case of static methods) class variable. All other
aspects of method semantics remain unchanged.

Like "synchronized", the "once" declaration is *not* inherited, although
calling a "once"-method from a subclass e.g. super.getVar(), continues to
exhibit the once-only behaviour.

Optimizing compilers and runtime environments can certainly utilize the "once"
declaration to perform additional optimizations. For example, the method only
need be "synchronized" until the first call to it has completed, and the monitor
released.
(Review ID: 130050)
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