Visitor

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The Chapter 18 discusses various approaches to implement visitor in an evolvable API. The learning path itself is important, but to stress the important point, here is the code for the final solution of the expression problem:

Code from Language.java:

public interface Expression {
    public abstract void visit(Visitor v);
}
public interface Plus extends Expression {
    public Expression getFirst();
    public Expression getSecond();
}
public interface Number extends Expression {
    public int getValue();
}
 
public static abstract class Visitor {
    Visitor() {}
 
    public static Visitor create(Version10 v) {
        return create10(v);
    }
 
    public interface Version10 {
        public boolean visitUnknown(Expression e);
        public void visitPlus(Plus s);
        public void visitNumber(Number n);
    }
 
    public abstract void dispatchPlus(Plus p);
    public abstract void dispatchNumber(Number n);
}
 

The solution is using Java interfaces to represent expression elements and yet it is fully evolvable (one can always define new expression element interface). Visitor is not just a single class, but one Java interface and one Java final class. Visitors written using this style are easily extensible. For example when adding support for Minus operation in version 2.0 one just adds:

Code from Language.java:

/** @since 2.0 */
public interface Minus extends Expression {
    public Expression getFirst();
    public Expression getSecond();
}
 
public static abstract class Visitor {
    Visitor() {}
    /** @since 2.0 */
    public static Visitor create(Version20 v) {
        return create20(v);
    }
 
    /** @since 2.0 */
    public interface Version20 extends Version10 {
        public void visitMinus(Minus m);
    }
 
 
    /** @since 2.0 */
    public abstract void dispatchNumber(Number n);
}
 

This is the most flexible solution. It uses a form of tripple dispatch - e.g. the actual visit method called (on some implementation of the visitor interface) is determined by the expression type, version of the expression language and implementation of the visitor interface:

Code from Language.java:

static Visitor create20(final Visitor.Version20 v) {
    return new Visitor() {
        @Override
        public void dispatchPlus(Plus p) {
            v.visitPlus(p);
        }
 
        @Override
        public void dispatchNumber(Number n) {
            v.visitNumber(n);
        }
 
        @Override
        public void dispatchMinus(Minus m) {
            v.visitMinus(m);
        }
 
        @Override
        public void dispatchReal(Real r) {
            v.visitUnknown(r);
        }
    };
}
 

This solution to the expression problem is another realization of the general principle to separate ClientAPI from ProviderAPI. Client part of the visitor can be enriched by new dispatch methods with every version. The interface part of visitor is immutable, a fixed point, which stays the same for those who implement it. Each version defines its own unique interface (according to the list of expression types it supports). The internals of the API then bridge the dispatch methods to appropriate interface visit methods.

TheAPIBook has been written prior to JDK 1.8 being mainstream. It is thus fair to ask does anything change with JDK8 on the recommendations presented here?

JDK8 and Default methods

Can the extensible visitor pattern be simplified by usage of default methods? No, not really. Usage of default methods only increases fuzziness and goes against cluelessness of the users of the API. A general example can be found at the default methods page, here is its application to the visitor case:

When introducing the version 2.0 of the interface, one might be tempted to add the visitMinus method into an existing interface:

public interface Version10 {
    public boolean visitUnknown(Expression e);
    public void visitPlus(Plus s);
    public void visitNumber(Number n);
 
    public default void visitMinus(Minus s) {
      visitUnknown(s);
    }
}

however that hurts the clarity of the specified version. When I see an implementation of the interface without the visitMinus method being overriden, what does that mean? Does that mean that the implementation was written against version 1.0? Or that it has been written against version 2.0, but the default method implementation is fine? This makes a difference as the arithmetica example demonstrates!

As such rather create new pure interface for each version of your language/expressions rather than relying on default methods. Don't be lazy, don't increase fuzziness of your API!

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