Getting 'null' when concatenating chars into string array

When this line execute

String [] stringArr = new String [array.length];

It create an array and initialize array element with null. So when you perform concat(stringArr[i] += "") it add null then data.

You need to change your method like this-

public static String [] charToString(char [][] array)
    {
        String [] stringArr = new String [array.length];

        for (int i = 0; i < array.length; i++) 
        {
            if(stringArr[i]==null){
                stringArr[i]="";
            }
            stringArr[i] += "";
            for (int j = 0; j < array[i].length; j++)
            {
                stringArr[i] += array[i][j];
            }
        }
        return stringArr;
    } 

This is not perhaps an exact fix for your code, but can take advantage of the String constructor which accepts a character array as input.

char [][] charArr = {
    {'a','b','c'},
    {'d','e','f'},
    {'g','h','i'}
};

StringBuilder sb = new StringBuilder();
for (int i=0; i < charArr.length; ++i) {
    if (i > 0) sb.append("\n");
    sb.append(new String(charArr[i]));
}
System.out.println(sb);

abc
def
ghi

// < java 8
public static String[] charToString(char[][] arr) {
    String[] res = new String[arr.length];

    for (int i = 0; i < arr.length; i++)
        res[i] = new String(arr[i]);

    return res;
}

// >= java 8
public static String[] charToString(char[][] arr) {
    return Arrays.stream(arr).map(String::new).toArray(String[]::new);
}

Input:

{
    { 'a', 'b', 'c' },
    { 'd', 'e', 'f' },
    { 'g', 'h', 'i' }
}

Output:

abc
def
ghi

Notes:

1. Bad practice to concatenate strings with loop; do use StringBuilder instead;
2. It is better to use out-of-box method, like. new String(char value[]) instead of build it using StringBuilder.
3. Streams is good, but sometimes it is too hard to read; do not hesitate to use POJO and loops in case it looks more readable.

Using streams one could update the method as:

private static String [] charToString(char[][] array) {
    return Arrays.stream(array) //forms a Stream<char[]> (all your rows)
            .map(String::new) // maps the char[] to String forming Stream<String>
            .toArray(String[]::new); // converts the stream to array String[]
}

and further use it as :

char[][] charArr =
            {{'a', 'b', 'c'},
                    {'d', 'e', 'f'},
                    {'g', 'h', 'i'}
            };

Arrays.stream(charToString(charArr)).forEach(System.out::println);

Output

abc
def
ghi