The threads of my executor are not executed in order

I'm trying to make a program write to a file using the RandomAccessFile class in a parallel and mutually exclusive way.

When compiling and executing works, the problem comes when, when I open the file, I see that the letters are in order but that some letters are not and only leave a space.

public class ficheroSeguro extends Thread implements Runnable{
//macro para el fichero
static final String MiArchivo = 
"C:/Users/Raul_/Desktop/PCTR 7/ficheroSeguroTexto.txt";
//cadena que pasamos al constructor
static String palabrasfichero;
//contador para ver el orden en que se ejecutan
int i;
public ficheroSeguro(String S,int i){
    this.palabrasfichero = S;
    this.i=i;
}

public synchronized void run(){
    long threadId = ficheroSeguro.currentThread().getId();
    System.out.println("Ejecutando el hilo: " +threadId+ "con valor: " +i);
    try{
    writeData(MiArchivo,palabrasfichero,i);
    }catch(IOException e){System.out.println("Error en metodo run");}
}



public static void writeData(String S, String data, int i) throws IOException {
    RandomAccessFile raf1 = new RandomAccessFile(MiArchivo, "rw");
    raf1.seek(i);
    raf1.writeChar(palabrasfichero.charAt(i));
    raf1.close();
}   



public static void main(String[] args) throws InterruptedException{
    Scanner sc = new Scanner(System.in);
    try{
        RandomAccessFile raf = new RandomAccessFile (MiArchivo,"rw");
        raf.setLength(0);
    } catch(FileNotFoundException e){System.out.println("Error al abrir");}
     catch(IOException e){System.out.println("Error al vaciar");}
    String cadena = sc.next();
    //ejecutor con tantos hilos como tamaño tenga la cadena
    ExecutorService exe= Executors.newFixedThreadPool(cadena.length());
    for(int i = 0; i<cadena.length();i++){
        exe.execute(new ficheroSeguro(cadena,i));
    }
    exe.shutdown();
    exe.awaitTermination(10, TimeUnit.MILLISECONDS);
}



}'

When I execute it and print on the screen the system.out of the run, the threads appear with a different order

Ejecutando el hilo: 14con valor: 1
Ejecutando el hilo: 12con valor: 0
Ejecutando el hilo: 16con valor: 2
Ejecutando el hilo: 18con valor: 3