HELP - dynamic creation of classes

[Phil Lesh]

Hi,
I'm trying to dynamically create classes. I have an interface called Algorithm in package "App.algorithms". There are algorithms in the same package implementing the interface. From a different package called "App.gui" I want to create a new object/instance of the algorithm with the name className. Here is part of the code:

Class algoClass = Class.forName(className);
Algorithm currentAlgorithm = (Algorithm)algoClass.newInstance();

I've tried setting the className to "App.algorithms.DijkstraShortestPath" and "DijkstraShortestPath" explicitly (where DijkstraShortestPath.java is a class in "App.algorithms") but it still fails on the second line. Am I missing something? Maybe I need to do something with ClassLoader? Please can you provide a bit of code. Thanks in advance.
Phil

[Phil Lesh]

Hi again,
The exception that the above code throws is the InstantiationException. How would I instantiate the class within the above code? Thanks again!
Phil

[Don Kiddick]

From the javadocs for newInstance ...
Throws:
IllegalAccessException - if the class or its nullary constructor is not accessible.
InstantiationException - if this Class represents an abstract class, an interface, an array class, a primitive type, or void; or if the class has no nullary constructor; or if the instantiation fails for some other reason.
ExceptionInInitializerError - if the initialization provoked by this method fails.
SecurityException - if there is no permission to create a new instance.

[Stan James]

Nullary is a new word to me. Get thee to a nullary? Does that just mean you have to have a no-parameter constructor? By the way, you have to have a no-parameter constructor.
If you must use a constructor with parameters, you have to get into reflection, and invoke it. Here's some code I have around:

    
public static Object createInstance( String aClassName, Class[] aArgTypes, Object[] aArgValues )
    throws Exception
    {
        aClassName = Argument.getSynonym( aClassName );
        if  ( aArgTypes.length == 1 )
            if ( aClassName.equals( aArgTypes[0] ) )
                return aArgValues[0];
        Constructor lCtor = getConstructor( aClassName, aArgTypes );
        return lCtor.newInstance( aArgValues );
    }
    private static Constructor getConstructor( String aClassName, Class[] aArgTypes )
    throws Exception
    {
        Class lClass = Class.forName( aClassName );
        Constructor lCtor = lClass.getConstructor( aArgTypes );
        if ( null == lCtor )
            throw new Exception( "Reflector.getConstructor No constructor found for " + aClassName + typesToString( aArgTypes ) );
        return lCtor;
    }    

See the whole project at Jim's Pages - Ooops. That zip is gone awol. I will try to remember to upload it at home tonite.
[ April 08, 2003: Message edited by: Stan James ]

[Layne Lund]

Isn't a nullary a school where pointers go to learn how to be null? Kind of like a nunary...

[Cindy Glass]

All of which means that you have to create an instance of one of the classes that implements the Algorithm interface and then cast it to the Algorithm type.
You can not create an instance of an interface directly.