HashMap Search for part of an key? [duplicate]

问题

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>?

回答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.

回答2:

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

Check this question Partial search in HashMap

回答3:

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

回答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.

回答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.

回答6:

With HashMap<String, Integer> you can only go through keySet() and do contains() for String keys and your pattern.

回答7:

As has been noted, there isn't a terribly efficient* way to do it with the datastructure you have specified. However, if you add an additional Map<Integer, List<String>> to keep track of the mapping from string length to the list of all keys with that length, then you will be able to do this very efficiently.

*Using just the Map<String, Integer>, you would need to iterate through the entire capacity of the larger map, whereas adding this supplemental datastructure would impose an O(1) lookup (assuming you used a HashMap) followed by iteration through just the result set, which is the fastest possible outcome.

回答8:

You can try this approach:

public Map<String,Integer> filterMap(Map<String, Integer> inputMap){
    Map<String, Integer> resultHashMap = new HashMap<String, Integer>();
        for (String key : inputMap.keySet()) {
            if(key.length()==5){
                resultHashMap.put(key,inputMap.get(key));
            }   
        }
        return resultHashMap;
    }

回答9:

For example:

public static void checkMap(Map mp) {
    Iterator it = mp.entrySet().iterator();
    while (it.hasNext()) {
        Map.Entry pairs = (Map.Entry)it.next();
        System.out.println(pairs.getKey().toLowerCase().contains("YourString"))
        }
}

This code will print true for those keys which contain your substring.

来源：https://stackoverflow.com/questions/16664808/hashmapstring-integer-search-for-part-of-an-key

标签

java

hashmap