Does the "negative part" consist of only a single digit? If so, I might view a String that can be parsed into two positive numbers separated by a dash (-). You may want to look at the random() method in the Math class. For more flexibility, you can use the Random class instead.
I'll let you look at the Java API docs (see the links above) and check out your text book for details in this area. If you generate the two numbers, you can use the Integer class to convert them to strings and concatenate them together.