How To Open Files When In Classes Instead Of Main Code

I have looked over other people code relating to files that are related to my actual studies in class. My teacher hasn't worked on a lot of file problems so I want to gain some understanding on it.

1.) I have this class with variables declared

import java.io.*;
public class FileDisplay {
private String filename;
FileWriter outputfile;
File Inputfile;

I'm aware that having a private variable would not allow that variable to be accessed outside the code. Vice versa, public variable would do that. The FileWriter and the File Inputfile is something I don't understand, especially have them in classes. What is the purpose of them?

2.)

public class FileDisplay {
private String filename;
FileWriter outputfile;
File Inputfile;
 
[Code] ....

I have made this code here that opens the output and input file. If I make another method, how would I display the text onto that same file. Hard to clarify for me, but in each method do I just keep creating :

FileWriter outputfile=new Filewriter(filename)