Module java.base
Package java.util

Class Dictionary<K,​V>

  • Direct Known Subclasses:
    Hashtable
    public abstract classDictionary<K,​V>extendsObject
    TheDictionary class is the abstract parent of any class, such asHashtable, which maps keys to values. Every key and every value is an object. In any oneDictionary object, every key is associated with at most one value. Given aDictionary 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, theequals 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    NOTE: This class is obsolete.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:
    1.0
    See Also:
    Map,Object.equals(java.lang.Object),Object.hashCode(),Hashtable

    • 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 theisEmpty 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 anEnumeration 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 theelements method is that anEnumeration 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 theisEmpty 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 thekey isnull.
        See Also:
        put(java.lang.Object, java.lang.Object)

      • put

        public abstract V put​(K key,V value)
        Maps the specifiedkey to the specifiedvalue in this dictionary. Neither the key nor the value can benull.

        If this dictionary already contains an entry for the specifiedkey, the value already in this dictionary for thatkey is returned, after modifying the entry to contain the new element.

        If this dictionary does not already have an entry for the specifiedkey, an entry is created for the specifiedkey andvalue, andnull is returned.

        Thevalue can be retrieved by calling theget method with akey that is equal to the originalkey.

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

      • remove

        public abstract V remove​(Object key)
        Removes thekey (and its correspondingvalue) from this dictionary. This method does nothing if thekey is not in this dictionary.
        Parameters:
        key - the key that needs to be removed.
        Returns:
        the value to which thekey had been mapped in this dictionary, ornull if the key did not have a mapping.
        Throws:
        NullPointerException - ifkey isnull.