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
    }
}