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## java.util
Class Dictionary

    java.lang.Object
      |
      +--java.util.Dictionary

Direct Known Subclasses:
  Hashtable

public abstract class Dictionary
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NOTE: This class is obsolete.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 extendidng this class.

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

Constructor Summary
`Dictionary()` Sole constructor.

Method Summary
`abstract Enumeration` | `elements()` Returns an enumeration of the values in this dictionary.
`abstract Object` | `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` | `keys()` Returns an enumeration of the keys in this dictionary.
`abstract Object` | `put(Object key, Object value)` Maps the specified `key` to the specified `value` in this dictionary.
`abstract Object` | `remove(Object key)` Removes the `key` (and its corresponding `value`) from this dictionary.
`abstract int` | `size()` Returns the number of entries (dinstint 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 (dinstint 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 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 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 Object 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 Object put(Object key,
                               Object 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 Object 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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For further API reference and developer documentation, see Java 2 SDK SE Developer Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.

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