Dictionary

AI-generated Key Takeaways

• The Dictionary class is an abstract parent for classes that map keys to values, like Hashtable, but it is obsolete and should not be used for new implementations.

• Developers should use the Map interface for new implementations instead of extending the Dictionary class.

• Each key in a Dictionary object is associated with at most one value, and any non-null object can be used as a key or value.

• The equals method should be used to determine key equality in Dictionary implementations.

• Dictionary provides methods to get, put, remove, and list keys and values, but it is recommended to use the Map interface for similar functionality in modern Java development.

public abstract class Dictionary extends Object

Known Direct Subclasses Hashtable<K, V> Hashtable<K, V> This class implements a hash table, which maps keys to values.

Known Indirect Subclasses Properties, Provider Properties The Properties class represents a persistent set of properties.  Provider This class represents a "provider" for the Java Security API, where a provider implements some or all parts of Java Security.

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NOTE: This class is obsolete.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.

See Also

• Map
• Object.equals(java.lang.Object)
• Object.hashCode()
• Hashtable

Public Constructor Summary

Dictionary() Sole constructor.

Public Method Summary

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.

Inherited Method Summary

From class java.lang.Object

Object	clone() Creates and returns a copy of this Object.
boolean	equals(Object obj) Compares this instance with the specified object and indicates if they are equal.
void	finalize() Invoked when the garbage collector has detected that this instance is no longer reachable.
final Class<?>	getClass() Returns the unique instance of Class that represents this object's class.
int	hashCode() Returns an integer hash code for this object.
final void	notify() Causes a thread which is waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
final void	notifyAll() Causes all threads which are waiting on this object's monitor (by means of calling one of the wait() methods) to be woken up.
String	toString() Returns a string containing a concise, human-readable description of this object.
final void	wait(long timeout, int nanos) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait(long timeout) Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object or until the specified timeout expires.
final void	wait() Causes the calling thread to wait until another thread calls the notify() or notifyAll() method of this object.

Public Constructors

public Dictionary ()

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

Public Methods

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

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)

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.

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

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)

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.

public abstract int size ()

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

Returns

• the number of keys in this dictionary.