HashMap<String, Integer> Search for part of an key?

Question

I am currently using HashMap<String, Integer> which is filled with keys of type String which are all, let's say, 5 chars long. How can I search for an specific key of 4 chars or less, which is part and at the beginning of some other keys and get all hits as a collection of <Key, Value>?

Solution 1

Iterate is your only option unless you create a custom data structure:

for (Entry<String, Integer> e : map.entrySet()) {
    if (e.getKey().startsWith("xxxx")) {
        //add to my result list
    }
}

If you need something more time efficient then you'd need an implementation of map where you are tracking these partial keys.

Solution 2

It seems like a use case for TreeMap rather than HashMap. The difference is that TreeMap preserves order. So you can find your partial match much quicker. You don't have to go through the whole map.

Check this question Partial search in HashMap

Solution 3

You cannot do this via HashMap, you should write your own implementation for Map for implementing string length based searching in a map.

Solution 4

Map<String, Integer> result = new HashMap<String, Integer>;
for(String key : yourMap.keySet()) {
    if(key.length() == 4){
        result.put(key, yourMap.get(key);
    }
}

After executing this code you have all key/value pairs with 4 letter keys in result.

Solution 5

Set<Entry<String, Integer>> s1 = map.entrySet();
    for (Entry<String, Integer> entry : s1) {
          if(entry.getKey().length == 4)
          //add it to a map;
}

First get the entry set to your hashmap. Iterate through the set and check the length of each key and add it to a map or use it as u want it.

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Comments

[Javier Diaz]

You have to iterate through all the HashMap, use getKey.length() and add or not (depending on the condition) to a List<>

[vikingsteve]

Have you looked at keySet() ?

[Juned Ahsan]

Is it too tough to solve, if yes for you, then at least share what have u tried so far?

[Thomas Jungblut]

What you are looking for is called a Trie.

[Menelaos]

Question: he is looking for entries that have 4 characters instead of 5 characters? Or he is looking for a series of 4 characters which may be within 5 characters?

[VoidStar]

I edited my question. It should be clear now

[Menelaos]

@Machtl Is the 4 characters at the beginning or end of string)? Can they be anywhere in the string?

[Menelaos]

Unless ofcourse, he changes the requirements, uses the key with only the 4 characters, and stores the other data in an object in value. Another option is to use a custom Key Object, with custom hashCode() method.

[Michael Aaron Safyan]

I wouldn't advise rolling one's own Map<>. That's probably overkill (and, besides, composition is often better than inheritance). However, I agree that the data structure, as-is, is not designed to do this.

[cyborg]

You don't need to implement all of Map. It is pretty easy to add extra stuff if you use the decorator pattern. So for example I want to log out every key added to a map? Fine, I make my LoggingMap implement Map and then you have to supply the actual instance of the Map it's going to log out. That would work perfectly well in this case except instead of logging you'd keep track of the keys you want in a list or something along those lines.

[harsh]

Yes right Map implementation can be extension (extends) of HashMap or other exiting implementations, override only what is required. My point was for this question Map seems to be a right data-structure but with some modified get (for length based retrieval) and put (for optimal get say O(1) again). As all other answers shows external logic as a solution, having a specific Map would enforce encapsulating that logic inside map itself.