Is there a way in Java for taking input directly in charArray?

I am using BufferedReader for taking input.

String s = new BufferedReader(new InputStreamReader(System.in)).readLine();

Here, in next part of the program, I may manipulate or change characters of above String, so as strings are immutable in Java, I either convert it to StringBuilder or charArray. I want to avoid that O(n) conversion for converting String to StringBuilder or charArray.
Is there any way for directly taking input in charArray or StringBuilder?

It you know how many characters there are, you can read the entire stream into a char array like this:

char[] chars = new char[nosChars];
BufferedReader br = new BufferedReader(source);
int nosRead = br.read(chars, 0, nosChars);
// Then do another read to check that you have read everything ...

If the input is actually a file and the file encoding is one that allows you to calculate the character count from the file size in bytes, then the above can be made to work.

However, if you don't know what nosChars is, or don't have a reliable but reasonable overestimate, then anything you can do will either involve an extra copying step or passing over the stream (somehow) twice to work out how big the char[] needs to be. Both are O(N).

But I suspect you will find that reading the input into a char[] is a bad idea. Editing data in a char[] (or a StringBuilder wrapped around a char[]) is liable to be O(N) for each character you insert or delete ... and N is the length of the entire input file. For many edits, it will be more efficient to start with a list of lines represented as String objects.

Finally, you should heed the standard advice about optimization. Don't optimize until:

• you know there is a real performance problem
• you know where the performance problem is
• you have a good way of measuring whether your optimization is effective.

Try this. Tell me if it's worked!

        Scanner sc = new Scanner(System.in);

        
        //Say you want store 5 letters
        char [] lettrs = new char[5];
        
        for (int i = 0; i < lettrs.length;i++){
            //This message will show until the loop finish
            System.out.println("Write one letter");
            //Taking input
            char c = sc.next().charAt(0);
            //storing input letter to array
            lettrs[i] = c;
        }

        //This for is to print the letters you stor in to the array
        for (int i = 0; i <lettrs.length ; i++) {
            System.out.println(lettrs[i]);
        }

With BufferedReader and dynamic char array

BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        System.out.println("Write a letter");
        char c = (char)br.read();
        char [] lettrs = {c};