Tell me more ×
Stack Overflow is a question and answer site for professional and enthusiast programmers. It's 100% free, no registration required.
up vote 23 down vote favorite
3

I need to convert a HashMap<String, Object> to an array; could anyone show me how it's done?

share|improve this question
3  
you want the keys, the values, or both? – harto Jul 7 '09 at 5:47

7 Answers

up vote 33 down vote accepted 
 hashMap.keySet().toArray(); // returns an array of keys
 hashMap.values().toArray(); // returns an array of values

Edit

Should be noted that the ordering of both arrays may not be the same, See oxbow_lakes answer for a better approach for iteration when the pair key/values are needed.

share|improve this answer
1  
beat me to it :( – jrharshath Jul 7 '09 at 5:48
gotta get the low hanging fruit where i can find it. :) – landon9720 Jul 7 '09 at 5:49
1  
@CrackerJack9 can you explain? – Jakobud Nov 2 '11 at 15:27
4  
The keys wont correspond to their values across the two arrays. – landon9720 Nov 2 '11 at 18:50
2  
@Jakobud landon9720 is correct...the order is psuedo-random, and it cannot be guaranteed that key[0] will correspond to value[0] after you convert the keys to a Set and the values to a Collection. While they are technically converted to arrays (and answers your question), the concept of the key-value pair has been lost - which is why this is a very misleading (and dangerous) answer.... – CrackerJack9 Nov 3 '11 at 6:43
show 4 more comments
up vote 30 down vote

If you want the keys and values, you can always do this via the entrySet:

hashMap.entrySet().toArray(); // returns a Map.Entry<K,V>[]

From each entry you can (of course) get both the key and value via the getKey and getValue methods

share|improve this answer
+1 that's exactly what I was looking for! – Kenny Cason Aug 23 '10 at 19:25
up vote 13 down vote

If you have HashMap<String, SomeObject> hashMap then

hashMap.keySet().toArray();

will return an Object[]. If instead you want an array of the type SomeObject, you could use:

hashMap.keySet().toArray(new SomeObject[0]);

share|improve this answer
I think you mean values() instead of keySet() for an array of SomeObject. – Paul Bellora Feb 8 '12 at 4:33
that is very, very useful! Thank you! – seinecle Mar 30 '12 at 8:48
You can also improve performance by specifying the array size upfront instead of using 0. See example here: stackoverflow.com/a/5061692/265877 – Alex Apr 6 at 21:33
up vote 7 down vote

To guarantee the correct order for each array of Keys and Values, use this (the other answers use individual Sets which offer no guarantee as to order.

Map<String, Object> map = new HashMap<String, Object>();
String[] keys = new String[map.size()];
Object[] values = new Object[map.size()];
int index = 0;
for (Map.Entry<String, Object> mapEntry : map.entrySet()) {
    keys[index] = mapEntry.getKey();
    values[index] = mapEntry.getValue();
    index++;
}
share|improve this answer
up vote 3 down vote

An alternative to CrackerJacks suggestion, if you want the HashMap to maintain order you could consider using a LinkedHashMap instead. As far as im aware it's functionality is identical to a HashMap but it is FIFO so it maintains the order in which items were added.

share|improve this answer
1  
Very good point, thanks for that insight, helped me out! :) – mgibson Feb 11 at 23:25
up vote 2 down vote 
Map<String, String> map = new HashMap<String, String>();
map.put("key1", "value1");
map.put("key2", "value2");

Object[][] twoDarray = new String[map.size()][2];

Object[] keys = map.keySet().toArray();
Object[] values = map.values().toArray();

for (int row = 0; row < twoDarray.length; row++) {
    twoDarray[row][0] = keys[row];
    twoDarray[row][1] = values[row];
}

for (int i = 0; i < twoDarray.length; i++) {
    for (int j = 0; j < twoDarray[i].length; j++) {
        System.out.println(twoDarray[i][j]);
    }
}
share|improve this answer
up vote 0 down vote

You may try this too.

public static String[][] getArrayFromHash(Hashtable<String,String> data){
        String[][] str = null;
        {
            Object[] keys = data.keySet().toArray();
            Object[] values = data.values().toArray();
            str = new String[keys.length][values.length];
            for(int i=0;i<keys.length;i++) {
                str[0][i] = (String)keys[i];
                str[1][i] = (String)values[i];
            }
        }
        return str;
    }

Here I am using String as return type. You may change it to required return type by you.

share|improve this answer

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.