How to freeze a HashMap to prevent further changes?

Question

The question is pretty much self-explanatory. I have a data structure (I mentioned a HashMap but it could be a Set or a List also) which I initially populate:

Map<String, String> map = new HashMap<String, String>();

for( something ) {
  map.put( something );
}

After the structure has been populated, I never want to add or delete any items:

map.freeze();

How could one achieve this using standard Java libraries?

If you are always populating it directly after initializing, why not use static initialization? [Here](http://stackoverflow.com/questions/507602/how-to-initialise-a-static-map-in-java) is a good example of how to do that. — colti, Jun 11 '13 at 19:09

Petr Janeček · Accepted Answer · 2013-06-12T08:36:26.473

20

The best you can do with standard JDK libraries is Collections.unmodifiableMap().

Note that you must drop the original map reference, because that reference can still be accessed and changed normally. If you passed the old reference to any other objects, they still will be able to change your map.

Best practice:

map = Collections.unmodifiableMap(map);

and make sure you didn't share the original map reference.

edited Jun 12 '13 at 08:36

answered Jun 11 '13 at 19:04

Petr Janeček

37,768
12
121
145

4

Not quite correct. If you're concerned that someone might mutate the original map, you can make a defensive copy of it. `map - Collections.unmodifiableMap(new HashMap(map));` Of course, this still leaves the question of whether any of the objects in the map are themselves mutable. – David Conrad Jun 11 '13 at 19:49

score 7 · Answer 2 · edited Jun 11 '13 at 19:14

7

It sounds like you would do very well with Guava's ImmutableMap. Which allows use of the Builder pattern to assemble and "freeze".

edited Jun 11 '13 at 19:14

Louis Wasserman

191,574
25
345
413

answered Jun 11 '13 at 19:06

Deadron

5,135
1
16
27

1

And this is the best solution there is if you can use 3rd party libraries. – Petr Janeček Jun 11 '13 at 19:08
5

Yes, but the questions asks: *How could one achieve this using **standard Java libraries**?* – arshajii Jun 11 '13 at 19:10
8

Sometimes the answer you want is not the answer you need. – Deadron Jun 11 '13 at 19:11
Please don't link to the old google-collections site; use the guava-libraries site. – Louis Wasserman Jun 11 '13 at 19:15

score 3 · Answer 3 · answered Jun 11 '13 at 19:05

Wrap it in a class and make it immutable. For example:

public class ImmutableMapWrapper {

    private Map<String, String> map = new HashMap<String, String>();

    public ImmutableMapWrapper() {
        for( something ) {
            this.map.put( something );
        }
    }
}

score 1 · Answer 4 · answered Jun 11 '13 at 19:06

1

Create an immutable HashMap:

HashMap <MyKey, MyValue> unmodifiableMap = Collections.unmodifiableMap(modifiableMap);

JavaDoc here.

Also, I think the Google data collections utils (Guava???) has an ImmutableMap type already.

answered Jun 11 '13 at 19:06

CodeChimp

8,016
5
41
79

How to freeze a HashMap to prevent further changes?

4 Answers4