Converting HashMap key values to Char Array in Java

Question

I have the following HashMap that I insert character-int pairs in a method and return this hashmap:

HashMap<Character, Integer> hmap = new HashMap<Character, Integer>();

for (...) {
    hmap.put(string[i], number[i]); 
}

However, when I want to convert this returned hashmap keys to char array, it includes also brackets and commas:

char[] charArray = myReturnedHashMap.keySet().toString().toCharArray(); 
//it returns 8 char "[ A , B , C , ]" instead of just "A B C"

So, how can I fix it?

Update: On the other hand, I am not sure if HashMap is a good idea to use in this scenario. I have a loop and I just need to return char and int value pairs. Then in the other method I convert char values to a chararray. Any idea?

Answer 1

Your problem is the call of 'keySet().toString()' - this creates the string representation of the map.

I think you want something like this:

Map<Character, Integer> hmap = new HashMap<Character, Integer>();
hmap.put('c', 1);
hmap.put('d', 2);
hmap.put('e', 3);

Character[] charArray = hmap.keySet().toArray(new Character[0]);

Edit: If you need the keys sorted, you can use a TreeMap instead of a HashMap. But be aware, if you are dealing with characters and expecting language specific sorting you need a Collator for sorting in the TreeMap.

Answer 2

This is an easy, very resuable (works with all kind of maps) and easily sortable solution:

char[] charArray = map.keySet().stream()
    .map(String::valueOf)
    .collect(Collectors.joining()).toCharArray();

Here's the fastest solution:

char[] chars = new char[map.size()];
int i = 0;
for (Character c : map.keySet()) {
    chars[i++] = c;
}

The keys in a HashMap aren't sorted. Use another map implementation or sort the stream .

Answer 3

You can use toArray() with the caveat that it'll return an Object[] array.

Object[] charArray = myReturnedHashMap.keySet().toArray();

test demo from jshell:

jshell> HashMap<Character, Integer> a = new HashMap<>()
a ==> {}

jshell> a.put('a', 1)
$11 ==> null

jshell> a.keySet()
$12 ==> [a]

jshell> a.keySet().to
toArray(     toString()   

jshell> a.keySet().toArray()
$13 ==> Object[1] { 'a' }

jshell> a.keySet().toArray()[0]
$14 ==> 'a'

jshell> (char)a.keySet().toArray()[0]
$15 ==> 'a'

Answer 4

The toString() in unnecessary in your case. You just want to take that Set and convert it to an Array. Something like this seems to work just fine:

    HashMap<Character, Integer> hmap = new HashMap<Character, Integer>();
    hmap.put('a',1);
    hmap.put('b',1);
    hmap.put('c',1);
    Set<Character> characters = hmap.keySet();
    Character[] chars = new Character[characters.size()];
    characters.toArray(chars);
    System.out.println(Arrays.toString(chars));

Answer 5

use toArray()

        Map<Character, Integer> map = new HashMap<Character, Integer>();
        map.put('a', 1);
        map.put('b', 1);
        Character[] charArray = map.keySet().toArray(Character[]::new);