Visitor in Java
Why read if you can watch?
Watch Visitor's video tutorialProblem."If you want to add a new Visitable object, you have to change the Visitor interface, and then implement that method in each of your Visitors."
Solution. With the ReflectiveVisitor, you only need one method in the Visitor interface - visit(Object). All other visit() methods can be added later as point-to-point coupling is required.
import java.lang.reflect.Method;
////////// The "element" hierarchy //////////
interface Element {
public void accept( ReflectiveVisitor v );
}
class This implements Element {
public void accept( ReflectiveVisitor v ) { v.visit( this ); }
public String thiss() { return "This"; }
}
class That implements Element {
public void accept( ReflectiveVisitor v ) { v.visit( this ); }
public String that() { return "That"; }
}
class TheOther implements Element {
public void accept( ReflectiveVisitor v ) { v.visit( this ); }
public String theOther() { return "TheOther"; }
}
////////// The "operation" hierarchy //////////
abstract class ReflectiveVisitor {
abstract public void visit( Object o );
public void visitTheOther( TheOther e ) {
System.out.println( "ReflectiveVisitor: do Base on " + e.theOther() );
}
// 1. Look for visitElementClassName() in the current class
// 2. Look for visitElementClassName() in superclasses
// 3. Look for visitElementClassName() in interfaces
// 4. Look for visitObject() in current class
protected Method getMethod( Class c ) {
Class newc = c;
Method m = null;
while (m == null && newc != Object.class) {
String method = newc.getName();
method = "visit" + method.substring( method.lastIndexOf('.') + 1 );
try {
m = getClass().getMethod( method, new Class[] { newc } );
} catch (NoSuchMethodException ex) {
newc = newc.getSuperclass();
} }
if (newc == Object.class) {
// System.out.println( "Searching for interfaces" );
Class[] interfaces = c.getInterfaces();
for (int i=0; i < interfaces.length; i++) {
String method = interfaces[i].getName();
method = "visit" + method.substring( method.lastIndexOf('.') + 1 );
try {
m = getClass().getMethod( method, new Class[] { interfaces[i] } );
} catch (NoSuchMethodException ex) { }
} }
if (m == null)
try {
m = getClass().getMethod( "visitObject", new Class[] { Object.class } );
} catch (Exception ex) { }
return m;
} }
class UpVisitor extends ReflectiveVisitor {
public void visit( Object o ) {
try {
getMethod( o.getClass() ).invoke( this, new Object[] { o } );
} catch (Exception ex) {
System.out.println( "UpVisitor - no appropriate visit() method" );
} }
public void visitThis( This e ) {
System.out.println( "UpVisitor: do Up on " + e.thiss() );
}
public void visitObject( Object e ) {
System.out.println( "UpVisitor: generic visitObject() method" );
} }
class DownVisitor extends ReflectiveVisitor {
public void visit( Object o ) {
try {
getMethod( o.getClass() ).invoke( this, new Object[] { o } );
} catch (Exception ex) {
System.out.println( "DownVisitor - no appropriate visit() method" );
} }
public void visitThat( That e ) {
System.out.println( "DownVisitor: do Down on " + e.that() );
} }
class VisitorDemo {
public static void main( String[] args ) {
Element[] list = { new This(), new That(), new TheOther() };
UpVisitor up = new UpVisitor();
DownVisitor down = new DownVisitor();
for (int i=0; i < list.length; i++)
list[i].accept( up );
for (int i=0; i < list.length; i++)
list[i].accept( down );
} }
UpVisitor: do Up on This
UpVisitor: generic visitObject() method
ReflectiveVisitor: do Base on TheOther
DownVisitor - no appropriate visit() method
DownVisitor: do Down on That
ReflectiveVisitor: do Base on TheOther
