Thread: How do you use class.forname?

I'm working on a project that needs to utilize external classes. A few years ago I used Class.forName to do this and I remember using an interface. However, I can't remember how this worked. Can someone tell me if they see any problems with the code below:

//Main.java

package directlink;
 
public class Main {
 
    Main() {
        try {
            Class c = Class.forName("directlink.classes.Dfile");
        } catch (ClassNotFoundException x) {
            x.printStackTrace();
        }
    }
 
    public static void main(String[] args) {
        new Main();
    }
}

//Command.java

package directlink.classes;
 
import java.util.Vector;
 
abstract class Command implements Commandinterface {
    Vector<String> options;
 
    Command() {
        this.options = new Vector<String>(10, 10);
    }
 
    public void setOptions(Vector<String>options) {
        this.options = options;
    }
}

//Commandinterface.java

package directlink.classes;
 
import java.util.Vector;
 
public interface Commandinterface {
    public void setOptions(Vector<String> name);
    public Vector<String[]> getActions();
    public void runAction(String action);
}

//Dfile.java

package directlink.classes;
import java.util.Vector;
 
public class Dfile extends Command implements Commandinterface {
    public Vector<String[]> getActions() {
        Vector<String[]> actions = new Vector<String[]>(5, 5);
        String[] command = new String[2];
        command[0] = "copy";
        command[1] = "from to";
        actions.add(command);
 
        command = new String[2];
        command[0] = "delete";
        command[1] = "from";
        actions.add(command);
 
        return actions;
    }
 
    public void runAction(String action) {
        if (action.equals("copy")) {
            //This is just to make sure Class.forName is working this is not a real command
            System.out.println("Copying file from to");
        }
    }
}

and this loads without error. However if I try to use:

c.runAction("copy");

I get the error:

Exception in thread "main" java.lang.RuntimeException: Uncompilable source code
at directlink.Main.<init>(Main.java:12)
at directlink.Main.main(Main.java:19)
Java Result: 1

I changed my code to:

package directlink;
 
public class Main {
 
    Main() throws InstantiationException, IllegalAccessException {
        try {
            final Class c = Class.forName("directlink.classes.Dfile");
            final Dfile dfile = (Dfile)c.newInstance();
            dfile.runAction("copy");
        } catch (ClassNotFoundException x) {
            x.printStackTrace();
        }
    }
 
    public static void main(String[] args) throws InstantiationException, IllegalAccessException {
        new Main();
    }
}

and I'm getting an Uncompilable source code - cannot find symbol error. I understand what you're saying (I think). However, doesn't your code eliminate the ability for dynamic classes. Right after we use forname the next line assumes you know the name of the class that will be instantiated. Doesn't this eliminate the ability to add new classes later.

exactly speaking Class.forName()will load the class binary to memory, but it could't new a instance automatically, so after the line Class.forName("directlink.classes.Dfile") you need to do c.newInstance()

I tried this way and it is running.I hope it may help you.

public class Main {
 
    Main() {
        try {
            Class c = Class.forName("testing.Dfile");
            c.getMethod("runAction", java.lang.String.class).invoke(c.newInstance(),"copy");
 
        } catch (ClassNotFoundException x) {
            x.printStackTrace();
        } catch (SecurityException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (NoSuchMethodException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IllegalArgumentException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (IllegalAccessException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (InvocationTargetException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		} catch (InstantiationException e) {
			// TODO Auto-generated catch block
			e.printStackTrace();
		}
    }
 
    public static void main(String[] args) {
        new Main();
    }
}