FilteringIterator.java

/*******************************************************************************
 * Copyright (c) 1998, 2015 Oracle and/or its affiliates. All rights reserved.
 * This program and the accompanying materials are made available under the
 * terms of the Eclipse Public License v1.0 and Eclipse Distribution License v. 1.0
 * which accompanies this distribution.
 * The Eclipse Public License is available at http://www.eclipse.org/legal/epl-v10.html
 * and the Eclipse Distribution License is available at
 * http://www.eclipse.org/org/documents/edl-v10.php.
 *
 * Contributors:
 *     Oracle - initial API and implementation from Oracle TopLink
******************************************************************************/
package org.eclipse.persistence.tools.workbench.utility.iterators;

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

import org.eclipse.persistence.tools.workbench.utility.ClassTools;
import org.eclipse.persistence.tools.workbench.utility.filters.Filter;


/**
 * A <code>FilteringIterator</code> wraps another <code>Iterator</code>
 * and uses a <code>Filter</code> to determine which elements in the
 * nested iterator are to be returned by calls to <code>next()</code>.
 * <p>
 * As an alternative to building a <code>Filter</code>, a subclass
 * of <code>FilteringIterator</code> can override the
 * <code>accept(Object)</code> method.
 * <p>
 * One, possibly undesirable, side-effect of using this iterator is that
 * the nested iterator's <code>next()</code> method will be invoked
 * <em>before</em> the filtered iterator's <code>next()</code>
 * method is invoked. This is because the "next" element must be
 * checked for whether it is to be accepted before the filtered iterator
 * can determine whether it has a "next" element (i.e. that the
 * <code>hasNext()</code> method should return <code>true</code>).
 * This also prevents a filtered iterator from supporting the optional
 * <code>remove()</code> method.
 */
public class FilteringIterator implements Iterator {
    private Iterator nestedIterator;
    private Filter filter;
    private Object next;

    private static final Object START = new Object();
    private static final Object END = new Object();

    /**
     * Construct an iterator with the specified nested
     * iterator and a filter that simply accepts every object.
     * Use this constructor if you want to override the
     * <code>accept(Object)</code> method instead of building
     * a <code>Filter</code>.
     */
    public FilteringIterator(Iterator nestedIterator) {
        this(nestedIterator, Filter.NULL_INSTANCE);
    }

    /**
     * Construct an iterator with the specified nested
     * iterator and filter.
     */
    public FilteringIterator(Iterator nestedIterator, Filter filter) {
        super();
        this.nestedIterator = nestedIterator;
        this.filter = filter;
        // postpone the pre-load until after the constructor has finished executing
        this.next = START;
    }

    /**
     * @see java.util.Iterator#hasNext()
     */
    public boolean hasNext() {
        if (this.next == START) {
            this.loadNext();
        }
        return this.next != END;
    }

    /**
     * @see java.util.Iterator#next()
     */
    public Object next() {
        if (this.next == START) {
            this.loadNext();
        }
        if (this.next == END) {
            throw new NoSuchElementException();
        }
        Object result = this.next;
        this.loadNext();
        return result;
    }

    /**
     * Because we need to pre-load the next element
     * to be returned, we cannot support the <code>remove()</code>
     * method.
     * @see java.util.Iterator#remove()
     */
    public void remove() {
        throw new UnsupportedOperationException();
    }

    /**
     * Load next with the next valid entry from the nested
     * iterator. If there are none, next is set to <code>END</code>.
     */
    private void loadNext() {
        this.next = END;
        while (this.nestedIterator.hasNext() && (this.next == END)) {
            this.next = this.nestedIterator.next();
            if ( ! this.accept(this.next)) {
                this.next = END;
            }
        }
    }

    /**
     * Return whether the <code>FilteringIterator</code>
     * should return the specified next element from a call to the
     * <code>next()</code> method.
     * <p>
     * This method can be overridden by a subclass as an
     * alternative to building a <code>Filter</code>.
     */
    protected boolean accept(Object o) {
        return this.filter.accept(o);
    }

    /**
     * @see java.lang.Object#toString()
     */
    public String toString() {
        return ClassTools.shortClassNameForObject(this) + '(' + this.nestedIterator + ')';
    }

}