| /* |
| * Licensed to the Apache Software Foundation (ASF) under one |
| * or more contributor license agreements. See the NOTICE file |
| * distributed with this work for additional information |
| * regarding copyright ownership. The ASF licenses this file |
| * to you 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 org.apache.beam.runners.core; |
| |
| import static org.apache.beam.vendor.guava.v20_0.com.google.common.base.Preconditions.checkNotNull; |
| import static org.apache.beam.vendor.guava.v20_0.com.google.common.base.Preconditions.checkState; |
| |
| import java.util.NoSuchElementException; |
| import javax.annotation.Nullable; |
| import org.apache.beam.sdk.util.common.Reiterator; |
| |
| /** |
| * A {@link Reiterator} that supports one-element lookahead during iteration. |
| * |
| * @param <T> the type of elements returned by this iterator |
| */ |
| public final class PeekingReiterator<T> implements Reiterator<T> { |
| private @Nullable T nextElement; |
| private boolean nextElementComputed; |
| private final Reiterator<T> iterator; |
| |
| public PeekingReiterator(Reiterator<T> iterator) { |
| this.iterator = checkNotNull(iterator); |
| } |
| |
| PeekingReiterator(PeekingReiterator<T> it) { |
| this.iterator = checkNotNull(checkNotNull(it).iterator.copy()); |
| this.nextElement = it.nextElement; |
| this.nextElementComputed = it.nextElementComputed; |
| } |
| |
| @Override |
| public boolean hasNext() { |
| computeNext(); |
| return nextElementComputed; |
| } |
| |
| @Override |
| public T next() { |
| T result = peek(); |
| nextElementComputed = false; |
| return result; |
| } |
| |
| /** |
| * {@inheritDoc} |
| * |
| * <p>If {@link #peek} is called, {@code remove} is disallowed until {@link #next} has been |
| * subsequently called. |
| */ |
| @Override |
| public void remove() { |
| checkState(!nextElementComputed, "After peek(), remove() is disallowed until next() is called"); |
| iterator.remove(); |
| } |
| |
| @Override |
| public PeekingReiterator<T> copy() { |
| return new PeekingReiterator<>(this); |
| } |
| |
| /** |
| * Returns the element that would be returned by {@link #next}, without actually consuming the |
| * element. |
| * |
| * @throws NoSuchElementException if there is no next element |
| */ |
| public T peek() { |
| computeNext(); |
| if (!nextElementComputed) { |
| throw new NoSuchElementException(); |
| } |
| return nextElement; |
| } |
| |
| private void computeNext() { |
| if (nextElementComputed) { |
| return; |
| } |
| if (!iterator.hasNext()) { |
| return; |
| } |
| nextElement = iterator.next(); |
| nextElementComputed = true; |
| } |
| } |