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?

Why are you converting int to String before putting it in the Map? — Sanjay T. Sharma
– Sanjay T. Sharma, Commented Sep 14, 2011 at 7:22
Yes, integers are perfectly fine for being used as keys in a Map. It's just that you will be putting an Integer instead of the primitive int in the Map but I don't think that should be a problem for you. Also, auto-boxing will help you cut down the conversion from int to Integer call. — Sanjay T. Sharma
– Sanjay T. Sharma, Commented Sep 14, 2011 at 7:26

dacwe · Accepted Answer · 2011-09-14 07:29:26Z

11

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

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());

Or you need to loop:

ArrayList<Integer> intKeys = new ArraList<Integer>();

for (String stringKey : valueHashMap.keySet())
     intKeys.add(Integer.parseInt(stringKey);

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).

edited Sep 14, 2011 at 7:29

answered Sep 14, 2011 at 7:22

dacwe

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2 Comments

Joachim Sauer Over a year ago

+1 using Long will help avoid the Year 2038 Problem.

stefana Over a year ago

There are some mistakes in option 2

pablochan · Accepted Answer · 2011-09-14 07:28:58Z

1

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));
}

edited Sep 14, 2011 at 7:28

answered Sep 14, 2011 at 7:22

pablochan

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3 Comments

Joachim Sauer Over a year ago

That's parsing and not casting!

pablochan Over a year ago

@Joachim Sauer: That's because my answer covered the general case (where you can sometimes cast) and my code was for this specific instance of the problem. Fixed it anyway to avoid confusion.

Joachim Sauer Over a year ago

the problem is that many people call any kind of type conversion "casting" and that is wrong and confuses people even further.

JB Nizet · Accepted Answer · 2011-09-14 07:28:01Z

0

You really have type problems. Why do you change the longs into Strings to store them in a map. Why not simply use Long, which needs less memory and is more descriptive. Then why use Integer.toString to transform a long into a String? By casting your long to an int, you risk loosing information by. Here's how the code should probably look like:

private Map<Long, People> valueHashMap = new Hashmap<Long, People>();

long timeSinceEpoch = System.currentTimeMillis()/1000;
valueHashMap.put(timeSinceEpoch, people_obj);
List<Long> longKeys = new ArrayList<Long>(valueHashMap.keySet());

answered Sep 14, 2011 at 7:28

JB Nizet

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Comments

Pokuri · Accepted Answer · 2011-09-14 07:29:04Z

0

You can use org.apache.commons.collections.Transformer class for that as follows.

List<Integer> intKeys  = (List<Integer>)CollectionUtils.collect(valueHashMap.keySet(), new Transformer() {
                                    @Override
                                    public Object transform(Object key) {
                                        return Integer.valueOf(key);
                                    }
                                }, new ArrayList<Integer>());

answered Sep 14, 2011 at 7:29

Pokuri

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