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/*
 * Copyright 2011,2012 Metamarkets Group Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.metamx.common.guava;

import java.io.Closeable;

/**
 * A Yielder is an object that tries to act like the yield() command/continuations in other languages.  It's not
 * necessarily good at this job, but it works.  I think.
 *
 * Essentially, you can think of a Yielder as a linked list of items where the Yielder gives you access to the current
 * head via get() and it will give you another Yielder representing the next item in the chain via next().  A Yielder
 * that isDone() may return anything from both get() and next(), there is no contract and depending on those return
 * values will likely lead to bugs.
 *
 * Once next is called, there is no guarantee and no requirement that references to old Yielder objects will continue
 * to obey the contract.
 *
 * Yielders are Closeable and *must* be closed in order to prevent resource leaks.  Once close() is called, the behavior
 * of the whole chain of Yielders is undefined.
 */
public interface Yielder extends Closeable
{
  /**
   * Gets the object currently held by this Yielder.  Can be called multiple times as long as next() is not called.
   *
   * Once next() is called on this Yielder object, all further operations on this object are undefined.
   *
   * @return the currently yielded object, null if done
   */
  public T get();

  /**
   * Gets the next Yielder in the chain. The argument is used as the accumulator value to pass along to start the
   * accumulation until the next yield() call or iteration completes.
   *
   * Once next() is called on this Yielder object, all further operations on this object are undefined.
   *
   * @param initValue the initial value to pass along to start the accumulation until the next yield() call or
   *                  iteration completes.
   * @return the next Yielder in the chain, or undefined if done
   */
  public Yielder next(T initValue);

  /**
   * Returns true if this is the last Yielder in the chain.  A Yielder that isDone() should have the final non-yielded
   * result from the accumulator (or the initValue to the previous next() call).  It will probably break your code
   * to call next() on a Yielder that is done and expect something good from it.
   *
   * Once next() is called on this Yielder object, all further operations on this object are undefined.
   *
   * @return true if this is the last Yielder in the chain, false otherwise
   */
  public boolean isDone();
}