java Get hashmap keys as integer array

Question

I have a hashmap like this

public HashMap <String,People> valueHashMap  = new Hashmap();

Here the key to my HashMap is time in seconds as string, ie I am adding value to hashmap like this

long timeSinceEpoch = System.currentTimeMillis()/1000;
valueHashMap.put(
                   Integer.toString((int)timeSinceEpoch)
                   , people_obj
                );

Now I want to get all keys in the hashmap into an array list of integer.

ArrayList<Integer> intKeys = valueHashMap.keys()...

Is there any way to do that?

Solution 1

There is no direct way of converting a list of Strings to a list of Integers:

1. Either you need to redefine your valueHashMap like this:

   public HashMap<Integer, People> valueHashMap  = new HashMap<Integer, People>();
   
   ....
   
   ArrayList<Integer> intKeys = new ArrayList<Integer>(valueHashMap.keySet());
   

2. Or you need to loop:

   ArrayList<Integer> intKeys = new ArraList<Integer>();
   
   for (String stringKey : valueHashMap.keySet())
        intKeys.add(Integer.parseInt(stringKey);
   

3. I would advice you however to use the Long as key instead:

   public HashMap<Long, People> valueHashMap  = new HashMap<Long, People>();
   

   then there would be no casting to int (and you can use (1) above with Long instead).

Solution 2

You can't cast a List of one type to a List of another type, so you have to iterate through the keys and parse each one.

for(String k : valueHashMap.keySet()) {
    intKeys.add(Integer.valueOf(k));
}