How to create dynamic String Array if we dont know number of strings in the beginning?

Question

I have a HashMap<String, String>. If I want to create string array of hashmap.values(), we can create it as

String[] strArray = new String[hashmap.size()]

But my problem is if hashmap values contain "A,B,C" then I need to add A and B and C to strArray.

why not first count the number of needed items (traversing the hashmap once), and than actually create the array and put the values into it? — Vlad
– Vlad, Commented Jun 21, 2012 at 12:27
I guess noone will write you an email, the answers will come in here... ;) Also, can you tell us, what you are trying to do with this array? If this needs to dynamically change, you can just use one of the available collections (like ArrayList<String>). — brimborium
– brimborium, Commented Jun 21, 2012 at 12:27

Peter Lawrey · Accepted Answer · 2012-06-21 12:30:42Z

21

Use an ArrayList.

List<String> myList = new ArrayList<String>();

No matter what the size is of your HashMap you can easily work with your ArrayList.

If you need an array, you can use

String[] arr = myList.toArray(new String[myList.size()]);

when you have finished.

edited Jun 21, 2012 at 12:30

Peter Lawrey

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answered Jun 21, 2012 at 12:26

Kazekage Gaara

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4 Comments

Peter Lawrey Over a year ago

+1 If you need an array, you can use String[] arr = myList.toArray(new String[myList.size()]) when you have finished.

Kazekage Gaara Over a year ago

@PeterLawrey edit my answer and add this point as well please. :-)

Praveen Hiremath Over a year ago

I think you are not able to understand my question. It may be becos i may not put the question in rite words. Any Way i ll exxplain it again. For Ex: HashMap Values are

Praveen Hiremath Over a year ago

Thanks Peter.. I think you are not able to understand my question. It may be becos i would not put the question in rite words i think. Any Way i ll exxplain it again. For Ex: Map<String, String> map = new HashMap <String, String>(); map.put("1", "A,B,C"); map.put("2", "P,Q,R...,Z"); So i need to create the string Array with values (A,B,C,P,Q,R...,Z). Note that the comma separated values in Map may spread upto any length(wic is dynamic). Please give me the suggestions.

Peter Lawrey · Accepted Answer · 2012-09-03 14:34:54Z

2

You can take a copy of the values whenever you need an array.

Map<Double, String> map = ...
String[] values = map.values().toArray(new String[map.size()]);

If you change the map (even if the size doesn't change), the array won't change and you need to take another copy. Do the values need to be unique?

So i need to create the string Array with values (A,B,C,P,Q,R...,Z).

In that case it appears you want to do the following.

Map<Double, String> map = ...
List<String> valueList = new ArrayList<>();
for(String value: map.values())
   valueList.addAll(Arrays.asList(value.split(",")));
String[] values = valueList.toArray(new String[valueList.size()]);

edited Sep 3, 2012 at 14:34

answered Jun 21, 2012 at 12:32

Peter Lawrey

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4 Comments

Praveen Hiremath Over a year ago

Thanks Peter.. I think you are not able to understand my question. It may be becos i would not put the question in rite words i think. Any Way i ll exxplain it again. For Ex: Map<String, String> map = new HashMap <String, String>(); map.put("1", "A,B,C"); map.put("2", "P,Q,R...,Z"); So i need to create the string Array with values (A,B,C,P,Q,R...,Z). Note that the comma separated values in Map may spread upto any length(wic is dynamic). Please give me the suggestions.

Peter Lawrey Over a year ago

Do you want an element in the array for every String between the commas? If so, I have added a solution for that.

Praveen Hiremath Over a year ago

Yes Peter Lawrey. I want an element in the array for every String between the commas. To Build String Array I need knw the SIZE(dynamic). Im not able to find the SIZE.

Peter Lawrey Over a year ago

You don't need to know the size in advance, Using a List first you can determine the size before creating the array (as it does in the example)

John Kane · Accepted Answer · 2012-06-21 12:28:17Z

0

Does it need to be an array? With Maps you can get the key of value sets.

answered Jun 21, 2012 at 12:28

John Kane

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2 Comments

Praveen Hiremath Over a year ago

Thanks for responding John. yes it needs to be array. And the values in the HashMap is comma separated. Those comma seperated values are splitted by comma and added to String Array Separately.

John Kane Over a year ago

You could mimic ArrayList set a default initial size and if it is exceeded use the array copy in system (I think) to copy the values to a larger array. Its not the most effecient but you seem to need an array

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