Question

I’m trying to implement the strategy pattern using reflection, i.e. instantiate a new Concrete Strategy object using it’s class name.

I want to have a configurable file with the class name in it. We have a database manager that will make changes available during run-time. Here’s what I have so far:

StrategyInterface intrf = null;
try {
    String className = (String)table.get(someId);
    intrf = (StrategyInterface) Class.forName(className).newInstance();
} catch (InstantiationException e) {
    e.printStackTrace();
} catch (IllegalAccessException e) {
    e.printStackTrace();
} catch (ClassNotFoundException e) {
    System.out.println(e.getMessage());
}
return intrf;

I have a Class ConcreteStrategy which implements StrategyInterface. I have a test running in whcih table.get(someID) returns the String "ConcreteStrategy".

My problem is that ClassNotFoundEception is thrown. Why is this happening, and how I could get ConcreteStrategy to be instantiated given a class name? I don’t want to use an if-else block because number of concrete strategy objects will increase with time and development.

EDIT: I fixed it the following way,

String className = (String)table.get(custId);
className = TrackingLimiter.class.getPackage().getName() + "." + className;
limiter = (TrackingLimiter) Class.forName(className).newInstance();

Answer 1

Are you sure you don’t forget package name and class ConcreteStrategy is available for classloader?