path: root/java/util/AbstractMap.java

/* AbstractMap.java -- Abstract implementation of most of Map
   Copyright (C) 1998, 1999, 2000, 2001, 2002, 2005  Free Software Foundation, Inc.

This file is part of GNU Classpath.

GNU Classpath is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
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any later version.

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WITHOUT ANY WARRANTY; without even the implied warranty of
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General Public License for more details.

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02110-1301 USA.

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As a special exception, the copyright holders of this library give you
permission to link this library with independent modules to produce an
executable, regardless of the license terms of these independent
modules, and to copy and distribute the resulting executable under
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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this exception to your version of the library, but you are not
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exception statement from your version. */


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 Bryce McKinlay
 * @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()
  {
  }

  /**
   * 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();

  /**
   * 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 a value, v, where <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;
  }

  /**
   * 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()
      {
	/**
	 * Retrieves the number of keys in the backing map.
	 *
	 * @return The number of keys.
	 */
        public int size()
        {
          return AbstractMap.this.size();
        }

	/**
	 * Returns true if the backing map contains the
	 * supplied key.
	 *
	 * @param key The key to search for.
	 * @return True if the key was found, false otherwise.
	 */
        public boolean contains(Object key)
        {
          return containsKey(key);
        }

	/**
	 * Returns an iterator which iterates over the keys
	 * in the backing map, using a wrapper around the
	 * iterator returned by <code>entrySet()</code>.
	 *
	 * @return An iterator over the keys.
	 */
        public Iterator iterator()
        {
          return new Iterator()
          {
	    /**
	     * The iterator returned by <code>entrySet()</code>.
	     */
            private final Iterator map_iterator = entrySet().iterator();

	    /**
	     * Returns true if a call to <code>next()</code> will
	     * return another key.
	     *
	     * @return True if the iterator has not yet reached
	     *         the last key.
	     */
            public boolean hasNext()
            {
              return map_iterator.hasNext();
            }

	    /**
	     * Returns the key from the next entry retrieved
	     * by the underlying <code>entrySet()</code> iterator.
	     *
	     * @return The next key.
	     */
            public Object next()
            {
              return ((Map.Entry) map_iterator.next()).getKey();
            }

	    /**
	     * Removes the map entry which has a key equal
	     * to that returned by the last call to
	     * <code>next()</code>.
	     *
	     * @throws UnsupportedOperationException if the
	     *         map doesn't support removal.
	     */
            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
   *         by this map.
   * @throws ClassCastException if a key or value is of the wrong type for
   *         adding to this map.
   * @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.
   * @throws NullPointerException if <code>m</code> is null.
   * @see #put(Object, Object)
   */
  public void putAll(Map m)
  {
    Iterator entries = m.entrySet().iterator();
    int pos = m.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.
   *         Null may also be returned if null values are allowed
   *         in the map and the value of this mapping is null.
   * @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--)
      {
        Map.Entry entry = (Map.Entry) entries.next();
        r.append(entry.getKey());
        r.append('=');
        r.append(entry.getValue());
        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()
      {
	/**
	 * Returns the number of values stored in
	 * the backing map.
	 *
	 * @return The number of values.
	 */
        public int size()
        {
          return AbstractMap.this.size();
        }

	/**
	 * Returns true if the backing map contains
	 * the supplied value.
	 *
	 * @param value The value to search for.
	 * @return True if the value was found, false otherwise.
	 */
        public boolean contains(Object value)
        {
          return containsValue(value);
        }

	/**
	 * Returns an iterator which iterates over the
	 * values in the backing map, by using a wrapper
	 * around the iterator returned by <code>entrySet()</code>.
	 *
	 * @return An iterator over the values.
	 */
        public Iterator iterator()
        {
          return new Iterator()
          {
	    /**
	     * The iterator returned by <code>entrySet()</code>.
	     */
            private final Iterator map_iterator = entrySet().iterator();

	    /**
	     * Returns true if a call to <code>next()</call> will
	     * return another value.
	     *
	     * @return True if the iterator has not yet reached
	     * the last value.
	     */
            public boolean hasNext()
            {
              return map_iterator.hasNext();
            }

	    /**
	     * Returns the value from the next entry retrieved
	     * by the underlying <code>entrySet()</code> iterator.
	     *
	     * @return The next value.
	     */
            public Object next()
            {
              return ((Map.Entry) map_iterator.next()).getValue();
            }

	    /**
	     * Removes the map entry which has a key equal
	     * to that returned by the last call to
	     * <code>next()</code>.
	     *
	     * @throws UnsupportedOperationException if the
	     *         map doesn't support removal.
	     */
            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();
  }

  /**
   * A class which implements Map.Entry. It is shared by HashMap, TreeMap,
   * Hashtable, and Collections. It is not specified by the JDK, but makes
   * life much easier.
   *
   * @author Jon Zeppieri
   * @author Eric Blake (ebb9@email.byu.edu)
   */
  // XXX - FIXME Use fully qualified implements as gcj 3.1 workaround.
  //       Bug still exists in 3.4.1
  static class BasicMapEntry implements Map.Entry
  {
    /**
     * The key. Package visible for direct manipulation.
     */
    Object key;

    /**
     * The value. Package visible for direct manipulation.
     */
    Object value;

    /**
     * Basic constructor initializes the fields.
     * @param newKey the key
     * @param newValue the value
     */
    BasicMapEntry(Object newKey, Object newValue)
    {
      key = newKey;
      value = newValue;
    }

    /**
     * Compares the specified object with this entry. Returns true only if
     * the object is a mapping of identical key and value. In other words,
     * this must be:<br>
     * <pre>(o instanceof Map.Entry)
     *       && (getKey() == null ? ((HashMap) o).getKey() == null
     *           : getKey().equals(((HashMap) o).getKey()))
     *       && (getValue() == null ? ((HashMap) o).getValue() == null
     *           : getValue().equals(((HashMap) o).getValue()))</pre>
     *
     * @param o the object to compare
     * @return <code>true</code> if it is equal
     */
    public final boolean equals(Object o)
    {
      if (! (o instanceof Map.Entry))
        return false;
      // Optimize for our own entries.
      if (o instanceof BasicMapEntry)
        {
          BasicMapEntry e = (BasicMapEntry) o;
          return (AbstractMap.equals(key, e.key)
                  && AbstractMap.equals(value, e.value));
        }
      Map.Entry e = (Map.Entry) o;
      return (AbstractMap.equals(key, e.getKey())
              && AbstractMap.equals(value, e.getValue()));
    }

    /**
     * Get the key corresponding to this entry.
     *
     * @return the key
     */
    public final Object getKey()
    {
      return key;
    }

    /**
     * Get the value corresponding to this entry. If you already called
     * Iterator.remove(), the behavior undefined, but in this case it works.
     *
     * @return the value
     */
    public final Object getValue()
    {
      return value;
    }

    /**
     * Returns the hash code of the entry.  This is defined as the exclusive-or
     * of the hashcodes of the key and value (using 0 for null). In other
     * words, this must be:<br>
     * <pre>(getKey() == null ? 0 : getKey().hashCode())
     *       ^ (getValue() == null ? 0 : getValue().hashCode())</pre>
     *
     * @return the hash code
     */
    public final int hashCode()
    {
      return (AbstractMap.hashCode(key) ^ AbstractMap.hashCode(value));
    }

    /**
     * Replaces the value with the specified object. This writes through
     * to the map, unless you have already called Iterator.remove(). It
     * may be overridden to restrict a null value.
     *
     * @param newVal the new value to store
     * @return the old value
     * @throws NullPointerException if the map forbids null values.
     * @throws UnsupportedOperationException if the map doesn't support
     *          <code>put()</code>.
     * @throws ClassCastException if the value is of a type unsupported
     *         by the map.
     * @throws IllegalArgumentException if something else about this
     *         value prevents it being stored in the map.
     */
    public Object setValue(Object newVal)
    {
      Object r = value;
      value = newVal;
      return r;
    }

    /**
     * This provides a string representation of the entry. It is of the form
     * "key=value", where string concatenation is used on key and value.
     *
     * @return the string representation
     */
    public final String toString()
    {
      return key + "=" + value;
    }
  } // class BasicMapEntry
}