jena-core/src/main/java/org/apache/jena/util/iterator/LazyIterator.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.jena.util.iterator;

import java.util.function.Consumer;

/** An ExtendedIterator that is created lazily.
 * This is useful when constructing an iterator is expensive and 
 * you'd prefer to delay doing it until certain it's actually needed.
 * For example, if you have <code>iterator1.andThen(iterator2)</code>
 * you could implement iterator2 as a LazyIterator.  
 * The sequence to be defined is defined by the subclass's definition 
 * of {@link #create()}.  That is called exactly once on the first attempt 
 * to iterate (i.e. use one of the <code>hasNext</code>, <code>next</code>,
 * <code>remove</code>, <code>removeNext</code> operations,
 *  maybe indirectly via <code>toList</code>).
 */
abstract public class LazyIterator<T> extends NiceIterator<T> {

	private ExtendedIterator<T> it = null;

	/** An ExtendedIterator that is created lazily. 
	 * This constructor has very low overhead - the real work is 
	 * delayed until the first attempt to use the iterator.
	 */
	public LazyIterator() {
	}

	@Override
    public boolean hasNext() {
		lazy();
		return it.hasNext();
	}

	@Override
    public T next() {
		lazy();
		return it.next();
	}

	@Override
	public void forEachRemaining(Consumer<? super T> action) {
		lazy();
		it.forEachRemaining(action);
	}

	@Override
    public void remove() {
		lazy();
		it.remove();
	}

	// removeNext() is implemented with next() and remove() so lazy is called.
        
    @Override
    public void close() {
        if ( it != null )
            it.close() ;
    }    
    
	private void lazy() {
		if (it == null)
			it = create();
	}

	/** The subclass must define this to return
	 * the ExtendedIterator to invoke. This method will be
	 * called at most once, on the first attempt to
	 * use the iterator.
	 * From then on, all calls to this will be passed
	 * through to the returned Iterator.
	 * @return The parent iterator defining the sequence.
	 */
	public abstract ExtendedIterator<T> create();

}