stack/core/src/main/java/org/usergrid/persistence/query/util/PeekingIterator.java

package org.usergrid.persistence.query.util;


import java.util.Iterator;


/**
 * A simple iterator that allows us to "peek" to the next value without actually popping it.
 * <p/>
 * Meant as a wrapper to an existing iterator
 */
public class PeekingIterator<T> implements Iterable<T>, Iterator<T> {

    private Iterator<T> delegate;
    private T peeked;


    public PeekingIterator( Iterator<T> delegate ) {
        this.delegate = delegate;
    }


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


    @Override
    public boolean hasNext() {
        return peeked != null || delegate.hasNext();
    }


    @Override
    public T next() {
        T toReturn = null;

        if ( peeked != null ) {
            toReturn = peeked;
            peeked = null;
        }
        else {
            toReturn = delegate.next();
        }

        return toReturn;
    }


    /** Peek ahead in the iterator.  Assumes a next is present and has been checked */
    public T peek() {
        if ( peeked == null && delegate.hasNext() ) {
            peeked = delegate.next();
        }
        return peeked;
    }


    @Override
    public void remove() {
        throw new UnsupportedOperationException( "Remove is unsupported" );
    }
}