/* Dictionary.java -- an abstract (and essentially worthless) 
   class which is Hashtable's superclass
   Copyright (C) 1998, 2001, 2002 Free Software Foundation, Inc.

This file is part of GNU Classpath.

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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
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this exception to your version of the library, but you are not
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package java.util;

/**
 * A Dictionary maps keys to values; <i>how</i> it does that is
 * implementation-specific.
 * 
 * This is an abstract class which has really gone by the wayside.
 * People at Javasoft are probably embarrassed by it.  At this point,
 * it might as well be an interface rather than a class, but it remains
 * this poor, laughable skeleton for the sake of backwards compatibility.
 * At any rate, this was what came before the {@link Map} interface 
 * in the Collections framework.
 *
 * @author Jon Zeppieri
 * @author Eric Blake (ebb9@email.byu.edu)
 * @see Map
 * @see Hashtable
 * @since 1.0
 * @status updated to 1.4
 */
public abstract class Dictionary
{
  // WARNING: Dictionary is a CORE class in the bootstrap cycle. See the
  // comments in vm/reference/java/lang/Runtime for implications of this fact.

  /**
   * Sole constructor (often called implicitly).
   */
  public Dictionary()
  {
  }

  /**
   * Returns an Enumeration of the values in this Dictionary.
   *
   * @return an Enumeration of the values
   * @see #keys()
   */
  public abstract Enumeration elements();

  /** 
   * Returns the value associated with the supplied key, or null
   * if no such value exists. Since Dictionaries are not allowed null keys
   * or elements, a null result always means the key is not present.
   *
   * @param key the key to use to fetch the value
   * @return the mapped value
   * @throws NullPointerException if key is null
   * @see #put(Object, Object)
   */
  public abstract Object get(Object key);

  /**
   * Returns true when there are no elements in this Dictionary.
   *
   * @return <code>size() == 0</code>
   */
  public abstract boolean isEmpty();

  /**
   * Returns an Enumeration of the keys in this Dictionary
   *
   * @return an Enumeration of the keys
   * @see #elements()
   */
  public abstract Enumeration keys();

  /**
   * Inserts a new value into this Dictionary, located by the
   * supplied key. Dictionary does not support null keys or values, so
   * a null return can safely be interpreted as adding a new key.
   *
   * @param key the key which locates the value
   * @param value the value to put into the Dictionary
   * @return the previous value of the key, or null if there was none
   * @throws NullPointerException if key or value is null
   * @see #get(Object)
   */
  public abstract Object put(Object key, Object value);

  /**
   * Removes from the Dictionary the value located by the given key. A null
   * return safely means that the key was not mapped in the Dictionary.
   *
   * @param key the key used to locate the value to be removed
   * @return the value associated with the removed key
   * @throws NullPointerException if key is null
   */
  public abstract Object remove(Object key);

  /**
   * Returns the number of values currently in this Dictionary.
   *
   * @return the number of keys in the Dictionary
   */
  public abstract int size();
} // class Dictionary