[RESOLVED] dynamically creating an instance of a class at runtime, experts needed

[Pouncer]

I tried to play around with this, after reading http://java.sun.com/developer/techni...ion/index.html

but I have a problem:

import java.lang.reflect.*;

public class Test {
    
    public static void main(String args[]) {
	try {
            Class c = Class.forName("C:\\Program Files\\Apache Software Foundation\\Tomcat 6.0\\webapps\\eg\\submissions\\msk9\\ClassTest.class");
            Method m[] = c.getDeclaredMethods();
            for (int i = 0; i < m.length; i++)
		System.out.println(m[i].toString());
	}
	catch (Throwable e) {
            System.err.println(e);
	}
    }

}

java.lang.ClassNotFoundException: C:\Program Files\Apache Software Foundation\Tomcat 6.0\webapps\eg\submissions\msk9\ClassTest.class

but the .class file is definetely there..

[ComputerJy]

A class path is not a name, you must add the path to the class path when compiling your code and use Class.forName("Qualified name");

[Pouncer]

i'm now using interfaces so i know what method each submitted computer players use.

ComputerPlayerInterface.java

import java.io.*;

public interface ComputerPlayerInterface {
    
    // Method signatures, which all submitted computer players must have.
    String getUsername();
}

TestPlayer.java

import java.io.*;

public class TestPlayer implements ComputerPlayerInterface{

    public String getUsername() {
	return "Tom";
    }
}

Test.java

public class Test {
    
    public static void main(String args[]) {
	try {
            Class c = Class.forName("TestPlayer");
            
	    String name = c.getUsername();

	    System.out.println(name);
	}
	catch (Exception e) {
            System.err.println(e);
	}
    }
}

i'm trying to get it to print Tom, but it just gives an error saying

Test.java:7: cannot find symbol
symbol : method getUsername()
location: class java.lang.Class
String name = c.getUsername();
^
1 error

computerJ, could you help me fix this? im very confused. the TestPlayer.class is in the same directory as the applicatiion, dont know where im going wrong

[ComputerJy]

This is how you should do it

public class Test
{
	public static void main(String[] args) throws Exception
	{
		Class<?> c = Class.forName("TestPlayer");
		Object o = c.newInstance();
		String x = (String) c.getMethod("getUsername", (Class[]) null).invoke(o, (Object[])null);
		System.out.println(x);
	}
}

[Pouncer]

This is how you should do it

public class Test
{
	public static void main(String[] args) throws Exception
	{
		Class<?> c = Class.forName("TestPlayer");
		Object o = c.newInstance();
		String x = (String) c.getMethod("getUsername", (Class[]) null).invoke(o, (Object[])null);
		System.out.println(x);
	}
}

thanks computerjy, its perfect. if the getUsername took parameters, what would i change?
and also, is there a way to make the coding look a bit more easy on the eye. like for e.g, some way to dynamically create the instance and then just do something like

String name = theclassinstance.getUsername();

just like we would normally do when dealing with classes?

[ComputerJy]

A direct way:

public class Test
{
	public static void main(String[] args) throws Exception
	{
		Class<?> c = Class.forName("TestPlayer");
		ComputerPlayerInterface o = (ComputerPlayerInterface) c.newInstance();
		String x = o.getUsername();
		System.out.println(x);
	}
}

And for example if your TestPlayer class had the method getNumber that converts an integer to string the call would look like

public class Test
{
	public static void main(String[] args) throws Exception
	{
		Class<?> c = Class.forName("TestPlayer");
		Object o = c.newInstance();
		String x = (String) c.getMethod("getNumber", new Class[] { Integer.class }).invoke(o,
				new Object[] { Integer.valueOf(3) });
		System.out.println(x);
	}
}

And you must make sure that the the parameter in the method getNumber is Integer not int

[Pouncer]

A direct way:

public class Test
{
	public static void main(String[] args) throws Exception
	{
		Class<?> c = Class.forName("TestPlayer");
		ComputerPlayerInterface o = (ComputerPlayerInterface) c.newInstance();
		String x = o.getUsername();
		System.out.println(x);
	}
}

absolutely perfect. thanks very much buddy.

also do you know anything about production code to check that the object instance does actually implement the interface?

[ComputerJy]

Try this method:

public class Test
{
	public static void main(String[] args) throws Exception
	{
		Class<?> c = Class.forName("TestPlayer");
		for (int i = 0; i < c.getInterfaces().length; i++)
		{
			if (c.getInterfaces()[i].getName().endsWith("ComputerPlayerInterface"))
			{
				System.out.println("Correct");
				return;
			}
		}
		System.out.println("Invalid class");
	}
}

[Pouncer]

awesome, thanks very much. you sure are great computerJy