path: root/java/util/AbstractMap.java

/* AbstractMap.java -- Abstract implementation of most of Map
   Copyright (C) 1998, 1999, 2000, 2001 Free Software Foundation, Inc.

This file is part of GNU Classpath.

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it under the terms of the GNU General Public License as published by
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02111-1307 USA.

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permission to link this library with independent modules to produce an
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terms of your choice, provided that you also meet, for each linked
independent module, the terms and conditions of the license of that
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package java.util;

/**
 * An abstract implementation of Map to make it easier to create your own
 * implementations. In order to create an unmodifiable Map, subclass
 * AbstractMap and implement the <code>entrySet</code> (usually via an
 * AbstractSet).  To make it modifiable, also implement <code>put</code>,
 * and have <code>entrySet().iterator()</code> support <code>remove</code>.
 * <p>
 *
 * It is recommended that classes which extend this support at least the
 * no-argument constructor, and a constructor which accepts another Map.
 * Further methods in this class may be overridden if you have a more
 * efficient implementation.
 *
 * @author Original author unknown
 * @author Eric Blake <ebb9@email.byu.edu>
 * @see Map
 * @see Collection
 * @see HashMap
 * @see LinkedHashMap
 * @see TreeMap
 * @see WeakHashMap
 * @see IdentityHashMap
 * @since 1.2
 * @status updated to 1.4
 */
public abstract class AbstractMap implements Map
{
  /** An "enum" of iterator types. */
  // Package visible for use by subclasses.
  static final int KEYS = 0,
                   VALUES = 1,
                   ENTRIES = 2;

  /**
   * The cache for {@link #keySet()}.
   */
  // Package visible for use by subclasses.
  Set keys;

  /**
   * The cache for {@link #values()}.
   */
  // Package visible for use by subclasses.
  Collection values;

  /**
   * The main constructor, for use by subclasses.
   */
  protected AbstractMap()
  {
  }

  /**
   * Remove all entries from this Map (optional operation). This default
   * implementation calls entrySet().clear(). NOTE: If the entry set does
   * not permit clearing, then this will fail, too. Subclasses often
   * override this for efficiency.  Your implementation of entrySet() should
   * not call <code>AbstractMap.clear</code> unless you want an infinite loop.
   *
   * @throws UnsupportedOperationException if <code>entrySet().clear()</code>
   *         does not support clearing.
   * @see Set#clear()
   */
  public void clear()
  {
    entrySet().clear();
  }

  /**
   * Create a shallow copy of this Map, no keys or values are copied. The
   * default implementation simply calls <code>super.clone()</code>.
   *
   * @return the shallow clone
   * @throws CloneNotSupportedException if a subclass is not Cloneable
   * @see Cloneable
   * @see Object#clone()
   */
  protected Object clone() throws CloneNotSupportedException
  {
    AbstractMap copy = (AbstractMap) super.clone();
    // Clear out the caches; they are stale.
    copy.keys = null;
    copy.values = null;
    return copy;
  }

  /**
   * Returns true if this contains a mapping for the given key. This
   * implementation does a linear search, O(n), over the
   * <code>entrySet()</code>, returning <code>true</code> if a match
   * is found, <code>false</code> if the iteration ends. Many subclasses
   * can implement this more efficiently.
   *
   * @param key the key to search for
   * @return true if the map contains the key
   * @throws NullPointerException if key is <code>null</code> but the map
   *         does not permit null keys
   * @see #containsValue(Object)
   */
  public boolean containsKey(Object key)
  {
    Iterator entries = entrySet().iterator();
    int pos = size();
    while (--pos >= 0)
      if (equals(key, ((Map.Entry) entries.next()).getKey()))
        return true;
    return false;
  }

  /**
   * Returns true if this contains at least one mapping with the given value.
   * This implementation does a linear search, O(n), over the
   * <code>entrySet()</code>, returning <code>true</code> if a match
   * is found, <code>false</code> if the iteration ends. A match is
   * defined as <code>(value == null ? v == null : value.equals(v))</code>
   * Subclasses are unlikely to implement this more efficiently.
   *
   * @param value the value to search for
   * @return true if the map contains the value
   * @see #containsKey(Object)
   */
  public boolean containsValue(Object value)
  {
    Iterator entries = entrySet().iterator();
    int pos = size();
    while (--pos >= 0)
      if (equals(value, ((Map.Entry) entries.next()).getValue()))
        return true;
    return false;
  }

  /**
   * Returns a set view of the mappings in this Map.  Each element in the
   * set must be an implementation of Map.Entry.  The set is backed by
   * the map, so that changes in one show up in the other.  Modifications
   * made while an iterator is in progress cause undefined behavior.  If
   * the set supports removal, these methods must be valid:
   * <code>Iterator.remove</code>, <code>Set.remove</code>,
   * <code>removeAll</code>, <code>retainAll</code>, and <code>clear</code>.
   * Element addition is not supported via this set.
   *
   * @return the entry set
   * @see Map.Entry
   */
  public abstract Set entrySet();

  /**
   * Compares the specified object with this map for equality. Returns
   * <code>true</code> if the other object is a Map with the same mappings,
   * that is,<br>
   * <code>o instanceof Map && entrySet().equals(((Map) o).entrySet();</code>
   *
   * @param o the object to be compared
   * @return true if the object equals this map
   * @see Set#equals(Object)
   */
  public boolean equals(Object o)
  {
    return (o == this ||
            (o instanceof Map &&
             entrySet().equals(((Map) o).entrySet())));
  }

  /**
   * Returns the value mapped by the given key. Returns <code>null</code> if
   * there is no mapping.  However, in Maps that accept null values, you
   * must rely on <code>containsKey</code> to determine if a mapping exists.
   * This iteration takes linear time, searching entrySet().iterator() of
   * the key.  Many implementations override this method.
   *
   * @param key the key to look up
   * @return the value associated with the key, or null if key not in map
   * @throws NullPointerException if this map does not accept null keys
   * @see #containsKey(Object)
   */
  public Object get(Object key)
  {
    Iterator entries = entrySet().iterator();
    int pos = size();
    while (--pos >= 0)
      {
        Map.Entry entry = (Map.Entry) entries.next();
        if (equals(key, entry.getKey()))
          return entry.getValue();
      }
    return null;
  }

  /**
   * Returns the hash code for this map. As defined in Map, this is the sum
   * of all hashcodes for each Map.Entry object in entrySet, or basically
   * entrySet().hashCode().
   *
   * @return the hash code
   * @see Map.Entry#hashCode()
   * @see Set#hashCode()
   */
  public int hashCode()
  {
    return entrySet().hashCode();
  }

  /**
   * Returns true if the map contains no mappings. This is implemented by
   * <code>size() == 0</code>.
   *
   * @return true if the map is empty
   * @see #size()
   */
  public boolean isEmpty()
  {
    return size() == 0;
  }

  /**
   * Returns a set view of this map's keys. The set is backed by the map,
   * so changes in one show up in the other. Modifications while an iteration
   * is in progress produce undefined behavior. The set supports removal
   * if entrySet() does, but does not support element addition.
   * <p>
   *
   * This implementation creates an AbstractSet, where the iterator wraps
   * the entrySet iterator, size defers to the Map's size, and contains
   * defers to the Map's containsKey. The set is created on first use, and
   * returned on subsequent uses, although since no synchronization occurs,
   * there is a slight possibility of creating two sets.
   *
   * @return a Set view of the keys
   * @see Set#iterator()
   * @see #size()
   * @see #containsKey(Object)
   * @see #values()
   */
  public Set keySet()
  {
    if (keys == null)
      keys = new AbstractSet()
      {
        public int size()
        {
          return AbstractMap.this.size();
        }

        public boolean contains(Object key)
        {
          return containsKey(key);
        }

        public Iterator iterator()
        {
          return new Iterator()
          {
            private final Iterator map_iterator = entrySet().iterator();

            public boolean hasNext()
            {
              return map_iterator.hasNext();
            }

            public Object next()
            {
              return ((Map.Entry) map_iterator.next()).getKey();
            }

            public void remove()
            {
              map_iterator.remove();
            }
          };
        }
      };
    return keys;
  }

  /**
   * Associates the given key to the given value (optional operation). If the
   * map already contains the key, its value is replaced. This implementation
   * simply throws an UnsupportedOperationException. Be aware that in a map
   * that permits <code>null</code> values, a null return does not always
   * imply that the mapping was created.
   *
   * @param key the key to map
   * @param value the value to be mapped
   * @return the previous value of the key, or null if there was no mapping
   * @throws UnsupportedOperationException if the operation is not supported
   * @throws ClassCastException if the key or value is of the wrong type
   * @throws IllegalArgumentException if something about this key or value
   *         prevents it from existing in this map
   * @throws NullPointerException if the map forbids null keys or values
   * @see #containsKey(Object)
   */
  public Object put(Object key, Object value)
  {
    throw new UnsupportedOperationException();
  }

  /**
   * Copies all entries of the given map to this one (optional operation). If
   * the map already contains a key, its value is replaced. This implementation
   * simply iterates over the map's entrySet(), calling <code>put</code>,
   * so it is not supported if puts are not.
   *
   * @param m the mapping to load into this map
   * @throws UnsupportedOperationException if the operation is not supported
   * @throws ClassCastException if a key or value is of the wrong type
   * @throws IllegalArgumentException if something about a key or value
   *         prevents it from existing in this map
   * @throws NullPointerException if the map forbids null keys or values, or
   *         if <code>m</code> is null.
   * @see #put(Object, Object)
   */
  public void putAll(Map m)
  {
    Iterator entries = m.entrySet().iterator();
    int pos = size();
    while (--pos >= 0)
      {
        Map.Entry entry = (Map.Entry) entries.next();
        put(entry.getKey(), entry.getValue());
      }
  }

  /**
   * Removes the mapping for this key if present (optional operation). This
   * implementation iterates over the entrySet searching for a matching
   * key, at which point it calls the iterator's <code>remove</code> method.
   * It returns the result of <code>getValue()</code> on the entry, if found,
   * or null if no entry is found. Note that maps which permit null values
   * may also return null if the key was removed.  If the entrySet does not
   * support removal, this will also fail. This is O(n), so many
   * implementations override it for efficiency.
   *
   * @param key the key to remove
   * @return the value the key mapped to, or null if not present
   * @throws UnsupportedOperationException if deletion is unsupported
   * @see Iterator#remove()
   */
  public Object remove(Object key)
  {
    Iterator entries = entrySet().iterator();
    int pos = size();
    while (--pos >= 0)
      {
        Map.Entry entry = (Map.Entry) entries.next();
        if (equals(key, entry.getKey()))
          {
            // Must get the value before we remove it from iterator.
            Object r = entry.getValue();
            entries.remove();
            return r;
          }
      }
    return null;
  }

  /**
   * Returns the number of key-value mappings in the map. If there are more
   * than Integer.MAX_VALUE mappings, return Integer.MAX_VALUE. This is
   * implemented as <code>entrySet().size()</code>.
   *
   * @return the number of mappings
   * @see Set#size()
   */
  public int size()
  {
    return entrySet().size();
  }

  /**
   * Returns a String representation of this map. This is a listing of the
   * map entries (which are specified in Map.Entry as being
   * <code>getKey() + "=" + getValue()</code>), separated by a comma and
   * space (", "), and surrounded by braces ('{' and '}'). This implementation
   * uses a StringBuffer and iterates over the entrySet to build the String.
   * Note that this can fail with an exception if underlying keys or
   * values complete abruptly in toString().
   *
   * @return a String representation
   * @see Map.Entry#toString()
   */
  public String toString()
  {
    Iterator entries = entrySet().iterator();
    StringBuffer r = new StringBuffer("{");
    for (int pos = size(); pos > 0; pos--)
      {
        // Append the toString value of the entries rather than calling
        // getKey/getValue. This is more efficient and it matches the JDK
        // behaviour.
        r.append(entries.next());
        if (pos > 1)
          r.append(", ");
      }
    r.append("}");
    return r.toString();
  }

  /**
   * Returns a collection or bag view of this map's values. The collection
   * is backed by the map, so changes in one show up in the other.
   * Modifications while an iteration is in progress produce undefined
   * behavior. The collection supports removal if entrySet() does, but
   * does not support element addition.
   * <p>
   *
   * This implementation creates an AbstractCollection, where the iterator
   * wraps the entrySet iterator, size defers to the Map's size, and contains
   * defers to the Map's containsValue. The collection is created on first
   * use, and returned on subsequent uses, although since no synchronization
   * occurs, there is a slight possibility of creating two collections.
   *
   * @return a Collection view of the values
   * @see Collection#iterator()
   * @see #size()
   * @see #containsValue(Object)
   * @see #keySet()
   */
  public Collection values()
  {
    if (values == null)
      values = new AbstractCollection()
      {
        public int size()
        {
          return AbstractMap.this.size();
        }

        public Iterator iterator()
        {
          return new Iterator()
          {
            private final Iterator map_iterator = entrySet().iterator();

            public boolean hasNext()
            {
              return map_iterator.hasNext();
            }

            public Object next()
            {
              return ((Map.Entry) map_iterator.next()).getValue();
            }

            public void remove()
            {
              map_iterator.remove();
            }
          };
        }
      };
    return values;
  }

  /**
   * Compare two objects according to Collection semantics.
   *
   * @param o1 the first object
   * @param o2 the second object
   * @return o1 == null ? o2 == null : o1.equals(o2)
   */
  // Package visible for use throughout java.util.
  // It may be inlined since it is final.
  static final boolean equals(Object o1, Object o2)
  {
    return o1 == null ? o2 == null : o1.equals(o2);
  }

  /**
   * Hash an object according to Collection semantics.
   *
   * @param o the object to hash
   * @return o1 == null ? 0 : o1.hashCode()
   */
  // Package visible for use throughout java.util.
  // It may be inlined since it is final.
  static final int hashCode(Object o)
  {
    return o == null ? 0 : o.hashCode();
  }
}