A random number in the range from -10 to 10 in Java.

Then, apparently, you need to throw away the book that you are studying from, if you can't write something like:

import java.util.*;
import java.lang.*;
import java.io.*;

/* Name of the class has to be "Main" only if the class is public. */
class Ideone
{
    public static void main (String[] args) throws java.lang.Exception
    {
        Random r = new Random(System.currentTimeMillis());
        for (int i = 0; i<1000;i++) {
            int q = r.nextInt(21) - 10;
            System.out.println(q);
        }
    }
}

And that's why there are 21 and 10-this is already a homework assignment.

Update

Random takes the seed parameter , which is a special parameter for generating a sequence. In many languages, the random function is made so that if the initial value is the same, then the sequence will be the same.

This is done for debugging purposes and determinism (that is, that everything is stable and repeatable) of programs.

But setting the current time as a seed (which is not repeated) makes it possible to generate different sequences.