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?

Solution 1

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.

Solution 2

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>

Solution 3

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

This is will do just fine.

Solution 4

map.keySet()

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

Solution 5

Since Java 8:

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

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Comments

[gonephishing]

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.

[Marcelo]

@gonephishing You can use the set as a parameter for the constructor of an ArrayList for example..

[Michael Sims]

Brilliant! Works like a chamo

[Martin Meeser]

this will raise a warning, no?