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

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

but I have a problem:

Code:

---

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..

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");

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

ComputerPlayerInterface.java

Code:

---

import java.io.*;

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

---

TestPlayer.java

Code:

---

import java.io.*;

public class TestPlayer implements ComputerPlayerInterface{

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

---

Test.java

Code:

---

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

This is how you should do it

Code:

---

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);
        }
}

---

Quote:

---

Originally Posted by ComputerJy

This is how you should do it

Code:

---

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?

A direct way:

Code:

---

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

Code:

---

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

Quote:

---

Originally Posted by ComputerJy

A direct way:

Code:

---

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?

Try this method:

Code:

---

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");
        }
}

---

awesome, thanks very much. you sure are great computerJy