How to update a value for a key in HashMap?

Question

I am having HashMap like this,

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

I am trying to do before adding an element in map,

1. Want to check whether the key exist or not, i can get it by using map.containsKey().
2. If the key exist, i want check the size of Set respective to that key.
3. If size <= 1 i want add an element in that set.

Solution 1

I wouldn't use containsKey and get as this means two lookups when you only need one.

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

Set<String> set = map.get(key);
if(set != null && set.size() <= 1) 
    set.add(some$value);

The only problem with this is that the value will always be null unless you set it somewhere so what you may want is

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

Set<String> set = map.get(key);
if(value != null)
    map.put(key, set = new HashSet<String>());
if (set.size() <= 1) 
    set.add(some$value);

It is unusual to have a set with a maximum size of 2. Is there any reason for this?

Solution 2

Sounds like this:

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

Set<String> value = map.get("key");
if(value != null) {        
    if(value.size() <= 1) {
        value.add("some value");
    }
} else {
    map.put("key", new HashSet<String>());
}

Now, either the last point was poorly worded (i.e. you want to update the Set associated with the key) or you really want to update the key itself, in which case you'd probably have to just remove it and add a new entry.

Solution 3

You could get the set from the map with map.get(String key).

Then test the size of the Set. If needed, add your element.

Now you can simply remove the old set from the map with map.remove(String key) and reinsert it with put(String, Set);

Comments

[Dominik Sandjaja]

if (map.containsKey("x")) {Set<String> s = map.get("x"); if (s.size() <= 1) { s.add("y");}} (oh, just misread that the key should be updated - why?!)

[Luiggi Mendoza]

Why would you want to update the key? You can't and you shouldn't think on do it

[David Grant]

I know the OP asked for it, but it would make more sense if value.size() was being compared with 0. But have an upvote anyway ;)

[Tudor]

@David Grant: You mean == 0 instead of <= 1?

[Tudor]

@David Grant: Dunno what he actually wants to do here. It's possible he wants to add something if the size is 0 or 1. I just left his original code.

[Fildor]

Would it improve something to just map.get(key) and then check for null, since I guess if there is no such key it should be added, right?

[Tudor]

@Fildor: Yeah, I guess it would be more efficient, since only a single lookup is done.

[Vinesh]

I want to store only 2 values for a key... So i have set with a maximum size of 2.