design an abstract class so that any one can extend it and use the extended class polymorphically

I want to have an abstract class like this:

public abstract Operator {

    public int[] operands;

    public Operator(int[] operands) {
        this.operands = operands;
    }

    public abstract int getOperatorResult();
}

And some operators to extend from it like:

public class Add extends Operator {

    public Add(int[] operands) {
        super(operands);
    }

    public int getOperatorResult() {
        return operands[0] + operands[1];
    }
}

And an OperatorCreator which gets an expression like "1+1" and returns the appropriate Operator's sub-class(in this case Add):

public class OperatorCreator {

    public static Operator getInstance(String expr) {
        ...
    }
}

The main problem: And I want to let others to design their own operators by extending Operator class. And use their operators polymorphically. something like this:

public class Main {

    public static void main(String[] args) {
        Operator op = OperatorCreator.getInstance("1-2");
        System.out.println(op.getOperatorResult());
    }
}

How can I do this? Suppose that I know the location of .class file in which the new Operator's sub-class is compiled.

Thanks!