Visitor Pattern

#behavioral #pattern #designpatterns #java

Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.

Participants

Visitor: declares a Visit operation for each class of ConcreteElement in the object structure. The operation's name and signature identifies the class that sends the Visit request to the visitor. That lets the visitor determine the concrete class of the element being visited. Then the visitor can access the elements directly through its particular interface
ConcreteVisitor: implements each operation declared by Visitor. Each operation implements a fragment of the algorithm defined for the corresponding class or object in the structure. ConcreteVisitor provides the context for the algorithm and stores its local state. This state often accumulates results during the traversal of the structure.
Element: defines an Accept operation that takes a visitor as an argument.
ConcreteElement: implements an Accept operation that takes a visitor as an argument
ObjectStructure: can enumerate its elements. May provide a high-level interface to allow the visitor to visit its elements. May either be a Composite (pattern) or a collection such as a list or a set

Code

public class Main {

    public static void main(String[] args) {
        ObjectStructure o = new ObjectStructure();
        o.attach(new ConcreteElementA());
        o.attach(new ConcreteElementB());
        ConcreteVisitor1 v1 = new ConcreteVisitor1();
        ConcreteVisitor2 v2 = new ConcreteVisitor2();
        o.accept(v1);
        o.accept(v2);
    }
}

public interface Visitor {
    void visitConcreteElementA(ConcreteElementA concreteElementA);

    void visitConcreteElementB(ConcreteElementB concreteElementB);
}

public class ConcreteVisitor1 implements Visitor {

    @Override
    public void visitConcreteElementA(ConcreteElementA concreteElementA) {
        System.out.println(
                concreteElementA.getClass().getSimpleName() + " visited by " + this.getClass().getSimpleName());
    }

    @Override
    public void visitConcreteElementB(ConcreteElementB concreteElementB) {
        System.out.println(
                concreteElementB.getClass().getSimpleName() + " visited by " + this.getClass().getSimpleName());
    }

}

public class ConcreteVisitor2 implements Visitor {

    @Override
    public void visitConcreteElementA(ConcreteElementA concreteElementA) {
        System.out.println(
                concreteElementA.getClass().getSimpleName() + " visited by " + this.getClass().getSimpleName());
    }

    @Override
    public void visitConcreteElementB(ConcreteElementB concreteElementB) {
        System.out.println(
                concreteElementB.getClass().getSimpleName() + " visited by " + this.getClass().getSimpleName());
    }

}

public interface Element {
    void accept(Visitor visitor);
}

public class ConcreteElementA implements Element {

    @Override
    public void accept(Visitor visitor) {
        visitor.visitConcreteElementA(this);
    }

    public void operationA() {
    }
}

public class ConcreteElementB implements Element {

    @Override
    public void accept(Visitor visitor) {
        visitor.visitConcreteElementB(this);
    }

    public void operationB() {
    }
}

public class ObjectStructure {
    private List<Element> elements = new ArrayList<>();

    public void attach(Element element) {
        elements.add(element);
    }

    public void detach(Element element) {
        elements.remove(element);
    }

    public void accept(Visitor visitor) {
        for (Element element : elements) {
            element.accept(visitor);
        }
    }
}

Output

ConcreteElementA visited by ConcreteVisitor1
ConcreteElementB visited by ConcreteVisitor1
ConcreteElementA visited by ConcreteVisitor2
ConcreteElementB visited by ConcreteVisitor2