java/util/AbstractCollection.java

/*
 * %W% %E%
 *
 * Copyright 2006 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package java.util;

/**
 * This class provides a skeletal implementation of the <tt>Collection</tt>
 * interface, to minimize the effort required to implement this interface. <p>
 *
 * To implement an unmodifiable collection, the programmer needs only to
 * extend this class and provide implementations for the <tt>iterator</tt> and
 * <tt>size</tt> methods.  (The iterator returned by the <tt>iterator</tt>
 * method must implement <tt>hasNext</tt> and <tt>next</tt>.)<p>
 *
 * To implement a modifiable collection, the programmer must additionally
 * override this class's <tt>add</tt> method (which otherwise throws an
 * <tt>UnsupportedOperationException</tt>), and the iterator returned by the
 * <tt>iterator</tt> method must additionally implement its <tt>remove</tt>
 * method.<p>
 *
 * The programmer should generally provide a void (no argument) and
 * <tt>Collection</tt> constructor, as per the recommendation in the
 * <tt>Collection</tt> interface specification.<p>
 *
 * The documentation for each non-abstract method in this class describes its
 * implementation in detail.  Each of these methods may be overridden if
 * the collection being implemented admits a more efficient implementation.<p>
 *
 * This class is a member of the
 * <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @author  Josh Bloch
 * @author  Neal Gafter
 * @version %I%, %G%
 * @see Collection
 * @since 1.2
 */

public abstract class AbstractCollection<E> implements Collection<E> {
    /**
     * Sole constructor.  (For invocation by subclass constructors, typically
     * implicit.)
     */
    protected AbstractCollection() {
    }

    // Query Operations

    /**
     * Returns an iterator over the elements contained in this collection.
     *
     * @return an iterator over the elements contained in this collection
     */
    public abstract Iterator<E> iterator();

    public abstract int size();

    /**
     * {@inheritDoc}
     *
     * <p>This implementation returns <tt>size() == 0</tt>.
     */
    public boolean isEmpty() {
        return size() == 0;
    }

    /**
     * {@inheritDoc}
     *
     * <p>This implementation iterates over the elements in the collection,
     * checking each element in turn for equality with the specified element.
     *
     * @throws ClassCastException   {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     */
    public boolean contains(Object o) {
        Iterator<E> e = iterator();
        if (o==null) {
            while (e.hasNext())
                if (e.next()==null)
                    return true;
        } else {
            while (e.hasNext())
                if (o.equals(e.next()))
                    return true;
        }
        return false;
    }

    /**
     * {@inheritDoc}
     *
     * <p>This implementation returns an array containing all the elements
     * returned by this collection's iterator, in the same order, stored in
     * consecutive elements of the array, starting with index {@code 0}.
     * The length of the returned array is equal to the number of elements
     * returned by the iterator, even if the size of this collection changes
     * during iteration, as might happen if the collection permits
     * concurrent modification during iteration.  The {@code size} method is
     * called only as an optimization hint; the correct result is returned
     * even if the iterator returns a different number of elements.
     *
     * <p>This method is equivalent to:
     *
     *  <pre> {@code
     * List<E> list = new ArrayList<E>(size());
     * for (E e : this)
     *     list.add(e);
     * return list.toArray();
     * }</pre>
     */
    public Object[] toArray() {
        // Estimate size of array; be prepared to see more or fewer elements
        Object[] r = new Object[size()];
        Iterator<E> it = iterator();
        for (int i = 0; i < r.length; i++) {
            if (! it.hasNext()) // fewer elements than expected
                return Arrays.copyOf(r, i);
            r[i] = it.next();
        }
        return it.hasNext() ? finishToArray(r, it) : r;
    }

    /**
     * {@inheritDoc}
     *
     * <p>This implementation returns an array containing all the elements
     * returned by this collection's iterator in the same order, stored in
     * consecutive elements of the array, starting with index {@code 0}.
     * If the number of elements returned by the iterator is too large to
     * fit into the specified array, then the elements are returned in a
     * newly allocated array with length equal to the number of elements
     * returned by the iterator, even if the size of this collection
     * changes during iteration, as might happen if the collection permits
     * concurrent modification during iteration.  The {@code size} method is
     * called only as an optimization hint; the correct result is returned
     * even if the iterator returns a different number of elements.
     *
     * <p>This method is equivalent to:
     *
     *  <pre> {@code
     * List<E> list = new ArrayList<E>(size());
     * for (E e : this)
     *     list.add(e);
     * return list.toArray(a);
     * }</pre>
     *
     * @throws ArrayStoreException  {@inheritDoc}
     * @throws NullPointerException {@inheritDoc}
     */
    public <T> T[] toArray(T[] a) {
        // Estimate size of array; be prepared to see more or fewer elements
        int size = size();
        T[] r = a.length >= size ? a :
                  (T[])java.lang.reflect.Array
                  .newInstance(a.getClass().getComponentType(), size);
        Iterator<E> it = iterator();

        for (int i = 0; i < r.length; i++) {
            if (! it.hasNext()) { // fewer elements than expected
                if (a != r)
                    return Arrays.copyOf(r, i);
                r[i] = null; // null-terminate
                return r;
            }
            r[i] = (T)it.next();
        }
        return it.hasNext() ? finishToArray(r, it) : r;
    }

    /**
     * Reallocates the array being used within toArray when the iterator
     * returned more elements than expected, and finishes filling it from
     * the iterator.
     *
     * @param r the array, replete with previously stored elements
     * @param it the in-progress iterator over this collection
     * @return array containing the elements in the given array, plus any
     *         further elements returned by the iterator, trimmed to size
     */
    private static <T> T[] finishToArray(T[] r, Iterator<?> it) {
        int i = r.length;
        while (it.hasNext()) {
            int cap = r.length;
            if (i == cap) {
                int newCap = ((cap / 2) + 1) * 3;
                if (newCap <= cap) { // integer overflow
                    if (cap == Integer.MAX_VALUE)
                        throw new OutOfMemoryError
                            ("Required array size too large");
                    newCap = Integer.MAX_VALUE;
                }
                r = Arrays.copyOf(r, newCap);
            }
            r[i++] = (T)it.next();
        }
        // trim if overallocated
        return (i == r.length) ? r : Arrays.copyOf(r, i);
    }

    // Modification Operations

    /**
     * {@inheritDoc}
     *
     * <p>This implementation always throws an
     * <tt>UnsupportedOperationException</tt>.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     * @throws IllegalStateException         {@inheritDoc}
     */
    public boolean add(E e) {
        throw new UnsupportedOperationException();
    }

    /**
     * {@inheritDoc}
     *
     * <p>This implementation iterates over the collection looking for the
     * specified element.  If it finds the element, it removes the element
     * from the collection using the iterator's remove method.
     *
     * <p>Note that this implementation throws an
     * <tt>UnsupportedOperationException</tt> if the iterator returned by this
     * collection's iterator method does not implement the <tt>remove</tt>
     * method and this collection contains the specified object.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     */
    public boolean remove(Object o) {
        Iterator<E> e = iterator();
        if (o==null) {
            while (e.hasNext()) {
                if (e.next()==null) {
                    e.remove();
                    return true;
                }
            }
        } else {
            while (e.hasNext()) {
                if (o.equals(e.next())) {
                    e.remove();
                    return true;
                }
            }
        }
        return false;
    }


    // Bulk Operations

    /**
     * {@inheritDoc}
     *
     * <p>This implementation iterates over the specified collection,
     * checking each element returned by the iterator in turn to see
     * if it's contained in this collection.  If all elements are so
     * contained <tt>true</tt> is returned, otherwise <tt>false</tt>.
     *
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @see #contains(Object)
     */
    public boolean containsAll(Collection<?> c) {
        Iterator<?> e = c.iterator();
        while (e.hasNext())
            if (!contains(e.next()))
                return false;
        return true;
    }

    /**
     * {@inheritDoc}
     *
     * <p>This implementation iterates over the specified collection, and adds
     * each object returned by the iterator to this collection, in turn.
     *
     * <p>Note that this implementation will throw an
     * <tt>UnsupportedOperationException</tt> unless <tt>add</tt> is
     * overridden (assuming the specified collection is non-empty).
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     * @throws IllegalArgumentException      {@inheritDoc}
     * @throws IllegalStateException         {@inheritDoc}
     *
     * @see #add(Object)
     */
    public boolean addAll(Collection<? extends E> c) {
        boolean modified = false;
        Iterator<? extends E> e = c.iterator();
        while (e.hasNext()) {
            if (add(e.next()))
                modified = true;
        }
        return modified;
    }

    /**
     * {@inheritDoc}
     *
     * <p>This implementation iterates over this collection, checking each
     * element returned by the iterator in turn to see if it's contained
     * in the specified collection.  If it's so contained, it's removed from
     * this collection with the iterator's <tt>remove</tt> method.
     *
     * <p>Note that this implementation will throw an
     * <tt>UnsupportedOperationException</tt> if the iterator returned by the
     * <tt>iterator</tt> method does not implement the <tt>remove</tt> method
     * and this collection contains one or more elements in common with the
     * specified collection.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     *
     * @see #remove(Object)
     * @see #contains(Object)
     */
    public boolean removeAll(Collection<?> c) {
        boolean modified = false;
        Iterator<?> e = iterator();
        while (e.hasNext()) {
            if (c.contains(e.next())) {
                e.remove();
                modified = true;
            }
        }
        return modified;
    }

    /**
     * {@inheritDoc}
     *
     * <p>This implementation iterates over this collection, checking each
     * element returned by the iterator in turn to see if it's contained
     * in the specified collection.  If it's not so contained, it's removed
     * from this collection with the iterator's <tt>remove</tt> method.
     *
     * <p>Note that this implementation will throw an
     * <tt>UnsupportedOperationException</tt> if the iterator returned by the
     * <tt>iterator</tt> method does not implement the <tt>remove</tt> method
     * and this collection contains one or more elements not present in the
     * specified collection.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     * @throws ClassCastException            {@inheritDoc}
     * @throws NullPointerException          {@inheritDoc}
     *
     * @see #remove(Object)
     * @see #contains(Object)
     */
    public boolean retainAll(Collection<?> c) {
        boolean modified = false;
        Iterator<E> e = iterator();
        while (e.hasNext()) {
            if (!c.contains(e.next())) {
                e.remove();
                modified = true;
            }
        }
        return modified;
    }

    /**
     * {@inheritDoc}
     *
     * <p>This implementation iterates over this collection, removing each
     * element using the <tt>Iterator.remove</tt> operation.  Most
     * implementations will probably choose to override this method for
     * efficiency.
     *
     * <p>Note that this implementation will throw an
     * <tt>UnsupportedOperationException</tt> if the iterator returned by this
     * collection's <tt>iterator</tt> method does not implement the
     * <tt>remove</tt> method and this collection is non-empty.
     *
     * @throws UnsupportedOperationException {@inheritDoc}
     */
    public void clear() {
        Iterator<E> e = iterator();
        while (e.hasNext()) {
            e.next();
            e.remove();
        }
    }


    //  String conversion

    /**
     * Returns a string representation of this collection.  The string
     * representation consists of a list of the collection's elements in the
     * order they are returned by its iterator, enclosed in square brackets
     * (<tt>"[]"</tt>).  Adjacent elements are separated by the characters
     * <tt>", "</tt> (comma and space).  Elements are converted to strings as
     * by {@link String#valueOf(Object)}.
     *
     * @return a string representation of this collection
     */
    public String toString() {
        Iterator<E> i = iterator();
        if (! i.hasNext())
            return "[]";

        StringBuilder sb = new StringBuilder();
        sb.append('[');
        for (;;) {
            E e = i.next();
            sb.append(e == this ? "(this Collection)" : e);
            if (! i.hasNext())
                return sb.append(']').toString();
            sb.append(", ");
        }
    }

}
Revision:	1.9
Committed:	Sun Jun 25 19:07:16 2006 UTC (17 years, 11 months ago) by jsr166
Branch:	MAIN
Changes since 1.8:	+70 -42 lines
Log Message:	6377302: AbstractCollection.toArray is not safe for concurrent collections
#	Content
1	/*
2	* %W% %E%
3	*
4	* Copyright 2006 Sun Microsystems, Inc. All rights reserved.
5	* SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
6	*/
7
8	package java.util;
9
10	/**
11	* This class provides a skeletal implementation of the <tt>Collection</tt>
12	* interface, to minimize the effort required to implement this interface. <p>
13	*
14	* To implement an unmodifiable collection, the programmer needs only to
15	* extend this class and provide implementations for the <tt>iterator</tt> and
16	* <tt>size</tt> methods. (The iterator returned by the <tt>iterator</tt>
17	* method must implement <tt>hasNext</tt> and <tt>next</tt>.)<p>
18	*
19	* To implement a modifiable collection, the programmer must additionally
20	* override this class's <tt>add</tt> method (which otherwise throws an
21	* <tt>UnsupportedOperationException</tt>), and the iterator returned by the
22	* <tt>iterator</tt> method must additionally implement its <tt>remove</tt>
23	* method.<p>
24	*
25	* The programmer should generally provide a void (no argument) and
26	* <tt>Collection</tt> constructor, as per the recommendation in the
27	* <tt>Collection</tt> interface specification.<p>
28	*
29	* The documentation for each non-abstract method in this class describes its
30	* implementation in detail. Each of these methods may be overridden if
31	* the collection being implemented admits a more efficient implementation.<p>
32	*
33	* This class is a member of the
34	* <a href="{@docRoot}/../technotes/guides/collections/index.html">
35	* Java Collections Framework</a>.
36	*
37	* @author Josh Bloch
38	* @author Neal Gafter
39	* @version %I%, %G%
40	* @see Collection
41	* @since 1.2
42	*/
43
44	public abstract class AbstractCollection<E> implements Collection<E> {
45	/**
46	* Sole constructor. (For invocation by subclass constructors, typically
47	* implicit.)
48	*/
49	protected AbstractCollection() {
50	}
51
52	// Query Operations
53
54	/**
55	* Returns an iterator over the elements contained in this collection.
56	*
57	* @return an iterator over the elements contained in this collection
58	*/
59	public abstract Iterator<E> iterator();
60
61	public abstract int size();
62
63	/**
64	* {@inheritDoc}
65	*
66	* <p>This implementation returns <tt>size() == 0</tt>.
67	*/
68	public boolean isEmpty() {
69	return size() == 0;
70	}
71
72	/**
73	* {@inheritDoc}
74	*
75	* <p>This implementation iterates over the elements in the collection,
76	* checking each element in turn for equality with the specified element.
77	*
78	* @throws ClassCastException {@inheritDoc}
79	* @throws NullPointerException {@inheritDoc}
80	*/
81	public boolean contains(Object o) {
82	Iterator<E> e = iterator();
83	if (o==null) {
84	while (e.hasNext())
85	if (e.next()==null)
86	return true;
87	} else {
88	while (e.hasNext())
89	if (o.equals(e.next()))
90	return true;
91	}
92	return false;
93	}
94
95	/**
96	* {@inheritDoc}
97	*
98	* <p>This implementation returns an array containing all the elements
99	* returned by this collection's iterator, in the same order, stored in
100	* consecutive elements of the array, starting with index {@code 0}.
101	* The length of the returned array is equal to the number of elements
102	* returned by the iterator, even if the size of this collection changes
103	* during iteration, as might happen if the collection permits
104	* concurrent modification during iteration. The {@code size} method is
105	* called only as an optimization hint; the correct result is returned
106	* even if the iterator returns a different number of elements.
107	*
108	* <p>This method is equivalent to:
109	*
110	* <pre> {@code
111	* List<E> list = new ArrayList<E>(size());
112	* for (E e : this)
113	* list.add(e);
114	* return list.toArray();
115	* }</pre>
116	*/
117	public Object[] toArray() {
118	// Estimate size of array; be prepared to see more or fewer elements
119	Object[] r = new Object[size()];
120	Iterator<E> it = iterator();
121	for (int i = 0; i < r.length; i++) {
122	if (! it.hasNext()) // fewer elements than expected
123	return Arrays.copyOf(r, i);
124	r[i] = it.next();
125	}
126	return it.hasNext() ? finishToArray(r, it) : r;
127	}
128
129	/**
130	* {@inheritDoc}
131	*
132	* <p>This implementation returns an array containing all the elements
133	* returned by this collection's iterator in the same order, stored in
134	* consecutive elements of the array, starting with index {@code 0}.
135	* If the number of elements returned by the iterator is too large to
136	* fit into the specified array, then the elements are returned in a
137	* newly allocated array with length equal to the number of elements
138	* returned by the iterator, even if the size of this collection
139	* changes during iteration, as might happen if the collection permits
140	* concurrent modification during iteration. The {@code size} method is
141	* called only as an optimization hint; the correct result is returned
142	* even if the iterator returns a different number of elements.
143	*
144	* <p>This method is equivalent to:
145	*
146	* <pre> {@code
147	* List<E> list = new ArrayList<E>(size());
148	* for (E e : this)
149	* list.add(e);
150	* return list.toArray(a);
151	* }</pre>
152	*
153	* @throws ArrayStoreException {@inheritDoc}
154	* @throws NullPointerException {@inheritDoc}
155	*/
156	public <T> T[] toArray(T[] a) {
157	// Estimate size of array; be prepared to see more or fewer elements
158	int size = size();
159	T[] r = a.length >= size ? a :
160	(T[])java.lang.reflect.Array
161	.newInstance(a.getClass().getComponentType(), size);
162	Iterator<E> it = iterator();
163
164	for (int i = 0; i < r.length; i++) {
165	if (! it.hasNext()) { // fewer elements than expected
166	if (a != r)
167	return Arrays.copyOf(r, i);
168	r[i] = null; // null-terminate
169	return r;
170	}
171	r[i] = (T)it.next();
172	}
173	return it.hasNext() ? finishToArray(r, it) : r;
174	}
175
176	/**
177	* Reallocates the array being used within toArray when the iterator
178	* returned more elements than expected, and finishes filling it from
179	* the iterator.
180	*
181	* @param r the array, replete with previously stored elements
182	* @param it the in-progress iterator over this collection
183	* @return array containing the elements in the given array, plus any
184	* further elements returned by the iterator, trimmed to size
185	*/
186	private static <T> T[] finishToArray(T[] r, Iterator<?> it) {
187	int i = r.length;
188	while (it.hasNext()) {
189	int cap = r.length;
190	if (i == cap) {
191	int newCap = ((cap / 2) + 1) * 3;
192	if (newCap <= cap) { // integer overflow
193	if (cap == Integer.MAX_VALUE)
194	throw new OutOfMemoryError
195	("Required array size too large");
196	newCap = Integer.MAX_VALUE;
197	}
198	r = Arrays.copyOf(r, newCap);
199	}
200	r[i++] = (T)it.next();
201	}
202	// trim if overallocated
203	return (i == r.length) ? r : Arrays.copyOf(r, i);
204	}
205
206	// Modification Operations
207
208	/**
209	* {@inheritDoc}
210	*
211	* <p>This implementation always throws an
212	* <tt>UnsupportedOperationException</tt>.
213	*
214	* @throws UnsupportedOperationException {@inheritDoc}
215	* @throws ClassCastException {@inheritDoc}
216	* @throws NullPointerException {@inheritDoc}
217	* @throws IllegalArgumentException {@inheritDoc}
218	* @throws IllegalStateException {@inheritDoc}
219	*/
220	public boolean add(E e) {
221	throw new UnsupportedOperationException();
222	}
223
224	/**
225	* {@inheritDoc}
226	*
227	* <p>This implementation iterates over the collection looking for the
228	* specified element. If it finds the element, it removes the element
229	* from the collection using the iterator's remove method.
230	*
231	* <p>Note that this implementation throws an
232	* <tt>UnsupportedOperationException</tt> if the iterator returned by this
233	* collection's iterator method does not implement the <tt>remove</tt>
234	* method and this collection contains the specified object.
235	*
236	* @throws UnsupportedOperationException {@inheritDoc}
237	* @throws ClassCastException {@inheritDoc}
238	* @throws NullPointerException {@inheritDoc}
239	*/
240	public boolean remove(Object o) {
241	Iterator<E> e = iterator();
242	if (o==null) {
243	while (e.hasNext()) {
244	if (e.next()==null) {
245	e.remove();
246	return true;
247	}
248	}
249	} else {
250	while (e.hasNext()) {
251	if (o.equals(e.next())) {
252	e.remove();
253	return true;
254	}
255	}
256	}
257	return false;
258	}
259
260
261	// Bulk Operations
262
263	/**
264	* {@inheritDoc}
265	*
266	* <p>This implementation iterates over the specified collection,
267	* checking each element returned by the iterator in turn to see
268	* if it's contained in this collection. If all elements are so
269	* contained <tt>true</tt> is returned, otherwise <tt>false</tt>.
270	*
271	* @throws ClassCastException {@inheritDoc}
272	* @throws NullPointerException {@inheritDoc}
273	* @see #contains(Object)
274	*/
275	public boolean containsAll(Collection<?> c) {
276	Iterator<?> e = c.iterator();
277	while (e.hasNext())
278	if (!contains(e.next()))
279	return false;
280	return true;
281	}
282
283	/**
284	* {@inheritDoc}
285	*
286	* <p>This implementation iterates over the specified collection, and adds
287	* each object returned by the iterator to this collection, in turn.
288	*
289	* <p>Note that this implementation will throw an
290	* <tt>UnsupportedOperationException</tt> unless <tt>add</tt> is
291	* overridden (assuming the specified collection is non-empty).
292	*
293	* @throws UnsupportedOperationException {@inheritDoc}
294	* @throws ClassCastException {@inheritDoc}
295	* @throws NullPointerException {@inheritDoc}
296	* @throws IllegalArgumentException {@inheritDoc}
297	* @throws IllegalStateException {@inheritDoc}
298	*
299	* @see #add(Object)
300	*/
301	public boolean addAll(Collection<? extends E> c) {
302	boolean modified = false;
303	Iterator<? extends E> e = c.iterator();
304	while (e.hasNext()) {
305	if (add(e.next()))
306	modified = true;
307	}
308	return modified;
309	}
310
311	/**
312	* {@inheritDoc}
313	*
314	* <p>This implementation iterates over this collection, checking each
315	* element returned by the iterator in turn to see if it's contained
316	* in the specified collection. If it's so contained, it's removed from
317	* this collection with the iterator's <tt>remove</tt> method.
318	*
319	* <p>Note that this implementation will throw an
320	* <tt>UnsupportedOperationException</tt> if the iterator returned by the
321	* <tt>iterator</tt> method does not implement the <tt>remove</tt> method
322	* and this collection contains one or more elements in common with the
323	* specified collection.
324	*
325	* @throws UnsupportedOperationException {@inheritDoc}
326	* @throws ClassCastException {@inheritDoc}
327	* @throws NullPointerException {@inheritDoc}
328	*
329	* @see #remove(Object)
330	* @see #contains(Object)
331	*/
332	public boolean removeAll(Collection<?> c) {
333	boolean modified = false;
334	Iterator<?> e = iterator();
335	while (e.hasNext()) {
336	if (c.contains(e.next())) {
337	e.remove();
338	modified = true;
339	}
340	}
341	return modified;
342	}
343
344	/**
345	* {@inheritDoc}
346	*
347	* <p>This implementation iterates over this collection, checking each
348	* element returned by the iterator in turn to see if it's contained
349	* in the specified collection. If it's not so contained, it's removed
350	* from this collection with the iterator's <tt>remove</tt> method.
351	*
352	* <p>Note that this implementation will throw an
353	* <tt>UnsupportedOperationException</tt> if the iterator returned by the
354	* <tt>iterator</tt> method does not implement the <tt>remove</tt> method
355	* and this collection contains one or more elements not present in the
356	* specified collection.
357	*
358	* @throws UnsupportedOperationException {@inheritDoc}
359	* @throws ClassCastException {@inheritDoc}
360	* @throws NullPointerException {@inheritDoc}
361	*
362	* @see #remove(Object)
363	* @see #contains(Object)
364	*/
365	public boolean retainAll(Collection<?> c) {
366	boolean modified = false;
367	Iterator<E> e = iterator();
368	while (e.hasNext()) {
369	if (!c.contains(e.next())) {
370	e.remove();
371	modified = true;
372	}
373	}
374	return modified;
375	}
376
377	/**
378	* {@inheritDoc}
379	*
380	* <p>This implementation iterates over this collection, removing each
381	* element using the <tt>Iterator.remove</tt> operation. Most
382	* implementations will probably choose to override this method for
383	* efficiency.
384	*
385	* <p>Note that this implementation will throw an
386	* <tt>UnsupportedOperationException</tt> if the iterator returned by this
387	* collection's <tt>iterator</tt> method does not implement the
388	* <tt>remove</tt> method and this collection is non-empty.
389	*
390	* @throws UnsupportedOperationException {@inheritDoc}
391	*/
392	public void clear() {
393	Iterator<E> e = iterator();
394	while (e.hasNext()) {
395	e.next();
396	e.remove();
397	}
398	}
399
400
401	// String conversion
402
403	/**
404	* Returns a string representation of this collection. The string
405	* representation consists of a list of the collection's elements in the
406	* order they are returned by its iterator, enclosed in square brackets
407	* (<tt>"[]"</tt>). Adjacent elements are separated by the characters
408	* <tt>", "</tt> (comma and space). Elements are converted to strings as
409	* by {@link String#valueOf(Object)}.
410	*
411	* @return a string representation of this collection
412	*/
413	public String toString() {
414	Iterator<E> i = iterator();
415	if (! i.hasNext())
416	return "[]";
417
418	StringBuilder sb = new StringBuilder();
419	sb.append('[');
420	for (;;) {
421	E e = i.next();
422	sb.append(e == this ? "(this Collection)" : e);
423	if (! i.hasNext())
424	return sb.append(']').toString();
425	sb.append(", ");
426	}
427	}
428
429	}