The java.lang.reflect package will do what you''re asking, but be aware that it''s sloooow. There are faster ways of calling methods dynamicly. Polymorphism, an indexed list of Runnable objects, etc.
As always, performance test your code, see what works best in your situation.

"If consquences dictate our course of action, it doesn''t matter what''s right, it''s only wrong if you get caught."
- Tool

This post might be a little long-winded, so beware.

To use an indexed list of Runnables is easy and doesn't require any multi-thread code.
java.lang.Runnable is a simple interface that declares a single function: public void run(). Classes that implement the Runnable interface are most commonly passed to new Threads so that the Thread can call the Runnable's run() method in a new thread. However, the runnable object is also a tool you can use to simulate passing pointers to functions like in C.
EXAMPLE: Lets say you're writing a script for an AI. There are three methods the script should be able to call "MoveForward()", "MoveLeft()", & "MoveRight()". First thing you do is create 3 classes that implement Runnable, on for each method:

  public class MForwardRunner implements Runnable {  //...  public void run () {    // Call your moving forward code  }  //...}public class MLeftRunner implements Runnable {  //...  public void run () {    // Call your moving left code  }  //...}public class MRightRunner implements Runnable {  //...  public void run () {    // Call your moving right code  }  //...}   

Next, create a Hashtable with all of your functions indexed by their name:

  Hashtable scriptFuncTbl = new Hashtable();scriptFuncTbl.put("MoveForward()", new MForwardRunner());scriptFuncTbl.put("MoveRight()", new MRightRunner());scriptFuncTbl.put("MoveLeft()", new MLeftRunner());   

Then, to call a function from a script, just look up the Runnable in the hashtable and run it

  void runScriptFunc (String funcName){  Runnable r = (Runnable)scriptFuncTbl.get(funcName);  if (r != null) r.run();}   

This should work fine, and it will probably be faster than the reflection API (but be sure and test, test, test), but this isn't how I'd nessicarily do it.

If I was building a scripting language to run in a Java app (which I have done before), I would think about the fact that Java itself is essintially a scripting language, it just get's compiled into byte codes beforehand. Java also has all of the features you could want in a full-featured scripting language, is heavily supported by an industry full of developers, and will probably beat the pants off any generalized language you or I could come up with and implement when it comes to speed.
So think about this. You create an abstract class called "AIScript" or whatever. In this class you implement a bunch of methods that the script should be able to call (like an IsEnemyInFrontOfMe() function or the MoveForward/Right/Left functions from earlier). Finally, you add one abstract method called "RunScript()" or something.
Then, to actually make a script, you create another class that sub-classes AIScript and implements the RunScript() function. What does this do for you?
1) Easy upkeep of the interface scripts have into your game. Simply change the AIScript class and viola! No hashtables, no reflection API.
2) You can completely skip writing the parser/lexer/etc. for your engine, saving mucho development time.
3) The compiled Java code will run faster than just about anything you could implement.

You could even have a little utility that would take code written by level designers in the format:

  // AI Script filewhile (IsAlive()){  if (IsEnemyInFrontOfMe())  {    Attack();  }  else  {    FaceTowardsEnemy();    MoveForward();  }}   

and flow it into a template file:

  class AISCRIPT_[ScriptFileName] extends AIScript{  public void runScript ()  {    [ScriptContents]  }}   

replacing "[ScriptFileName]" with the name of the script file and "[ScriptContents]" with the contents of the script. Compile and you have a class file that is loadable into your game. This also makes it easier for users to mod your game since the "scripting language" you're using is completely documented in hundreds of books and thousands of online tutorials.

Just food for thought (the length of this post would imply a feast for thought). If you have any further questions about this stuff just post here or email me at wayfarerx@hotmail.com.

"If consquences dictate our course of action, it doesn't matter what's right, it's only wrong if you get caught."
- Tool

Edited by - wayfarerx on September 10, 2001 4:51:03 PM