Visitor.java

Visitor

The interpreter (and the evaluator) are performing structural recursion on the inductive data structure Program and Expression, respectively. We want to define one interpreter function that behaves differently depending on the argument. In functional languages this is done with Pattern Matching and in Java this is typically implemented using the Visitor Pattern.

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package interpreter;
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This Visitor is implemented using Reflection. That is kind of cheating, but simplifies the classical Visitor pattern a lot. Of course the performance is bad, but performance is not an issue in this little demonstration and actually there are a lot of other performance issues as well. 

public abstract class Visitor<T> {
    @SuppressWarnings("unchecked")
    public T visit(Object object) {
        try {
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Get the name of the class of the given object, search for a method with this name in self and call it. 

            return (T) this.getClass().getMethod("visit" + object.getClass().getSimpleName(), object.getClass()).invoke(this, object);
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }
}