Converting HashMap into Array

Question

I have a HashMap which I want to convert into a array. Which the code below, I get following: [[2, 11, 10, 9], [value1, value2, value3, value4], [null, null, null, null], [null, null, null, null]] The "null" entries give me a null pointer exception. Why is it two times as big as it should be? I just want the real entries: [[2, 11, 10, 9], [value1, value2, value3, value4]]. What did I wrong?

String[][] test = getArrayFromHash(hashmap);

    public static String[][] getArrayFromHash(HashMap<String, String> hashMap){
        String[][] str = null;
        {
            Object[] keys = hashMap.keySet().toArray();
            Object[] values = hashMap.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;
    }

Thanks!

score 4 · Accepted Answer · edited May 23 '17 at 12:27

4

Refer this link:::

    String[][] arr = new String[hashmap.size()][2];
    Set entries = hashmap.entrySet();
    Iterator entriesIterator = entries.iterator();

    int i = 0;
    while(entriesIterator.hasNext()){

        Map.Entry mapping = (Map.Entry) entriesIterator.next();

        arr[i][0] = mapping.getKey().toString();
        arr[i][1] = mapping.getValue().toString();

        i++;
    }

UPdated:::: to get two 1D arrays

String[] arr1 = new String[hashmap.size()];
String[] arr2 = new String[hashmap.size()];
        Set entries = hashmap.entrySet();
        Iterator entriesIterator = entries.iterator();

        int i = 0;
        while(entriesIterator.hasNext()){

            Map.Entry mapping = (Map.Entry) entriesIterator.next();

            arr1[i] = mapping.getKey().toString();
            arr2[i] = mapping.getValue().toString();

            i++;
        }

edited May 23 '17 at 12:27

Community

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1

answered May 26 '12 at 09:28

Shankar Agarwal

34,573
7
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Thanks a lot! How can I make Strings from these Objekts? I want to put all values into a Spinner, therefore I need Strings. – user1390816 May 26 '12 at 09:59
I get this exception:05-26 14:02:27.473: W/System.err(22784): java.lang.ClassCastException: [[Ljava.lang.Object; – user1390816 May 26 '12 at 12:01
can you post your complete logcat? – Shankar Agarwal May 26 '12 at 12:17
mapping.getKey().toString(); replace with mapping.getKey(); and try – Shankar Agarwal May 26 '12 at 12:18
The Error was on the conversion from the 2D-array to a 1D array. This was the code: for(int i = 0, k = 0; i < test.length; i++) for(int j = 0; j < test[0].length; j++, k++) colors[k] = test[i][j]; – user1390816 May 26 '12 at 12:19
you need 2D array or two 1D arrays? – Shankar Agarwal May 26 '12 at 12:21
I need two 1D- arrays, but the first try was one 1D-array – user1390816 May 26 '12 at 12:22

score 3 · Answer 2 · answered May 26 '12 at 09:30

3

Allocate array of proper size like

str = new String[2][values.length];

Moreover you should assert that values and keys have the same length. And also that matching entries appear at the same position. Something like

String[] keys = hashMap.keySet().toArray(new String[0]);
String[] values = new String[keys.length];
String[][] str = {keys, values};
for(int i=0;i<keys.length;i++) values[i] = hashMap.get(keys[i]);
return str;

should do (not tested).

answered May 26 '12 at 09:30

maaartinus

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Short, concise, and it looks like it will work to me. And just in case you didn't see my prev comment, I deleted my answer. I realized that Sets don't maintain order, and wasn't sure if I could cast to LinkedHashSet and maintain order before the cast. I see you address that here by using the keys in the keys array to get the actual values from the map itself. Very clever. +1 – jamesmortensen May 26 '12 at 09:41
Thx. Using an `EntrySet` may be faster. It has the overhead of creating `Map.Entry`, but eliminates the lookups. – maaartinus May 27 '12 at 11:19

score 3 · Answer 3 · answered May 26 '12 at 09:31

I'm not sure if keyset and values will necessary return the values in the same order (it's not specified in the implementation doc so I'd say it's not guaranteed).

try working with entrysets:

Set<Map.Entry<String, String>> set = hm.entrySet();
int i = 0;
for (Map.Entry<String,String> me : set) {
  arr[i][0] = me.getKey();
  arr[i][1] = me.getValue()};
  i++;
}

Converting HashMap into Array

3 Answers3