Question

 FileInputStream fis = new FileInputStream("./abc.txt");

I want to replace the quoted value by a string

I also want to fill the string with different filenames in a switch statement so that I can open different files and perform repeated operations on the files data.

how do I go about this?

Answer 1

FileInputStream fis = new FileInputStream(filename);

Note that there is no String.valueOf(String). If there was, it would be pointless as it would likely just return the String you passed it.