Java 2 thread random numbers with mutex

Hey guys i need some help to understand how to do this exercise:

"Write a program in which two threads write random numbers on a global vector of integers and size 100. These threads should use mutexes to get the value that indicates the position to be filled in the vector. This value starts at -1 and each thread should increase it before you use it . When the vector is filled in the original thread must be agreed by a condition variable ."

I use google translate.

I dont want you to solve my exercise i want a help about work with mutexes. I search here and understand a litle bit with the chicken history "What is a mutex?" but I don't know how to do it. This is like a challenge that the teacher made for us and he didn't explain about mutex.

here it's the beginning of my code with only 1 thread for now.

import java.util.Random; public class ThreadRandomNumbersClass {

public static class RunnableT1 implements Runnable{
    @Override
    public void run(){
        int i;
        int array[] = new int[100];
        Random rn = new Random();
        for(i=0; i<100; i++){
            array[i] = rn.nextInt();
            System.out.println(array[i]);
                            }
                    }
                                                    }

public static void main(String[] args) {
    // TODO Auto-generated method stub
    Thread t1 = new Thread(new RunnableT1());

    t1.start();

                                        }

}