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Commons Collections example source code file (FilterListIterator.java)

This example Commons Collections source code file (FilterListIterator.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Commons Collections tags/keywords

filterlistiterator, filterlistiterator, listiterator, nosuchelementexception, object, object, predicate, predicate, unsupportedoperationexception, unsupportedoperationexception, util

The Commons Collections FilterListIterator.java source code

/*
 *  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.commons.collections.iterators;

import java.util.ListIterator;
import java.util.NoSuchElementException;

import org.apache.commons.collections.Predicate;

/** 
 * Decorates another {@link ListIterator} using a predicate to filter elements.
 * <p>
 * This iterator decorates the underlying iterator, only allowing through
 * those elements that match the specified {@link Predicate Predicate}.
 *
 * @since Commons Collections 2.0
 * @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $
 * 
 * @author Rodney Waldhoff
 */
public class FilterListIterator implements ListIterator {

    /** The iterator being used */
    private ListIterator iterator;
    
    /** The predicate being used */
    private Predicate predicate;

    /** 
     * The value of the next (matching) object, when 
     * {@link #nextObjectSet} is true. 
     */
    private Object nextObject;

    /** 
     * Whether or not the {@link #nextObject} has been set
     * (possibly to <code>null). 
     */
    private boolean nextObjectSet = false;   

    /** 
     * The value of the previous (matching) object, when 
     * {@link #previousObjectSet} is true. 
     */
    private Object previousObject;

    /** 
     * Whether or not the {@link #previousObject} has been set
     * (possibly to <code>null). 
     */
    private boolean previousObjectSet = false;   

    /** 
     * The index of the element that would be returned by {@link #next}.
     */
    private int nextIndex = 0;
    
    //-----------------------------------------------------------------------
    /**
     * Constructs a new <code>FilterListIterator that will not function
     * until {@link #setListIterator(ListIterator) setListIterator}
     * and {@link #setPredicate(Predicate) setPredicate} are invoked.
     */
    public FilterListIterator() {
        super();
    }

    /**
     * Constructs a new <code>FilterListIterator that will not 
     * function until {@link #setPredicate(Predicate) setPredicate} is invoked.
     *
     * @param iterator  the iterator to use
     */
    public FilterListIterator(ListIterator iterator ) {
        super();
        this.iterator = iterator;
    }

    /**
     * Constructs a new <code>FilterListIterator.
     *
     * @param iterator  the iterator to use
     * @param predicate  the predicate to use
     */
    public FilterListIterator(ListIterator iterator, Predicate predicate) {
        super();
        this.iterator = iterator;
        this.predicate = predicate;
    }

    /**
     * Constructs a new <code>FilterListIterator that will not function
     * until {@link #setListIterator(ListIterator) setListIterator} is invoked.
     *
     * @param predicate  the predicate to use.
     */
    public FilterListIterator(Predicate predicate) {
        super();
        this.predicate = predicate;
    }

    //-----------------------------------------------------------------------
    /** Not supported. */
    public void add(Object o) {
        throw new UnsupportedOperationException("FilterListIterator.add(Object) is not supported.");
    }

    public boolean hasNext() {
        if(nextObjectSet) {
            return true;
        } else {
            return setNextObject();
        }
    }

    public boolean hasPrevious() {
        if(previousObjectSet) {
            return true;
        } else {
            return setPreviousObject();
        }
    }

    public Object next() {
        if(!nextObjectSet) {
            if(!setNextObject()) {
                throw new NoSuchElementException();
            }
        }
        nextIndex++;
        Object temp = nextObject;
        clearNextObject();
        return temp;
    }

    public int nextIndex() {
        return nextIndex;
    }

    public Object previous() {
        if(!previousObjectSet) {
            if(!setPreviousObject()) {
                throw new NoSuchElementException();
            }
        }
        nextIndex--;
        Object temp = previousObject;
        clearPreviousObject();
        return temp;
    }

    public int previousIndex() {
        return (nextIndex-1);
    }

    /** Not supported. */
    public void remove() {
        throw new UnsupportedOperationException("FilterListIterator.remove() is not supported.");
    }

    /** Not supported. */
    public void set(Object o) {
        throw new UnsupportedOperationException("FilterListIterator.set(Object) is not supported.");
    }

    //-----------------------------------------------------------------------
    /** 
     * Gets the iterator this iterator is using.
     * 
     * @return the iterator.
     */
    public ListIterator getListIterator() {
        return iterator;
    }

    /** 
     * Sets the iterator for this iterator to use.
     * If iteration has started, this effectively resets the iterator.
     * 
     * @param iterator  the iterator to use
     */
    public void setListIterator(ListIterator iterator) {
        this.iterator = iterator;
    }

    //-----------------------------------------------------------------------
    /** 
     * Gets the predicate this iterator is using.
     * 
     * @return the predicate.
     */
    public Predicate getPredicate() {
        return predicate;
    }

    /** 
     * Sets the predicate this the iterator to use.
     * 
     * @param predicate  the transformer to use
     */
    public void setPredicate(Predicate predicate) {
        this.predicate = predicate;
    }

    //-----------------------------------------------------------------------
    private void clearNextObject() {
        nextObject = null;
        nextObjectSet = false;
    }

    private boolean setNextObject() {
        // if previousObjectSet,
        // then we've walked back one step in the 
        // underlying list (due to a hasPrevious() call)
        // so skip ahead one matching object
        if(previousObjectSet) {
            clearPreviousObject();
            if(!setNextObject()) {
                return false;
            } else {
                clearNextObject();
            }
        }

        while(iterator.hasNext()) {
            Object object = iterator.next();
            if(predicate.evaluate(object)) {
                nextObject = object;
                nextObjectSet = true;
                return true;
            }
        }
        return false;
    }

    private void clearPreviousObject() {
        previousObject = null;
        previousObjectSet = false;
    }

    private boolean setPreviousObject() {
        // if nextObjectSet,
        // then we've walked back one step in the 
        // underlying list (due to a hasNext() call)
        // so skip ahead one matching object
        if(nextObjectSet) {
            clearNextObject();
            if(!setPreviousObject()) {
                return false;
            } else {
                clearPreviousObject();
            }
        }

        while(iterator.hasPrevious()) {
            Object object = iterator.previous();
            if(predicate.evaluate(object)) {
                previousObject = object;
                previousObjectSet = true;
                return true;
            }
        }
        return false;
    }

}

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