Java The Complete Reference, Seventh Edition

472 Part II: The Java Library

LinkedHashMapadds only one method to those defined byHashMap. This method is

removeEldestEntry( )and it is shown here:

protected boolean removeEldestEntry(Map.Entry<K, V>e)

This method is called byput( )andputAll( ). The oldest entry is passed ine.By default, this

method returnsfalseand does nothing. However, if you override this method, then you can

have theLinkedHashMapremove the oldest entry in the map. To do this, have your override

returntrue. To keep the oldest entry, returnfalse.

The IdentityHashMap Class

IdentityHashMapextendsAbstractMapand implements theMapinterface. It is similar to

HashMapexcept that it uses reference equality when comparing elements.IdentityHashMap

is a generic class that has this declaration:

class IdentityHashMap<K, V>

Here,Kspecifies the type of key, andVspecifies the type of value. The API documentation

explicitly states thatIdentityHashMapis not for general use.

The EnumMap Class

EnumMapextendsAbstractMapand implementsMap. It is specifically for use with keys of

anenumtype. It is a generic class that has this declaration:

class EnumMap<K extends Enum<K>, V>

Here,Kspecifies the type of key, andVspecifies the type of value. Notice thatKmust extend

Enum<K>, which enforces the requirement that the keys must be of anenumtype.

EnumMapdefines the following constructors:

EnumMap(Class<K>kType)

EnumMap(Map<K,? extends V>m)

EnumMap(EnumMap<K,? extends V>em)

The first constructor creates an emptyEnumMapof typekType.The second creates an

EnumMapmap that contains the same entries asm.The third creates anEnumMapinitialized

with the values inem.

EnumMapdefines no methods of its own.

Comparators

BothTreeSetandTreeMapstore 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 aComparatorwhen you construct the set or map. Doing so gives you the

ability to govern precisely how elements are stored within sorted collections and maps.

Comparatoris a generic interface that has this declaration:

interface Comparator<T>

Here,Tspecifies the type of objects being compared.