UnmodifiableMap Java Collections vs ImmutableMap Google

Question

Context  I need to return a reference to a map that I m using for a data cache  and I d like to make sure nobody can modify their reference   Question  I ve seen lots of references to UnmodifiableMap and ImmutableMap online  but I don t see anything comparing contrasting them   I figure there is a good reason that Google Guava created their own version - can someone tell me what it is

User · Answer

Guava Documentation     The JDK provides Collections unmodifiableXXX methods  but in our opinion  these can be unwieldy and verbose  unpleasant to use everywhere you want to make defensive copies unsafe  the returned collections are only truly immutable if nobody holds a reference to the original collection inefficient  the data structures still have all the overhead of mutable collections  including concurrent modification checks  extra space in hash tables  etc

User · Answer

An unmodifiable map may still change  It is only a view on a modifiable map  and changes in the backing map will be visible through the unmodifiable map  The unmodifiable map only prevents modifications for those who only have the reference to the unmodifiable view   Map lt String  String gt  realMap   new HashMap lt String  String gt     realMap put  A    B     Map lt String  String gt  unmodifiableMap   Collections unmodifiableMap realMap       This is not possible  It would throw an     UnsupportedOperationException   unmodifiableMap put  C    D        This is still possible  realMap put  E    F        The change in the  realMap  is now also visible    in the  unmodifiableMap   So the unmodifiableMap    has changed after it has been created  unmodifiableMap get  E       Will return  F      In contrast to that  the ImmutableMap of Guava is really immutable  It is a true copy of a given map  and nobody may modify this ImmutableMap in any way    Update    As pointed out in a comment  an immutable map can also be created with the standard API using  Map lt String  String gt  immutableMap        Collections unmodifiableMap new LinkedHashMap lt String  String gt  realMap       This will create an unmodifiable view on a true copy of the given map  and thus nicely emulates the characteristics of the ImmutableMap without having to add the dependency to Guava

User · Answer

ImmutableMap does not accept null values whereas Collections unmodifiableMap   does  In addition it will never change after construction  while UnmodifiableMap may  From the JavaDoc      An immutable  hash-based Map with reliable user-specified iteration order  Does not permit null keys or values       Unlike Collections unmodifiableMap java util Map   which is a view of a separate map which can still change  an instance of ImmutableMap contains its own data and will never change  ImmutableMap is convenient for public static final maps   constant maps   and also lets you easily make a  defensive copy  of a map provided to your class by a caller

User · Answer

Have a look at ImmutableMap JavaDoc  doc  There is information about that there      Unlike Collections unmodifiableMap java util Map   which is a view of a separate map which can still change  an instance of ImmutableMap contains its own data and will never change  ImmutableMap is convenient for public static final maps   constant maps   and also lets you easily make a  defensive copy  of a map provided to your class by a caller

[java] UnmodifiableMap (Java Collections) vs ImmutableMap (Google)

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