The pattern belongs to the behavioral category of the design patterns.
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.
Wikipedia says:
In object-oriented programming and software engineering, 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 the structures. It is one way to follow the open/closed principle.
In plain words:
The visitor allows adding new virtual functions to a family of classes, without modifying the classes.
The class diagram will be:
The task:
Let's consider to inspect a database, web service and an application with a visitor.
Let's define a service specification:
public interface Service {
void accept(final ServiceVisitor visitor);
}We need to define a service visitor also:
public interface ServiceVisitor {
void visit(final Service service, final String name);
}Let's create services:
final class DbService implements Service {
@Override
public void accept(final ServiceVisitor visitor) {
visitor.visit(this, "Database service");
}
}final class WebService implements Service {
private final String name;
public WebService(final String name) {
this.name = name;
}
@Override
public void accept(final ServiceVisitor visitor) {
visitor.visit(this, name);
}
}final class Application implements Service {
private final List<Service> services;
public Application() {
this.services = List.of(new DbService(), new WebService("Web service"));
}
@Override
public void accept(final ServiceVisitor visitor) {
for (final Service service : services) {
service.accept(visitor);
}
visitor.visit(this, "Application");
}
}We need to implement a visitor:
final class PerformVisitor implements ServiceVisitor {
@Override
public void visit(final Service service, final String name) {
System.out.println("Visited: " + name);
}
}And then it can be used as:
final var app = new Application();
app.accept(new PerformVisitor());
app.accept((service, name) -> System.out.println("Test visited: " + name));