Visitor is a behavioral pattern that uses a visitor class which changes the executing algorithm of an element class. By this way, execution algorithm of element can vary as and when a visitor varies. Element object has to accept the visitor object so that object handles the operation on the element object.

In the following example there is a ComputerPart interface defining accept operation along with concrete classes implementing ComputerPart : Keyboard, Mouse, Monitor, Computer. Another interface, ComputerPartVisitor, defines a visitor class operations. Computer uses concrete visitor to do corresponding action. VisitorPatternDemo will use Computer and ComputerPartVisitor classes to demonstrate use of visitor pattern.

Step 1 : Define an interface to represent element

public interface ComputerPart {
    default void accept(ComputerPartVisitor computerPartVisitor, ComputerPart computerPart) {
        computerPartVisitor.visit(computerPart);
    }
}

Step 2 : Create concrete classes implementing the above interface

public class Keyboard implements ComputerPart {}

public class Monitor implements ComputerPart {}

public class Mouse implements ComputerPart {}

public class Computer implements ComputerPart {
    ComputerPart [] parts;

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

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

Step 3 : Define an interface to represent visitor

public interface ComputerPartVisitor {
    void visit(ComputerPart computerPart);
}

Step 4 : Create concrete visitor implementing the above interface

public class ComputerPartDisplayVisitor implements ComputerPartVisitor {
    @Override
    public void visit(ComputerPart computerPart) {
        if (computerPart instanceof Computer) {
            System.out.println("Displaying Computer.");
        } else if (computerPart instanceof Keyboard) {
            System.out.println("Displaying Keyboard.");
        } else if (computerPart instanceof Mouse) {
            System.out.println("Displaying Mouse.");
        } else if (computerPart instanceof Monitor) {
            System.out.println("Displaying Monitor.");
        }
    }
}

Step 5 : Use ComputerPartDisplayVisitor to display parts of Computer

public class VisitorPatternDemo {
    public static void main(String[] args) {
        ComputerPart computer = new Computer();
        computer.accept(new ComputerPartDisplayVisitor(), computer);
    }
}

The output will be :

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