An Iterator that wraps a number of Iterators

    
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import java.util.Iterator;
import java.util.List;

/**
 * An JoinedIterator is an Iterator that wraps a number of Iterators.
 *
 * This class makes multiple iterators look like one to the caller.
 * When any method from the Iterator interface is called, the JoinedIterator
 * will delegate to a single underlying Iterator. The JoinedIterator will
 * invoke the Iterators in sequence until all Iterators are exhausted.
 *
 */
public class JoinedIterator implements Iterator {

  private static final Iterator[] ITERATORS = {};

  // wrapped iterators
  private Iterator[] iterators;

  // index of current iterator in the wrapped iterators array
  private int currentIteratorIndex;

  // the current iterator
  private Iterator currentIterator;

  // the last used iterator
  private Iterator lastUsedIterator;

  public JoinedIterator(List iterators) {
    this( (Iterator[]) iterators.toArray(ITERATORS) );
  }

  public JoinedIterator(Iterator[] iterators) {
    if( iterators==null )
      throw new NullPointerException("Unexpected NULL iterators argument");
    this.iterators = iterators;
  }

  public JoinedIterator(Iterator first, Iterator second) {
    this( new Iterator[] { first, second } );
  }

  public boolean hasNext() {
    updateCurrentIterator();
    return currentIterator.hasNext();
  }

  public Object next() {
    updateCurrentIterator();
    return currentIterator.next();
  }

  public void remove() {
    updateCurrentIterator();
    lastUsedIterator.remove();
  }


  // call this before any Iterator method to make sure that the current Iterator
  // is not exhausted
  protected void updateCurrentIterator() {

    if (currentIterator == null) {
      if( iterators.length==0  ) {
        currentIterator = EmptyIterator.emptyIterator;
      }
      else {
        currentIterator = iterators[0];
      }
      // set last used iterator here, in case the user calls remove
      // before calling hasNext() or next() (although they shouldn't)
      lastUsedIterator = currentIterator;
    }

    while (! currentIterator.hasNext() && currentIteratorIndex < iterators.length - 1) {
      currentIteratorIndex++;
      currentIterator = iterators[currentIteratorIndex];
    }
  }

}
/*   Copyright 2004 BEA Systems, Inc.
*
*   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.
*/


class EmptyIterator implements java.util.Iterator {
 public static final EmptyIterator emptyIterator = new EmptyIterator();
 public boolean hasNext() { return false; }
 public Object next() { return null; }
 public void remove() {
   throw new UnsupportedOperationException();
 }
}

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An Iterator that wraps a number of Iterators : Iterator « Collections Data Structure « Java

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