How to return a list of keys from a Hash Map?

Question

I'm currently trying to make a program that conjugates verbs into Spanish. I've created a Hash Table that contains a key and an instantiation of the object Verb. The key is a string that has the infinitive form of the verb (for example, "hablar"). This is the code that I have so far for the hash map:

public class VerbHashMap {

    HashMap<String, Verb> verbHashMap;

    public VerbHashMap(){
        verbHashMap = new HashMap();
    }   
}

Each verb's key in the HashMap is based on the infinitive form of the verb. For example, the string "hablar" is the key for a Spanish verb. The class Verb has a method called getInfinitive() which returns a string that contains the infinitive form of the verb.

public boolean addVerb(Verb verb){
    if(verbHashMap.containsValue(verb.getInfinitive()){
        return false;
    }
    else{
        verbHashMap.put(verb.getInfinitive(), verb);
        return true;
    }
}

The question is what is the most efficient way to create a method that returns a list of all the verbs in the Hash Map in alphabetical order? Should I have the method return an ArrayList which includes the keys of all the objects in the Hash Map? Or is there a much more efficient way to go about this?

The way your `addVerb()` method checks for the existence of the value first, shouldn't the verb be the key? — Marcelo, Jul 05 '11 at 22:55
This is probably the simplest question I've asked on SO, yet it has gotten me by far the most upvotes. — idungotnosn, Aug 16 '19 at 16:35

score 110 · Accepted Answer · answered Jul 05 '11 at 22:57

110

Use the keySet() method to return a set with all the keys of a Map.

If you want to keep your Map ordered you can use a TreeMap.

answered Jul 05 '11 at 22:57

Marcelo

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How can I get a List of keys and not a Set ? Because I need to access the individual keys after the Map is made. – gonephishing Jun 17 '15 at 09:59
8

@gonephishing You can use the set as a parameter for the constructor of an ArrayList for example.. – Marcelo Jun 24 '15 at 18:51

score 104 · Answer 2 · edited May 23 '17 at 12:03

104

Using map.keySet(), you can get a set of keys. Then convert this set into List by:

List<String> l = new ArrayList<String>(map.keySet());

And then use l.get(int) method to access keys.

PS:- source- Most concise way to convert a Set<String> to a List<String>

edited May 23 '17 at 12:03

Community

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answered May 12 '16 at 07:43

Anu

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Brilliant! Works like a chamo – Michael Sims Dec 03 '18 at 09:12
this will raise a warning, no? – Martin Meeser Dec 13 '19 at 10:01

score 14 · Answer 3 · answered Mar 05 '20 at 18:52

14

List<String> yourList = new ArrayList<>(map.keySet());

This is will do just fine.

answered Mar 05 '20 at 18:52

rajat meena

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Cengiz · Answer 4 · 2020-01-07T09:32:46.380

9

Since Java 8:

List<String> myList = map.keySet().stream().collect(Collectors.toList());

edited Jan 07 '20 at 09:32

answered Jan 07 '20 at 09:26

Cengiz

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score 9 · Answer 5 · answered Jul 05 '11 at 22:57

9

map.keySet()

will return you all the keys. If you want the keys to be sorted, you might consider a TreeMap

answered Jul 05 '11 at 22:57

Kal

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score -1 · Answer 6 · answered Jul 20 '19 at 11:15

 for(int i=0;i<ytFiles.size();i++){

                    int key = ytFiles.keyAt(i);
                    Log.e("key", String.valueOf(key));
                    String format = ytFiles.get(key).getFormat().toString();
                    String url = ytFiles.get(key).getUrl();
                    Log.e("url",url);
                }

you can get key by method keyat and you have to pass the index then it will return key at that particular index. this loop will get all the key

How to return a list of keys from a Hash Map?

6 Answers6

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