Convert HashMap key value (String) to Vector <String>?

Question

I have a hashmap declared as :

HashMap<String, Double> hm = new HashMap<String, Double>();

I'm declaring a Vector as :

 Vector<String> productList = new Vector<String>();

Now, I'm trying to add the keys into the vector :

  Set set = hm.entrySet();
  // Get an iterator
  Iterator i = set.iterator();
  // Display elements
  while(i.hasNext()) {
     Map.Entry me = (Map.Entry)i.next();
     //System.out.print(me.getKey() + ": ");
     productList.add(me.getKey());
     //System.out.println(me.getValue());
  }

//hm is the HashMap holding the keys and values.

When I compile the code it gives the following error:

ProductHandler.java:52: error: no suitable method found for add(Object)
     productList.add(me.getKey());
                ^
method Collection.add(String) is not applicable
  (argument mismatch; Object cannot be converted to String)

Do we need to convert the values to String type before trying to add it to the vector? Am I missing out on something?

You can use Vector<String> productList = new Vector<String>(hm.keySet()); — αƞjiβ
– αƞjiβ, Commented Apr 2, 2016 at 2:43

Elliott Frisch · Accepted Answer · 2016-04-02 02:37:35Z

5

First, you can do it in one line with the Vector(Collection<? extends E) constructor and a call to Map.keySet() like

Vector<String> productList = new Vector<>(hm.keySet());

Second, you should prefer an ArrayList¹

List<String> productList = new ArrayList<>(hm.keySet());

^{1_{Unless you're sharing this list with multiple threads, adding synchronization with a Vector is a cost.}}

answered Apr 2, 2016 at 2:37

Elliott Frisch

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3 Comments

221b Over a year ago

Thanks that worked! Just curious, how does a Vector map the values of a Set after the hm.keySet() returns the keys?

Elliott Frisch Over a year ago

Here it's equivalent to adding all of the elements in the keySet to the Collection.

221b Over a year ago

Got it! Thanks! :)

Eran · Accepted Answer · 2016-04-02 02:37:09Z

2

Don't use raw types.

Your code can be much simpler with enhanced for loop :

  for(String key : hm.keySet()) {
     productList.add(key);
  }

or

  for(Map.Entry<String,Double> entry : hm.entrySet()) {
     productList.add(entry.getKey());
  }

answered Apr 2, 2016 at 2:37

Eran

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