2931. Design Pattern - Visitor
Visitor Pattern

Behavioral Pattern: Visitor Pattern.

1. Visitor Pattern

The Visitor pattern represents an operation to be performed on the elements of an object structure without changing the classes on which it operates.

2. Example

2.1 Computer Part

public interface ComputerPart {
   public void accept(ComputerPartVisitor computerPartVisitor);
}

public class Keyboard implements ComputerPart {

   @Override
   public void accept(ComputerPartVisitor computerPartVisitor) {
      computerPartVisitor.visit(this);
   }
}

public class Monitor implements ComputerPart {

   @Override
   public void accept(ComputerPartVisitor computerPartVisitor) {
      computerPartVisitor.visit(this);
   }
}

public class Mouse implements ComputerPart {

   @Override
   public void accept(ComputerPartVisitor computerPartVisitor) {
      computerPartVisitor.visit(this);
   }
}

public class Computer implements ComputerPart {

   ComputerPart[] parts;

   public Computer(){
      parts = new ComputerPart[] {new Mouse(), new Keyboard(), new Monitor()};		
   }

   @Override
   public void accept(ComputerPartVisitor computerPartVisitor) {
      for (int i = 0; i < parts.length; i++) {
         parts[i].accept(computerPartVisitor);
      }
      computerPartVisitor.visit(this);
   }
}

2.2 Visitor

public interface ComputerPartVisitor {
    public void visit(Computer computer);
    public void visit(Mouse mouse);
    public void visit(Keyboard keyboard);
    public void visit(Monitor monitor);
}

public class ComputerPartDisplayVisitor implements ComputerPartVisitor {

   @Override
   public void visit(Computer computer) {
      System.out.println("Displaying Computer.");
   }

   @Override
   public void visit(Mouse mouse) {
      System.out.println("Displaying Mouse.");
   }

   @Override
   public void visit(Keyboard keyboard) {
      System.out.println("Displaying Keyboard.");
   }

   @Override
   public void visit(Monitor monitor) {
      System.out.println("Displaying Monitor.");
   }
}

2.3 Client

public class Client {
    public void run() {
        ComputerPart computer = new Computer();
        computer.accept(new ComputerPartDisplayVisitor());
    }
}

Output.

Displaying Mouse.
Displaying Keyboard.
Displaying Monitor.
Displaying Computer.

3. Source Files

4. References