package net.dafydd.demo;
/**
 * Dynamic loading example application.<br>
 *
 * MOTIVATION: Although Java looks like a simplified C++, dynamically loading code
 * is an example of a task that's easy in Java and comparatively difficult in C++.
 * This is a problem for C++ because it  has low-level dependencies - meaning
 * that loading code at runtime is more involved and very platform dependent.
 *
 * FEATURES: 
 * Loads first command-line argument, or if no first arg, loads <tt>foo.class</tt>
 * Invokes <tt>.toString()</tt> on loaded object, and then prints
 * out a list of the interfaces implemented by the class which was loaded.
 * NOTE: that references to "Class" are to java.lang.Class (because java.lang is 
 * implicitly imported into every java file.
 *
 * @author Dafydd Rees
 * @version 2.0
 */
public class LoadAndListInterface {

	/** A main function for the app. If no argument is supplied,
	  * the function will try to load a class named <tt>foo</tt>.
	  * If there are arguments, then an attempt to load the classname
	  * specified by the first arugment is made.
	  */
        public static void main(String argv[]) {
		String 	cname;				// name of class to load
		Class   cnamecls;			// the class object for cname's class
		int	ifCount=0;			// the number of interfaces implemented by cname's class

		if (argv.length>0) cname = argv[0]; 	// set up classname
			else cname = "foo";

			try {				// for Exceptions
				cnamecls = Class.forName(cname);	// get the Class class for the cname
			
				Object o = cnamecls.newInstance();	
									// NOTE: using o below implicitly calls o.toString()
									//       due to definition of println method
				System.out.println("Loaded "+cname+" and as a string is "+o+".");
				
				ifCount =printInterfaces(cname);			// Try to print out the interface names...
				System.out.println("Number of interfaces "+ifCount+".");
			} catch (Throwable e) {
			// Potentially many conditions here:
			//	ClassNotFoundException
			//	InstantiationException
			//	IllegalAccessException
			// Also a potential for NoSuchMethodError due to say nonexistence of
			// a 0-arg constructor.
			// we ignore the distinctions and just "bail out".
				System.out.println("Failed to load " + cname + ".");
				System.out.println("Because of  "+e);
			}
		}

	/** Print out a list of the interfaces implemented a class.
	  * The output is written on <tt>System.out</tt>.
	  * @param cname the name of the class (as a string) from which the interfaces are to be listed.
	  * @return the number of interfaces implemented.
	  * @exception java.lang.ClassNotFoundException  Thrown when the class represented by the
	  *             <tt>cname</tt> is not found.
	  */
	protected static int printInterfaces(String cname) 
			throws   ClassNotFoundException {
		int i;
		Class ifList[] ;	// the array of class objects that represent each interface in cname

		ifList = ((Class.forName(cname)).getInterfaces());
		System.out.println("Interfaces implemented:\n");
		for (i=0; i<ifList.length; i++) {
			System.out.println(" - " + ifList[i]);
			}

		return ifList.length;
		}
}