Stop and continue while evaluation an expression tree

Answer 1

The position in the tree can be specified using a list of child numbers. Your evaluates would look something like this (quickly written; not compiled/tested):

public class Context {
    private Context parent; // set in constructor
    private int child;  // with get/set
}

public ReturnType evaluate(Context context) 
    context.setChild(context.getChild() + 1);
    context = new Context(context); // push a new context for calls
    // do evaluation - when calling kids, pass context            
}

The above should track the context as a list of numbers that tell which child you're currently processing.

Rather than implement that in each node type, I'd recommend writing a decorator (or a superclass using a template method) to do it for you, such as:

// decorator
public class ContextTrackerEvaluator<T> implements Evaluator<T> {
    private Evaluator realEvaluator;
    public ContextTrackerEvaluator(Evaluator realEvaluator) {
        this.realEvaluator = realEvaluator;
    }
    public T evaluate(Context context) {
        context.setChild(context.getChild() + 1);
        context = new Context(context); // push a new context for calls
        realEvaluator.evaluate(context);
    }
}

// OR superclass w/ template method
public class EvaluatorBase<T> {
    public final T evaluate(Context context) {
        context.setChild(context.getChild() + 1);
        context = new Context(context); // push a new context for calls
        doEvaluate(context);
    }
    // subclasses override doEvaluate to do their real work
    protected abstract T doEvaluate(Context context);
}

If you use the decorator, make sure you decorate every node...

That gives you access to the context list.

You can then add a "stop context" parameter that you can compare (so you'd have two contexts passed in and you could compare them to see if they match.

Hope this helps! -- Scott