gistfile1.java

import java.util.ArrayList;
import java.util.List;


public class HandlerChain<IN, OUT> {
  private List<Handler<?, ?>> handlers = new ArrayList<Handler<?, ?>>();

  private HandlerChain() {
  }

  private static <IN, OUT> HandlerChain<IN, OUT> create() {
    return new HandlerChain<IN, OUT>();
  }

  public static <IN, OUT> HandlerChain<IN, OUT> create(Handler<IN, OUT> h) {
    HandlerChain<IN, OUT> hc = create();
    hc.handlers.add(h);
    return hc;
  }

  private static <IN, T, OUT> HandlerChain<IN, OUT> create(HandlerChain<IN, T> hc, Handler<T, OUT> h) {
    HandlerChain<IN, OUT> result = new HandlerChain<IN, OUT>();
    result.handlers.addAll(hc.handlers);
    result.handlers.add(h);
    return result;
  }

  public <T> HandlerChain<IN, T> add(Handler<OUT, T> h) {
    return create(this, h);
  }

  public OUT process(IN in) {
    Object o = in;
    for (Handler<?, ?> h : handlers) {
      o = handle(h, o);
    }
    return cast(o);
  }

  @SuppressWarnings({ "unchecked", "rawtypes" })
  private Object handle(Handler h, Object in) {
    return h.handle(in);
  }
  
  @SuppressWarnings("unchecked")
  private <T> T cast(Object o) {
    return (T) o;
  }

}