flink-runtime/src/main/java/org/apache/flink/runtime/util/EmptyIterator.java

/*
 * 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.flink.runtime.util;

import java.util.Iterator;
import java.util.NoSuchElementException;

/** An empty iterator that never returns anything. */
public final class EmptyIterator<E> implements Iterator<E>, Iterable<E> {

    /** The singleton instance. */
    private static final EmptyIterator<Object> INSTANCE = new EmptyIterator<Object>();

    /**
     * Gets a singleton instance of the empty iterator.
     *
     * @param <E> The type of the objects (not) returned by the iterator.
     * @return An instance of the iterator.
     */
    public static <E> EmptyIterator<E> get() {
        @SuppressWarnings("unchecked")
        EmptyIterator<E> iter = (EmptyIterator<E>) INSTANCE;
        return iter;
    }

    /**
     * Always returns false, since this iterator is empty.
     *
     * @see java.util.Iterator#hasNext()
     */
    @Override
    public boolean hasNext() {
        return false;
    }

    /**
     * Always throws a {@link java.util.NoSuchElementException}.
     *
     * @see java.util.Iterator#next()
     */
    @Override
    public E next() {
        throw new NoSuchElementException();
    }

    /**
     * Throws a {@link java.lang.UnsupportedOperationException}.
     *
     * @see java.util.Iterator#remove()
     */
    @Override
    public void remove() {
        throw new UnsupportedOperationException();
    }

    @Override
    public Iterator<E> iterator() {
        return this;
    }
}