Question

public class TextFileExtractor{
    public static String[] fileExtractor(String[] s){

        StringBuilder sb=new StringBuilder();

            for(int i=0;i<=s.length;i++){
                if(s[i].endsWith(".txt")){
                    sb.append(s[i]);
                    sb.append(',');
                }
            }
        String str=sb.toString();
        String[] sa=str.split(",");

        return sa;
    }
}

import java.util.*;

class P8Test{

    public static void main(String[] args) 
    {   Scanner scn=new Scanner(System.in);
        System.out.print("Enter String Array:");
        String[] data=new String[(scn.nextLine()).length()];
        data=(scn.nextLine()).split(",");

            System.out.print("all Files");
                System.out.print(Arrays.toString(data));
                    System.out.print("Text Files");

        String[] res=TextFileExtractor.fileExtractor(data);
        System.out.print(Arrays.toString(res));
    }
}

Answer 1

You are out of array because in for you use i<=s.length but max index of array is s.length-1

But I am interested in this

String[] data = new String[(scn.nextLine()).length()];
data = (scn.nextLine()).split(",");

Why are you creating empty array and then replace it with new one?
I think you wanted to create something more like this

String[] data = scn.nextLine().split(",");

Earlier you ware creating array based on second line of user input because you ware invoking scn.nextLine() twice.