// onjava/TaskManager.java
// (c)2016 MindView LLC: see Copyright.txt
// We make no guarantees that this code is fit for any purpose.
// Visit http://mindviewinc.com/Books/OnJava/ for more book information.
// Managing and executing a queue of tasks.
package onjava;
import java.util.concurrent.*;
import java.util.*;

public class TaskManager<R,C extends Callable<R>>
extends ArrayList<TaskItem<R,C>> {
  private ExecutorService exec =
    Executors.newSingleThreadExecutor();
  public void add(C task) {
    add(new TaskItem<>(exec.submit(task),task));
  }
  public List<R> getResults() {
    Iterator<TaskItem<R,C>> items = iterator();
    List<R> results = new ArrayList<>();
    while(items.hasNext()) {
      TaskItem<R,C> item = items.next();
      if(item.future.isDone()) {
        try {
          results.add(item.future.get());
        } catch(InterruptedException |
                ExecutionException e) {
          throw new RuntimeException(e);
        }
        items.remove();
      }
    }
    return results;
  }
  public List<String> purge() {
    Iterator<TaskItem<R,C>> items = iterator();
    List<String> results = new ArrayList<>();
    while(items.hasNext()) {
      TaskItem<R,C> item = items.next();
      // Leave completed tasks for results reporting:
      if(!item.future.isDone()) {
        results.add("Cancelling " + item.task);
        item.future.cancel(true); // Can interrupt
        items.remove();
      }
    }
    return results;
  }
}