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Commons Collections example source code file (SingletonListIterator.java)

This example Commons Collections source code file (SingletonListIterator.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

illegalstateexception, illegalstateexception, listiterator, nosuchelementexception, nosuchelementexception, object, object, resettablelistiterator, singletonlistiterator, singletonlistiterator, unsupportedoperationexception, util

The Commons Collections SingletonListIterator.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.ResettableListIterator;

/**
 * <code>SingletonIterator is an {@link ListIterator} over a single 
 * object instance.
 *
 * @since Commons Collections 2.1
 * @version $Revision: 646777 $ $Date: 2008-04-10 13:33:15 +0100 (Thu, 10 Apr 2008) $
 * 
 * @author Stephen Colebourne
 * @author Rodney Waldhoff
 */
public class SingletonListIterator implements ListIterator, ResettableListIterator {

    private boolean beforeFirst = true;
    private boolean nextCalled = false;
    private boolean removed = false;
    private Object object;

    /**
     * Constructs a new <code>SingletonListIterator.
     *
     * @param object  the single object to return from the iterator
     */
    public SingletonListIterator(Object object) {
        super();
        this.object = object;
    }

    /**
     * Is another object available from the iterator?
     * <p>
     * This returns true if the single object hasn't been returned yet.
     * 
     * @return true if the single object hasn't been returned yet
     */
    public boolean hasNext() {
        return beforeFirst && !removed;
    }

    /**
     * Is a previous object available from the iterator?
     * <p>
     * This returns true if the single object has been returned.
     * 
     * @return true if the single object has been returned
     */
    public boolean hasPrevious() {
        return !beforeFirst && !removed;
    }

    /**
     * Returns the index of the element that would be returned by a subsequent
     * call to <tt>next.
     *
     * @return 0 or 1 depending on current state. 
     */
    public int nextIndex() {
        return (beforeFirst ? 0 : 1);
    }

    /**
     * Returns the index of the element that would be returned by a subsequent
     * call to <tt>previous. A return value of -1 indicates that the iterator is currently at
     * the start.
     *
     * @return 0 or -1 depending on current state. 
     */
    public int previousIndex() {
        return (beforeFirst ? -1 : 0);
    }

    /**
     * Get the next object from the iterator.
     * <p>
     * This returns the single object if it hasn't been returned yet.
     *
     * @return the single object
     * @throws NoSuchElementException if the single object has already 
     *    been returned
     */
    public Object next() {
        if (!beforeFirst || removed) {
            throw new NoSuchElementException();
        }
        beforeFirst = false;
        nextCalled = true;
        return object;
    }

    /**
     * Get the previous object from the iterator.
     * <p>
     * This returns the single object if it has been returned.
     *
     * @return the single object
     * @throws NoSuchElementException if the single object has not already 
     *    been returned
     */
    public Object previous() {
        if (beforeFirst || removed) {
            throw new NoSuchElementException();
        }
        beforeFirst = true;
        return object;
    }

    /**
     * Remove the object from this iterator.
     * @throws IllegalStateException if the <tt>next or previous 
     *        method has not yet been called, or the <tt>remove method 
     *        has already been called after the last call to <tt>next
     *        or <tt>previous.
     */
    public void remove() {
        if(!nextCalled || removed) {
            throw new IllegalStateException();
        } else {
            object = null;
            removed = true;
        }
    }
    
    /**
     * Add always throws {@link UnsupportedOperationException}.
     *
     * @throws UnsupportedOperationException always
     */
    public void add(Object obj) {
        throw new UnsupportedOperationException("add() is not supported by this iterator");
    }
    
    /**
     * Set sets the value of the singleton.
     *
     * @param obj  the object to set
     * @throws IllegalStateException if <tt>next has not been called 
     *          or the object has been removed
     */
    public void set(Object obj) {
        if (!nextCalled || removed) {
            throw new IllegalStateException();
        }
        this.object = obj;
    }
    
    /**
     * Reset the iterator back to the start.
     */
    public void reset() {
        beforeFirst = true;
        nextCalled = false;
    }
    
}

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