One liner with java-8:

String str = "testString";

//[t, e, s, t, S, t, r, i, n, g]
Character[] charObjectArray = 
    str.chars().mapToObj(c -> (char)c).toArray(Character[]::new); 

What it does is:

• get an IntStream of the characters (you may want to also look at codePoints())
• map each 'character' value to Character (you need to cast to actually say that its really a char, and then Java will box it automatically to Character)
• get the resulting array by calling toArray()

You can do that like this

char[] allchar = new char[total]; // Your code till here is proper

// Copying the contents of the 2d array to a new 1d array
int counter = 0; // Counter as the index of your allChar array
for (int i = 0; i < a1.length; i++) { 
    for (int j = 0; j < a1[i].length; j++) { // nested for loop - typical 2d array format
        allchar[counter++] = a1[i][j]; // copying it to the new array
    }
}

Here's a solution that:

• doesn't use any imports – except for java.util.Arrays so that it can print out the character array for verification, but that's only for this answer, doesn't have anything to do with the code solution itself.
• builds an array of Character, and not java.lang.String as you posted in your code – your expected output ["h", "e", "l", "l", "o"] could be an array of Strings (double-quotes) but also looks like it would work as characters

import java.util.Arrays;

String s = "hello";
Character[] chars = new Character[s.length()];
for (int i = 0; i < s.length(); i++) {
    chars[i] = s.charAt(i);
}
System.out.println("chars[]: " + Arrays.toString(chars));

Here's the output:

chars[]: [h, e, l, l, o]

String strName = "name";
String[] strArray = new String[] {strName};
System.out.println(strArray[0]); //prints "name"

The second line allocates a String array with the length of 1. Note that you don't need to specify a length yourself, such as:

String[] strArray = new String[1];

instead, the length is determined by the number of elements in the initalizer. Using

String[] strArray = new String[] {strName, "name1", "name2"};

creates an array with a length of 3.

String text = String.copyValueOf(data);

or

String text = String.valueOf(data);

is arguably better (encapsulates the new String call).