collections.java

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    /**
     * The single key.
     * @serial the singleton key
     */
    private final Object k;

    /**
     * The corresponding value.
     * @serial the singleton value
     */
    private final Object v;

    /**
     * Cache the entry set.
     */
    private transient Set entries;

    /**
     * Construct a singleton.
     * @param key the key
     * @param value the value
     */
    SingletonMap(Object key, Object value)
    {
      k = key;
      v = value;
    }

    /**
     * There is a single immutable entry.
     */
    public Set entrySet()
    {
      if (entries == null)
        entries = singleton(new AbstractMap.BasicMapEntry(k, v)
        {
          public Object setValue(Object o)
          {
            throw new UnsupportedOperationException();
          }
        });
      return entries;
    }

    // The remaining methods are optional, but provide a performance
    // advantage by not allocating unnecessary iterators in AbstractMap.
    /**
     * Single entry.
     */
    public boolean containsKey(Object key)
    {
      return equals(key, k);
    }

    /**
     * Single entry.
     */
    public boolean containsValue(Object value)
    {
      return equals(value, v);
    }

    /**
     * Single entry.
     */
    public Object get(Object key)
    {
      return equals(key, k) ? v : null;
    }

    /**
     * Calculate the hashcode directly.
     */
    public int hashCode()
    {
      return hashCode(k) ^ hashCode(v);
    }

    /**
     * Return the keyset.
     */
    public Set keySet()
    {
      if (keys == null)
        keys = singleton(k);
      return keys;
    }

    /**
     * The size: always 1!
     */
    public int size()
    {
      return 1;
    }

    /**
     * Return the values. Technically, a singleton, while more specific than
     * a general Collection, will work. Besides, that's what the JDK uses!
     */
    public Collection values()
    {
      if (values == null)
        values = singleton(v);
      return values;
    }

    /**
     * Obvious string.
     */
    public String toString()
    {
      return "{" + k + "=" + v + "}";
    }
  } // class SingletonMap

  /**
   * Sort a list according to the natural ordering of its elements. The list
   * must be modifiable, but can be of fixed size. The sort algorithm is
   * precisely that used by Arrays.sort(Object[]), which offers guaranteed
   * nlog(n) performance. This implementation dumps the list into an array,
   * sorts the array, and then iterates over the list setting each element from
   * the array.
   *
   * @param l the List to sort
   * @throws ClassCastException if some items are not mutually comparable
   * @throws UnsupportedOperationException if the List is not modifiable
   * @throws NullPointerException if some element is null
   * @see Arrays#sort(Object[])
   */
  public static void sort(List l)
  {
    sort(l, null);
  }

  /**
   * Sort a list according to a specified Comparator. The list must be
   * modifiable, but can be of fixed size. The sort algorithm is precisely that
   * used by Arrays.sort(Object[], Comparator), which offers guaranteed
   * nlog(n) performance. This implementation dumps the list into an array,
   * sorts the array, and then iterates over the list setting each element from
   * the array.
   *
   * @param l the List to sort
   * @param c the Comparator specifying the ordering for the elements, or
   *        null for natural ordering
   * @throws ClassCastException if c will not compare some pair of items
   * @throws UnsupportedOperationException if the List is not modifiable
   * @throws NullPointerException if null is compared by natural ordering
   *        (only possible when c is null)
   * @see Arrays#sort(Object[], Comparator)
   */
  public static void sort(List l, Comparator c)
  {
    Object[] a = l.toArray();
    Arrays.sort(a, c);
    ListIterator i = l.listIterator(a.length);
    for (int pos = a.length; --pos >= 0; )
      {
	i.previous();
	i.set(a[pos]);
      }
  }

  /**
   * Swaps the elements at the specified positions within the list. Equal
   * positions have no effect.
   *
   * @param l the list to work on
   * @param i the first index to swap
   * @param j the second index
   * @throws UnsupportedOperationException if list.set is not supported
   * @throws IndexOutOfBoundsException if either i or j is < 0 or >=
   *         list.size()
   * @since 1.4
   */
  public static void swap(List l, int i, int j)
  {
    l.set(i, l.set(j, l.get(i)));
  }


  /**
   * Returns a synchronized (thread-safe) collection wrapper backed by the
   * given collection. Notice that element access through the iterators
   * is thread-safe, but if the collection can be structurally modified
   * (adding or removing elements) then you should synchronize around the
   * iteration to avoid non-deterministic behavior:<br>
   * <pre>
   * Collection c = Collections.synchronizedCollection(new Collection(...));
   * ...
   * synchronized (c)
   *   {
   *     Iterator i = c.iterator();
   *     while (i.hasNext())
   *       foo(i.next());
   *   }
   * </pre><p>
   *
   * Since the collection might be a List or a Set, and those have incompatible
   * equals and hashCode requirements, this relies on Object's implementation
   * rather than passing those calls on to the wrapped collection. The returned
   * Collection implements Serializable, but can only be serialized if
   * the collection it wraps is likewise Serializable.
   *
   * @param c the collection to wrap
   * @return a synchronized view of the collection
   * @see Serializable
   */
  public static Collection synchronizedCollection(Collection c)
  {
    return new SynchronizedCollection(c);
  }

  /**
   * The implementation of {@link #synchronizedCollection(Collection)}. This
   * class name is required for compatibility with Sun's JDK serializability.
   * Package visible, so that collections such as the one for
   * Hashtable.values() can specify which object to synchronize on.
   *
   * @author Eric Blake <[e-mail]>
   */
  static class SynchronizedCollection
    implements Collection, Serializable
  {
    /**
     * Compatible with JDK 1.4.
     */
    private static final long serialVersionUID = 3053995032091335093L;

    /**
     * The wrapped collection. Package visible for use by subclasses.
     * @serial the real collection
     */
    final Collection c;

    /**
     * The object to synchronize on.  When an instance is created via public
     * methods, it will be this; but other uses like SynchronizedMap.values()
     * must specify another mutex. Package visible for use by subclasses.
     * @serial the lock
     */
    final Object mutex;

    /**
     * Wrap a given collection.
     * @param c the collection to wrap
     * @throws NullPointerException if c is null
     */
    SynchronizedCollection(Collection c)
    {
      this.c = c;
      mutex = this;
      if (c == null)
        throw new NullPointerException();
    }

    /**
     * Called only by trusted code to specify the mutex as well as the
     * collection.
     * @param sync the mutex
     * @param c the collection
     */
    SynchronizedCollection(Object sync, Collection c)
    {
      this.c = c;
      mutex = sync;
    }

    public boolean add(Object o)
    {
      synchronized (mutex)
        {
          return c.add(o);
        }
    }

    public boolean addAll(Collection col)
    {
      synchronized (mutex)
        {
          return c.addAll(col);
        }
    }

    public void clear()
    {
      synchronized (mutex)
        {
          c.clear();
        }
    }

    public boolean contains(Object o)
    {
      synchronized (mutex)
        {
          return c.contains(o);
        }
    }

    public boolean containsAll(Collection c1)
    {
      synchronized (mutex)
        {
          return c.containsAll(c1);
        }
    }

    public boolean isEmpty()
    {
      synchronized (mutex)
        {
          return c.isEmpty();
        }
    }

    public Iterator iterator()
    {
      synchronized (mutex)
        {
          return new SynchronizedIterator(mutex, c.iterator());
        }
    }

    public boolean remove(Object o)
    {
      synchronized (mutex)
        {
          return c.remove(o);
        }
    }

    public boolean removeAll(Collection col)
    {
      synchronized (mutex)
        {
          return c.removeAll(col);
        }
    }

    public boolean retainAll(Collection col)
    {
      synchronized (mutex)
        {
          return c.retainAll(col);
        }
    }

    public int size()
    {
      synchronized (mutex)
        {
          return c.size();
        }
    }

    public Object[] toArray()
    {
      synchronized (mutex)
        {
          return c.toArray();
        }
    }

    public Object[] toArray(Object[] a)
    {
      synchronized (mutex)
        {
          return c.toArray(a);
        }
    }

    public String toString()
    {
      synchronized (mutex)
        {
          return c.toString();
        }
    }
  } // class SynchronizedCollection

  /**
   * The implementation of the various iterator methods in the
   * synchronized classes. These iterators must "sync" on the same object
   * as the collection they iterate over.
   *
   * @author Eric Blake <[e-mail]>
   */
  private static class SynchronizedIterator implements Iterator
  {
    /**
     * The object to synchronize on. Package visible for use by subclass.
     */
    final Object mutex;

    /**
     * The wrapped iterator.
     */
    private final Iterator i;

    /**
     * Only trusted code creates a wrapper, with the specified sync.
     * @param sync the mutex
     * @param i the wrapped iterator
     */
    SynchronizedIterator(Object sync, Iterator i)
    {
      this.i = i;
      mutex = sync;
    }

    public Object next()
    {
      synchronized (mutex)
        {
          return i.next();
        }
    }

    public boolean hasNext()
    {
      synchronized (mutex)
        {
          return i.hasNext();
        }
    }

    public void remove()
    {
      synchronized (mutex)
        {
          i.remove();
        }
    }
  } // class SynchronizedIterator

  /**
   * Returns a synchronized (thread-safe) list wrapper backed by the
   * given list. Notice that element access through the iterators
   * is thread-safe, but if the list can be structurally modified
   * (adding or removing elements) then you should synchronize around the
   * iteration to avoid non-deterministic behavior:<br>
   * <pre>
   * List l = Collections.synchronizedList(new List(...));
   * ...
   * synchronized (l)
   *   {
   *     Iterator i = l.iterator();
   *     while (i.hasNext())
   *       foo(i.next());
   *   }
   * </pre><p>
   *
   * The returned List implements Serializable, but can only be serialized if
   * the list it wraps is likewise Serializable. In addition, if the wrapped
   * list implements RandomAccess, this does too.
   *
   * @param l the list to wrap
   * @return a synchronized view of the list
   * @see Serializable
   * @see RandomAccess
   */
  public static List synchronizedList(List l)
  {
    if (l instanceof RandomAccess)
      return new SynchronizedRandomAccessList(l);
    return new SynchronizedList(l);
  }

  /**
   * The implementation of {@link #synchronizedList(List)} for sequential
   * lists. This class name is required for compatibility with Sun's JDK
   * serializability. Package visible, so that lists such as Vector.subList()
   * can specify which object to synchronize on.
   *
   * @author Eric Blake <[e-mail]>
   */
  static class SynchronizedList extends SynchronizedCollection
    implements List
  {
    /**
     * Compatible with JDK 1.4.
     */
    private static final long serialVersionUID = -7754090372962971524L;

    /**
     * The wrapped list; stored both here and in the superclass to avoid
     * excessive casting. Package visible for use by subclass.
     * @serial the wrapped list
     */
    final List list;

    /**
     * Wrap a given list.
     * @param l the list to wrap
     * @throws NullPointerException if l is null
     */
    SynchronizedList(List l)
    {
      super(l);
      list = l;
    }

    /**
     * Called only by trusted code to specify the mutex as well as the list.
     * @param sync the mutex
     * @param l the list
     */
    SynchronizedList(Object sync, List l)
    {
      super(sync, l);
      list = l;
    }