Visitor pattern implementation in case of source code un-availability

Question

One of the reasons to consider the Visitor_pattern:

A practical result of this separation is the ability to add new operations to existing object structu

Answer 1

If your library does not has accept methods you need to do it with instanceof. (Normally you do twice single-dispatching in Java to emulate double dispatching; but here we use instanceof to emulate double dispatching).

Here is the example:

interface Library {
  public void get1();
  public void get2();
}

public class Library1 implements Library {
  public void get1() { ... }
  public void get2() { ... }
}

public class Library2 implements Library {
  public void get1() { ... }
  public void get2() { ... }
}

interface Visitor {
   default void visit(Library1 l1) {}
   default void visit(Library2 l2) {}

   default void visit(Library l) {
      // add here instanceof for double dispatching
      if (l instanceof Library1) {
          visit((Library1) l);
      }
      else if (l instanceof Library2) {
          visit((Library2) l);
      }
   }
}

// add extra print methods to the library
public class PrinterVisitor implements Visitor {
   void visit(Library1 l1) {
       System.out.println("I am library1");
   } 
   void visit(Library2 l2) {
       System.out.println("I am library2");
   }        
}

and now in any method you can write:

Library l = new Library1();
PrinterVisitor pv = new PrinterVisitor();
pv.visit(l);

and it will print to you "I am library1";