Visitor_Pattern_Exemple

package com.rac021.visitor ;

/**
 *
 * @author ryahiaoui
 */

import java.util.HashMap ;
import java.util.function.Function ;


public interface Visitor {
    
  public interface Human {   }
  public class     Child implements Human {  }
  public class     Adult implements Human {  }
  
  public class Applier<R> {
      
    private final HashMap< Class<?>, Function<Object, R>> map = new HashMap<>() ;
    
    public <T> Applier<R> when(Class<T> type, Function< T, R > function ) {
      map.put(type, obj -> function.apply(type.cast(obj))) ;
      return this ;
    }
    public R call(Object receiver) {
      return map.getOrDefault(receiver.getClass(),
              obj -> { throw new IllegalArgumentException("Error " + obj); })
          .apply(receiver) ;
    }
  }

  public static void main(String[] args) {
      
    Applier<String> applier = new Applier<>() ;
    applier.when(Child.class, child -> " I'm a Child")
           .when(Adult.class, adult -> "I'm an Adult") ;

    Human human = new Child()         ;
    String who  = applier.call(human) ; 
    System.out.println(who)           ;
    
  }
}