Reflection.java

/*
<APPLET CODE="Reflection.class" WIDTH=400 HEIGHT=100></APPLET>
*/

import java.awt.*;
import javax.swing.*;

import java.lang.reflect.*;

/**
 * An example of using reflection in Java.
 *
 * @author Jim Glenn
 * @version 0.1 10/15/2007 Beware?
 */

public class Reflection extends JApplet
{
    public void paint(Graphics g)
    {
	g.drawString("The easy way", 20, 20);

	try
	    {
		Class gClass = Graphics.class; // the Class object for Graphics


		// set up the argument list for getMethod (the list
		// of Classes of arguments for the method we're trying
		// to get

		Class[] drawArgs = new Class[3];
		drawArgs[0] = String.class;
		drawArgs[1] = int.class;
		drawArgs[2] = int.class;

		Method draw = gClass.getMethod("drawString", drawArgs);
		// or could use variable argument lists:
		// draw = gClass.getMethod("drawString", String.class, int.class, int.class);
		
		// just for fun, we'll invoke Math.max(20, 40) using
		// reflection to compute the y-value for our second
		// string

		Method max = Math.class.getMethod("max", int.class, int.class);

		// note the autoboxing for the arguments to invoke; the
		// return value can't be unboxed until it is cast from Object
		int xPos = 20;
		int yPos = (Integer)(max.invoke(null, 40, xPos));
		
		draw.invoke(g, "The hard way", xPos, yPos);
		// draw.invoke(g, new Object[] {"The hard way", xPos, yPos});
	    }
	catch (NoSuchMethodException e)
	    {
		// would get here if either getMethod invocation failed
		e.printStackTrace(System.err);
	    }
	/*
	catch (SecurityException e) // optional catch block
	    {
		// reflection can't get around access restrictions
		// (protected/private/package) or security restructions
		// (applet restrictions, etc.)
		e.printStackTrace(System.err);
	    }
	*/
	catch (IllegalAccessException e)
	    {
		// see above
		e.printStackTrace(System.err);
	    }
	/*
	catch (IllegalArgumentException e) // optional catch block
	    {
		// invoke checks argument lists (including the first
		// argument passed as this) just like the compiler does
		e.printStackTrace(System.err);
	    }
	*/
	catch (InvocationTargetException e)
	    {
		// the method invoke invoked might throw an exception;
		// we could access that exception here
		e.printStackTrace(System.err);
	    }
    }
}