JUnit 4 testing for a class that separates file

Question

I'm trying to write a test class with jUnit 4 to test Separating class. Separating class has 3 methods to open a file, separate its content into 3 files and close the file. The SeparatingTest doesn't work. It separates the content of the file but it doesn't pass the test. I am new to jUnit test. Can you help me with that please? separating class:

import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.*;
import java.util.*;

public class Separating {

    public static BufferedReader input;
    public static PrintWriter output14;
    public static PrintWriter output15;
    public static PrintWriter output16;
    
    
    public static void openFiles() throws FileNotFoundException, IOException {

        input= new BufferedReader(new FileReader("numbers.txt"));
        output14 = new PrintWriter(new FileOutputStream("numbers14.txt"));
        output15 = new PrintWriter(new FileOutputStream("numbers15.txt"));
        output16 = new PrintWriter(new FileOutputStream("numbers16.txt"));

    }

    public void separate(BufferedReader input, PrintWriter output14,
               PrintWriter output15, PrintWriter output16) throws IOException, InvalidUoBException {

        String line;

        while ((line = input.readLine()) != null) {

            if (line.startsWith("14")) {
                output14.write(line + "\n");
                count14++;

            } else if (line.startsWith("15")) {
                output15.write(line + "\n");
                count15++;

            } else if (line.startsWith("16")) {
                output16.write(line + "\n");
                count16++;
            } else {
                throw new InvalidUoBException("not 14, 15, 16");
            }
            

        }
    }

    public static void closeFiles() throws IOException {

        input.close();
        output14.close();
        output15.close();
        output16.close();
    }
    

    public static void main(String[] args) throws IOException {
        try {
            Separating br = new Separating();
            openFiles();
            br.separate(input, output14, output15, output16);
            closeFiles();

            System.out.println("Successfuly separated the file");
        } catch (InvalidUoBException e) {
            System.out.println("UoB number does not start with 14, 15 or 16!");
        } catch (FileNotFoundException e) {
            System.out.println("File was not found");
            System.out.println("or could not be opened.");

        } catch (IOException e) {
            System.out.println("Error reading from file.");

        }

    }
}

test class:


import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.FileReader;
import java.io.IOException;
import java.io.PrintWriter;
import static junit.framework.TestCase.assertEquals;
import org.junit.After;
import org.junit.AfterClass;
import org.junit.Before;
import org.junit.BeforeClass;
import org.junit.Test;
import static org.junit.Assert.*;

public class SeparatingTest extends Separating {

    public SeparatingTest() {
    }
    public static BufferedReader inputTest;
    public static PrintWriter outputTest14;
    public static PrintWriter outputTest15;
    public static PrintWriter outputTest16;

    public static BufferedReader inputTest14;
    public static BufferedReader inputTest15;
    public static BufferedReader inputTest16;

    @BeforeClass
    public static void setUpClass() {

    }

    @AfterClass
    public static void tearDownClass() {
    }

    @Before
    public void setUp() throws FileNotFoundException {

        inputTest = new BufferedReader(new FileReader("testnumbers.txt"));
        outputTest14 = new PrintWriter(new FileOutputStream("testnumbers14.txt"));
        outputTest15 = new PrintWriter(new FileOutputStream("testnumbers15.txt"));
        outputTest16 = new PrintWriter(new FileOutputStream("testnumbers16.txt"));

        inputTest14 = new BufferedReader(new FileReader("testnumbers14.txt"));
        inputTest15 = new BufferedReader(new FileReader("testnumbers15.txt"));
        inputTest16 = new BufferedReader(new FileReader("testnumbers16.txt"));

    }

    @After
    public void tearDown() throws IOException {
        inputTest.close();
        outputTest14.close();
        outputTest15.close();
        outputTest16.close();

        inputTest14.close();
        inputTest15.close();
        inputTest16.close();

    }

    @Test
    public void testSeparate() throws Exception {
        /*testnumbers.txt: 14451024
                           15752601
                           16148923
*/

        Separating br200 = new Separating();
        br200.separateStudents(inputTest, outputTest14, outputTest15, outputTest16);

        assertEquals(inputTest14.readLine(), "18451024");
        assertEquals(inputTest15.readLine(), "15752601");
        assertEquals(inputTest16.readLine(), "16148923");

    }

}