什么是不可修改的地图（真的）是必要的？

Question

I have a map of constants, like this:

private static Map<String, Character> _typesMap =
        new HashMap<String, Character>() {
        {
            put ("string", 'S');
            put ("normalizedString", 'N');
            put ("token", 'T');
            // (...)
        }

Do I really need to use Collections.unmodifiableMap() to create this map? What is the advantage of using it? Are there any disadvantages of not using it, besides the obvious fact that they are not really becoming constant?

Answer 1

Collections.unmodifiableMap guarantees that the map will not be modified. It's mostly useful if you want to return a read-only view of an internal map from a method call, eg:

class A {
    private Map importantData;

    public Map getImportantData() {
        return Collections.unmodifiableMap(importantData);
    }
}

This gives you a fast method that does not risk the client changing your data. It's much faster and more memory efficient than returning a copy of the map. If the client really does want to modify the returned value then they can copy it themselves, but changes to the copy won't be reflected in A's data.

If you are not returning map references to anyone else then don't bother making it unmodifiable unless you are paranoid about making it immutable. You can probably trust yourself to not change it.

Answer 2

Cameron Skinner's statement above that "Collections.unmodifiableMap guarantees that the map will not be modified" is actually only partly true in general, although it happens to be accurate for the specific example in the question (only because the Character object is immutable). I'll explain with an example.

Collections.unmodifiableMap actually only gives you protection that the references to objects held in the map cannot be changed. It does that by restricting the 'put' into the map that it returns. However, the original encapsulated map can still be modified from outside the class because Collections.unmodifiableMap does not make any copies of the contents of the map.

In the question posted by Paulo, the Character objects held in the map are luckily unmodifiable. However, in general this may not be true and the unmodifiability advertised by Collections.unmodifiableMap should not be the only safeguard. For instance, see the example below.

import java.awt.Point;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

public class SeeminglyUnmodifiable {
   private Map<String, Point> startingLocations = new HashMap<>(3);

   public SeeminglyUnmodifiable(){
      startingLocations.put("LeftRook", new Point(1, 1));
      startingLocations.put("LeftKnight", new Point(1, 2));
      startingLocations.put("LeftCamel", new Point(1, 3));
      //..more locations..
   }

   public Map<String, Point> getStartingLocations(){
      return Collections.unmodifiableMap(startingLocations);
   }

   public static void main(String [] args){
     SeeminglyUnmodifiable  pieceLocations = new SeeminglyUnmodifiable();
     Map<String, Point> locations = pieceLocations.getStartingLocations();

     Point camelLoc = locations.get("LeftCamel");
     System.out.println("The LeftCamel's start is at [ " + camelLoc.getX() +  ", " + camelLoc.getY() + " ]");

     //Try 1. update elicits Exception
     try{
        locations.put("LeftCamel", new Point(0,0));  
     } catch (java.lang.UnsupportedOperationException e){
        System.out.println("Try 1 - Could not update the map!");
     }

     //Try 2. Now let's try changing the contents of the object from the unmodifiable map!
     camelLoc.setLocation(0,0);

     //Now see whether we were able to update the actual map
     Point newCamelLoc = pieceLocations.getStartingLocations().get("LeftCamel");
     System.out.println("Try 2 - Map updated! The LeftCamel's start is now at [ " + newCamelLoc.getX() +  ", " + newCamelLoc.getY() + " ]");       }
}

When you run this example, you see:

The LeftCamel's start is at [ 1.0, 3.0 ]
Try 1 - Could not update the map!
Try 2 - Map updated! The LeftCamel's start is now at [ 0.0, 0.0 ]

The startingLocations map is encapsulated and only returned by leveraging Collections.unmodifiableMap in the getStartingLocations method. However, the scheme is subverted by getting access to any object and then changing it, as seen in "Try 2" in the code above. Suffice to say that one can only rely on Collections.unmodifiableMap to give a truly unmodifiable map IF the objects held in the map are themselves immutable. If they're not, we'd want to either copy the objects in the map or otherwise restrict access to the object's modifier methods, if possible.

Answer 3

Wrapping the Map is to ensure the caller won't change the collection. While this is useful in testing, you really should find this sort of bug there, it may not be so useful in production. A simple workaround is to have your own wrapper like.

public static <K,V> Map<K,V> unmodifiableMap(Map<K,V> map) {
   assert (map = Collections.unmodifiableMap(map)) != null;
   return map;
}

This only wraps the map when assertions are turned on.