java/src/main/com/google/appengine/api/datastore/AbstractIterator.java

   1 // Copyright 2009 Google Inc. All Rights Reserved.
   2 package com.google.appengine.api.datastore;
   3
   4 import java.util.Iterator;
   5 import java.util.NoSuchElementException;
   6
   7 /**
   8  * @see com.google.common.collect.AbstractIterator
   9  */
  10 abstract class AbstractIterator<T> implements Iterator<T> {
  11   private State state = State.NOT_READY;
  12
  13   enum State {
  14     READY,
  15     NOT_READY,
  16     DONE,
  17     FAILED,
  18   }
  19
  20   private T next;
  21
  22   protected abstract T computeNext();
  23
  24   protected final T endOfData() {
  25     state = State.DONE;
  26     return null;
  27   }
  28
  29   @Override
  30   public final boolean hasNext() {
  31     if (state == State.FAILED) {
  32       throw new IllegalStateException();
  33     }
  34     switch (state) {
  35       case DONE:
  36         return false;
  37       case READY:
  38         return true;
  39       default:
  40     }
  41     return tryToComputeNext();
  42   }
  43
  44   private boolean tryToComputeNext() {
  45     state = State.FAILED;
  46     next = computeNext();
  47     if (state != State.DONE) {
  48       state = State.READY;
  49       return true;
  50     }
  51     return false;
  52   }
  53
  54   @Override
  55   public final T next() {
  56     if (!hasNext()) {
  57       throw new NoSuchElementException();
  58     }
  59     state = State.NOT_READY;
  60     return next;
  61   }
  62
  63   @Override
  64   public void remove() {
  65     throw new UnsupportedOperationException();
  66   }
  67 }