UnmodifiableMap (Java Collections) vs ImmutableMap (Google)

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.

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.

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.

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<String, String> realMap = new HashMap<String, String>();
realMap.put("A", "B");

Map<String, String> 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<String, String> immutableMap = 
    Collections.unmodifiableMap(new LinkedHashMap<String, String>(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.