LinkedHashMap adds only one method to those defined by HashMap. This method is
removeEldestEntry( ) and it is shown here:
protected boolean removeEldestEntry(Map.Entry<K, V> e)
This method is called by put( ) and putAll( ). The oldest entry is passed in e. By default, this
method returns false and does nothing. However, if you override this method, then you can
have the LinkedHashMap remove the oldest entry in the map. To do this, have your override
return true. To keep the oldest entry, return false.
The IdentityHashMap Class
IdentityHashMap extends AbstractMap and implements the Map interface. It is similar to
HashMap except that it uses reference equality when comparing elements. IdentityHashMap
is a generic class that has this declaration:
class IdentityHashMap<K, V>
Here, K specifies the type of key, and V specifies the type of value. The API documentation
explicitly states that IdentityHashMap is not for general use.
The EnumMap Class
EnumMap extends AbstractMap and implements Map. It is specifically for use with keys of
an enum type. It is a generic class that has this declaration:
class EnumMap<K extends Enum<K>, V>
Here, K specifies the type of key, and V specifies the type of value. Notice that K must extend
Enum<K>, which enforces the requirement that the keys must be of an enum type.
EnumMap defines the following constructors:
EnumMap(Map<K, ? extends V> m)
EnumMap(EnumMap<K, ? extends V> em)
The first constructor creates an empty EnumMap of type kType. The second creates an
EnumMap map that contains the same entries as m. The third creates an EnumMap initialized
with the values in em.
EnumMap defines no methods of its own.
Both TreeSet and TreeMap store elements in sorted order. However, it is the comparator that
defines precisely what "sorted order" means. By default, these classes store their elements
by using what Java refers to as "natural ordering," which is usually the ordering that you
would expect (A before B, 1 before 2, and so forth). If you want to order elements a different
way, then specify a Comparator when you construct the set or map. Doing so gives you the
ability to govern precisely how elements are stored within sorted collections and maps.
Comparator is a generic interface that has this declaration:
Here, T specifies the type of objects being compared.
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