java.util

## Class Dictionary<K,V>

- java.lang.Object
- - java.util.Dictionary<K,V>

- Direct Known Subclasses:
    Hashtable

      public abstract class Dictionary<K,V>
      extends Object

  The `Dictionary` class is the abstract parent of any class, such as `Hashtable`, which maps keys to values. Every key and every value is an object. In any one `Dictionary` object, every key is associated with at most one value. Given a `Dictionary` and a key, the associated element can be looked up. Any non-`null` object can be used as a key and as a value.

  As a rule, the `equals` method should be used by implementations of this class to decide if two keys are the same.

  NOTE: This class is obsolete. New implementations should implement the Map interface, rather than extending this class.

  Since:
    JDK1.0
  See Also:
    `Map`, `Object.equals(java.lang.Object)`, `Object.hashCode()`, `Hashtable`

- - ### Constructor Summary

    Constructors
    Constructor and Description
    `Dictionary()` Sole constructor.

  - ### Method Summary

    Methods
    Modifier and Type | Method and Description
    --- | ---
    `abstract Enumeration<V>` | `elements()` Returns an enumeration of the values in this dictionary.
    `abstract V` | `get(Object key)` Returns the value to which the key is mapped in this dictionary.
    `abstract boolean` | `isEmpty()` Tests if this dictionary maps no keys to value.
    `abstract Enumeration<K>` | `keys()` Returns an enumeration of the keys in this dictionary.
    `abstract V` | `put(K key, V value)` Maps the specified `key` to the specified `value` in this dictionary.
    `abstract V` | `remove(Object key)` Removes the `key` (and its corresponding `value`) from this dictionary.
    `abstract int` | `size()` Returns the number of entries (distinct keys) in this dictionary.

    - ### Methods inherited from class java.lang.Object

      `clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait`

- - ### Constructor Detail

    - #### Dictionary

          public Dictionary()

      Sole constructor. (For invocation by subclass constructors, typically implicit.)

  - ### Method Detail

    - #### size

          public abstract int size()

      Returns the number of entries (distinct keys) in this dictionary.
      Returns:
        the number of keys in this dictionary.

    - #### isEmpty

          public abstract boolean isEmpty()

      Tests if this dictionary maps no keys to value. The general contract for the `isEmpty` method is that the result is true if and only if this dictionary contains no entries.
      Returns:
        `true` if this dictionary maps no keys to values; `false` otherwise.

    - #### keys

          public abstract Enumeration<K> keys()

      Returns an enumeration of the keys in this dictionary. The general contract for the keys method is that an `Enumeration` object is returned that will generate all the keys for which this dictionary contains entries.
      Returns:
        an enumeration of the keys in this dictionary.
      See Also:
        `elements()`, `Enumeration`

    - #### elements

          public abstract Enumeration<V> elements()

      Returns an enumeration of the values in this dictionary. The general contract for the `elements` method is that an `Enumeration` is returned that will generate all the elements contained in entries in this dictionary.
      Returns:
        an enumeration of the values in this dictionary.
      See Also:
        `keys()`, `Enumeration`

    - #### get

          public abstract V get(Object key)

      Returns the value to which the key is mapped in this dictionary. The general contract for the `isEmpty` method is that if this dictionary contains an entry for the specified key, the associated value is returned; otherwise, `null` is returned.
      Parameters:
        `key` - a key in this dictionary. `null` if the key is not mapped to any value in this dictionary.
      Returns:
        the value to which the key is mapped in this dictionary;
      Throws:
        `NullPointerException` - if the `key` is `null`.
      See Also:
        `put(java.lang.Object, java.lang.Object)`

    - #### put

          public abstract V put(K key,
              V value)

      Maps the specified `key` to the specified `value` in this dictionary. Neither the key nor the value can be `null`.

      If this dictionary already contains an entry for the specified `key`, the value already in this dictionary for that `key` is returned, after modifying the entry to contain the new element.

      If this dictionary does not already have an entry for the specified `key`, an entry is created for the specified `key` and `value`, and `null` is returned.

      The `value` can be retrieved by calling the `get` method with a `key` that is equal to the original `key`.

      Parameters:
        `key` - the hashtable key.
        `value` - the value.
      Returns:
        the previous value to which the `key` was mapped in this dictionary, or `null` if the key did not have a previous mapping.
      Throws:
        `NullPointerException` - if the `key` or `value` is `null`.
      See Also:
        `Object.equals(java.lang.Object)`, `get(java.lang.Object)`

    - #### remove

          public abstract V remove(Object key)

      Removes the `key` (and its corresponding `value`) from this dictionary. This method does nothing if the `key` is not in this dictionary.
      Parameters:
        `key` - the key that needs to be removed.
      Returns:
        the value to which the `key` had been mapped in this dictionary, or `null` if the key did not have a mapping.
      Throws:
        `NullPointerException` - if `key` is `null`.

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