com.google.common.collect.AbstractSequentialIterator.java Source code

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Here is the source code for com.google.common.collect.AbstractSequentialIterator.java

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/*
 * Copyright (C) 2010 The Guava Authors
 *
 * 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.google.common.collect;

import com.google.common.annotations.GwtCompatible;

import java.util.NoSuchElementException;

import javax.annotation.Nullable;

/**
 * This class provides a skeletal implementation of the {@code Iterator}
 * interface for sequences whose next element can always be derived from the
 * previous element. Null elements are not supported, nor is the
 * {@link #remove()} method.
 *
 * <p>Example: <pre>   {@code
 *
 *   Iterator<Integer> powersOfTwo =
 *       new AbstractSequentialIterator<Integer>(1) {
 *         protected Integer computeNext(Integer previous) {
 *           return (previous == 1 << 30) ? null : previous * 2;
 *         }
 *       };}</pre>
 *
 * @author Chris Povirk
 * @since 12.0 (in Guava as {@code AbstractLinkedIterator} since 8.0)
 */
@GwtCompatible
public abstract class AbstractSequentialIterator<T> extends UnmodifiableIterator<T> {
    private T nextOrNull;

    /**
     * Creates a new iterator with the given first element, or, if {@code
     * firstOrNull} is null, creates a new empty iterator.
     */
    protected AbstractSequentialIterator(@Nullable T firstOrNull) {
        this.nextOrNull = firstOrNull;
    }

    /**
     * Returns the element that follows {@code previous}, or returns {@code null}
     * if no elements remain. This method is invoked during each call to
     * {@link #next()} in order to compute the result of a <i>future</i> call to
     * {@code next()}.
     */
    protected abstract T computeNext(T previous);

    @Override
    public final boolean hasNext() {
        return nextOrNull != null;
    }

    @Override
    public final T next() {
        if (!hasNext()) {
            throw new NoSuchElementException();
        }
        try {
            return nextOrNull;
        } finally {
            nextOrNull = computeNext(nextOrNull);
        }
    }
}