/ Design Patterns / Behavioral patterns / Visitor

Visitor Design Pattern in Java

Visitor design pattern

  1. Add an accept(Visitor) method to the "element" hierarchy
  2. Create a "visitor" base class w/ a visit() method for every "element" type
  3. Create a "visitor" derived class for each "operation" to do on "elements"
  4. Client creates "visitor" objects and passes each to accept() calls
interface Element {
    void accept(Visitor v);
}

class FOO implements Element {
    public void accept(Visitor v) {
        v.visit(this);
    }

    public String getFOO() {
        return "FOO";
    }
}

class BAR implements Element {
    public void   accept( Visitor v ) {
        v.visit( this );
    }

    public String getBAR() {
        return "BAR";
    }
}

class BAZ implements Element {
    public void accept(Visitor v) {
        v.visit(this);
    }

    public String getBAZ() {
        return "BAZ";
    }
}

interface Visitor {
    void visit(FOO foo);
    void visit(BAR bar);
    void visit(BAZ baz);
}

class UpVisitor implements Visitor {
    public void visit(FOO foo) {
        System.out.println("do Up on " + foo.getFOO());
    }

    public void visit(BAR bar) {
        System.out.println("do Up on " + bar.getBAR());
    }

    public void visit(BAZ baz) {
        System.out.println( "do Up on " + baz.getBAZ() );
    }
}

class DownVisitor implements Visitor {
    public void visit(FOO foo) {
        System.out.println("do Down on " + foo.getFOO());
    }

    public void visit(BAR bar) {
        System.out.println("do Down on " + bar.getBAR());
    }

    public void visit(BAZ baz ) {
        System.out.println("do Down on " + baz.getBAZ());
    }
}

public class VisitorDemo {
    public static void main( String[] args ) {
        Element[] list = {new FOO(), new BAR(), new BAZ()};
        UpVisitor up = new UpVisitor();
        DownVisitor down = new DownVisitor();
        for (Element element : list) {
            element.accept(up);
        }
        for (Element element : list) {
            element.accept(down);
        }
    }
}

Output

do Up on FOO
do Up on BAR
do Up on BAZ
do Down on FOO
do Down on BAR
do Down on BAZ

Code examples

Java Visitor in Java Visitor in Java: Double dispatch (within a single hierarchy) Visitor in Java Visitor in Java
C++ Visitor in C++: Before and after Visitor in C++ Visitor in C++: Recovering lost type information
PHP Visitor in PHP
Delphi Visitor in Delphi
Python Visitor in Python