Java：使用空键隐藏条目的地图视图？

Question

I have a Map<String,String> which contains a special value with a null key. It's not so easy to change things so I don't have this.

Is there a way to do either of the following:

• make a view of this map that is also a Map<String,String> which hides the null-key entry
• make a view of the map's keySet() which hides the null-key entry

By "view" I mean that changes in the map produce changes in the view.

I assume the latter is easier but I don't know how to do it.

Answer 1

The simple method is to extend your map and override the proper methods to check for null key values. These would be:

containsKey(Object key)
entrySet() 
get(Object key) 
keySet() 
put(K key, V value) 
putAll(Map<? extends K,? extends V> m) 

You could also implement your own object extending Map and pass your Map-with-null in the constructor and perform check/controls when one of the above methods are called.

If you are open to external libraries, you could take a look at PredicatedMap .

Answer 2

If the value with the null key is a special case and not meant to be used when you access the entries, why is it in the map? Is there a reason why you decided to put it in the map instead of having different string variable?

It sounds like it would be easier to use a different approach instead of trying to extend the map to have some special properties for this one case. This would also would be more explicit in showing that this string value is a special value and not something that could trip up future developers who have to look at this code after you.

Answer 3

I used Guava's Maps.filterKeys() ; that worked fine.

 Maps.filterKeys(mymap, new Predicate<String>() {
    @Override public boolean apply(String key) { return key != null; } 
 }),