Design Patterns - Visitor Pattern

In Visitor pattern, we use a visitor class which changes the executing algorithm of an element class. By this way, execution algorithm of element can vary as and when visitor varies. This pattern comes under behavior pattern category. As per the pattern, element object has to accept the visitor object so that visitor object handles the operation on the element object.

Implementation

We are going to create a ComputerPart interface defining accept opearation.Keyboard, Mouse, Monitor and Computer are concrete classes implementing ComputerPart interface. We will define another interface ComputerPartVisitor which will define a visitor class operations. Computer uses concrete visitor to do corresponding action.

VisitorPatternDemo, our demo class, will use Computer and ComputerPartVisitor classes to demonstrate use of visitor pattern.

Step 1

Define an interface to represent element.

ComputerPart.java

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

Step 2

Create concrete classes extending the above class.

Keyboard.java

pubpc class Keyboard implements ComputerPart {

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

Monitor.java

pubpc class Monitor implements ComputerPart {

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

Mouse.java

pubpc class Mouse implements ComputerPart {

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

Computer.java

pubpc class Computer implements ComputerPart {
	
   ComputerPart[] parts;

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


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

Step 3

Define an interface to represent visitor.

ComputerPartVisitor.java

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

Step 4

Create concrete visitor implementing the above class.

ComputerPartDisplayVisitor.java

pubpc class ComputerPartDisplayVisitor implements ComputerPartVisitor {

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

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

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

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

Step 5

Use the ComputerPartDisplayVisitor to display parts of Computer.

VisitorPatternDemo.java

pubpc class VisitorPatternDemo {
   pubpc static void main(String[] args) {

      ComputerPart computer = new Computer();
      computer.accept(new ComputerPartDisplayVisitor());
   }
}

Step 6

Verify the output.

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