Design Patterns - Visitor

Design Patterns | Visitor

References
Example

References
- Definition: (source: http://en.wikipedia.org/wiki/Visitor_pattern)
  
  The visitor design pattern is a way of separating an algorithm from an object structure on which it operates.
  A practical result of this separation is the ability to add new operations to existing object structures without modifying those structures.
- Class diagram: (source: http://en.wikipedia.org/wiki/Visitor_pattern)

Example

The Visitor:

public interface Visitor {
    void visit(final Debit debit);

    void visit(final Credit credit);
}

The Concrete Visitor:

public class VisitorImpl implements Visitor {
    public void visit(final Debit debit) {
        System.out.println("Visit Debit ...");
    }

    public void visit(final Credit credit) {
        System.out.println("Visit Credit ...");
    }
}

The Element:

public interface Payment {
    void accept(final Visitor visitor);
}

The Concrete Element:

Implementation 1:

public class Debit implements Payment {
    public void accept(final Visitor visitor) {
        visitor.visit(this);
    }
}

Implementation 2:

public class Credit implements Payment {
    public void accept(final Visitor visitor) {
        visitor.visit(this);
    }
}

A simple class to test the Visitor design pattern:

public class VisitorPatternTest {
    public static void main(String[] args) {
        final Payment debit = new Debit();
        final Payment credit = new Credit();

        final Visitor visitor1 = new VisitorImpl();

        debit.accept(visitor1);
        credit.accept(visitor1);
    }
}