/*
* 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.atlas.iterator;
import java.util.Iterator ;
import java.util.NoSuchElementException ;
import java.util.Queue ;
/** PeekIterator - is one slot ahead from the wrapped iterator */
public class PeekIterator<T> implements Iterator<T>
{
private final Iterator<T> iter ;
private boolean finished = false ;
// Slot always full when iterator active. Null is a valid element.
private T slot ;
public static <T> PeekIterator<T> create(PeekIterator<T> iter) { return iter ; }
public static <T> PeekIterator<T> create(Iterator<T> iter)
{
if ( iter instanceof PeekIterator<?> )
return (PeekIterator<T>)iter ;
return new PeekIterator<>(iter) ;
}
public PeekIterator(Iterator<T> iter)
{
this.iter = iter ;
fill() ;
}
private void fill()
{
if ( finished ) return ;
if ( iter.hasNext() )
slot = iter.next();
else
{
finished = true ;
slot = null ;
}
}
@Override
public boolean hasNext()
{
if ( finished )
return false ;
return true ;
}
/** Peek the next element or return null
* @see Queue#peek
*/
public T peek()
{
if ( finished )
return null ;
return slot ;
}
/** Peek the next element or throw NoSuchElementException */
public T element()
{
if ( finished )
throw new NoSuchElementException() ;
return slot ;
}
@Override
public T next()
{
if ( finished )
throw new NoSuchElementException() ;
T x = slot ;
// Move on now so the slot is loaded for peek.
fill() ;
return x ;
}
@Override
public void remove()
{ throw new UnsupportedOperationException() ; }
}