Samling av studenter i ett univeristet

Hej! 

Jag har fått följande error:

Exception in thread "main" java.lang.Error: Unresolved compilation problem:
files cannot be resolved

at University.listAllFiles(University.java:49)
at University.main(University.java:61)

Jag vet inte vad problemet är 

Min kod är följande:

import java.util.ArrayList;

public class University {

private ArrayList<String> students;

public University() {

students = new ArrayList<>();

}

public void addStudents(String addStudents) {
if( addStudents.length() > 0) {
students.add(addStudents); }

}

public int amountOfStudents() {
System.out.println(students.size());
return (students.size());

}
public void removeStudents(int index) {
if( index < students.size() && index >= 0) {
students.remove(index); }

}

public String getNameOfStudentWithTheIndex(int index) {

return (students.get(index));
}
public boolean theStudentIsHere(String name) {

return students.contains(name);

}

public void theUniversityIsclosed() {

students.clear();
}
public void listAllFiles() {
System.out.println(files.get(0));
}

public static void main(String[] args) {
University KTH = new University();
University MTH = new University();
KTH.addStudents("Zayn");
KTH.addStudents("Sara");
KTH.addStudents("Albert");
KTH.listAllFiles();

}
}