Альтернативное решение.

import java.io.*;
import java.util.Scanner;

public class Solution {
    public static String firstFileName;
    public static String secondFileName;

    //add your code here - добавьте код тут

    static {
        Scanner scanner = new Scanner(System.in);

        firstFileName = scanner.nextLine();
        secondFileName = scanner.nextLine();

        scanner.close();
    }

    public static void main(String[] args) throws InterruptedException {
        systemOutPrintln(firstFileName);
        systemOutPrintln(secondFileName);
    }

    public static void systemOutPrintln(String fileName) throws InterruptedException {
        ReadFileInterface f = new ReadFileThread();
        f.setFileName(fileName);
        f.start();
        //add your code here - добавьте код тут
        f.join();
        System.out.println(f.getFileContent());
    }

    public interface ReadFileInterface {

        void setFileName(String fullFileName);

        String getFileContent();

        void join() throws InterruptedException;

        void start();
    }

    //add your code here - добавьте код тут

    public static class ReadFileThread extends Thread implements ReadFileInterface {
        private String fullFileName;
        private String fileContent = "";

        @Override
        public void setFileName(String fullFileName) {
            this.fullFileName = fullFileName;
        }

        @Override
        public String getFileContent() {
            return fileContent;
        }

        @Override
        public void run() {
            Scanner scanner = new Scanner(fullFileName);
            while (scanner.hasNextLine()) {
                fileContent += scanner.nextLine() + " ";
            }
            scanner.close();
        }
    }
}

@Override
public void run() {
    try {
        BufferedReader reader = new BufferedReader(new FileReader(new File(fullFileName)));
        while (reader.ready()) {
            fileContent += reader.readLine() + " ";
        }
        reader.close();
    } catch (IOException e) {
        e.printStackTrace();
    }
}