Recursively dividing up square regions

I'm trying to divide square into 4 smaller quadrants by randomly selecting a point in the square and drawing two lines through it, and then dividing each of those quadrants up into 4, and so on, until the width/height is 1, in a recursive way. My code is here:

public static void draw(int x0, int y0, int xmax, int ymax) {

if (Math.abs(xmax - x0) > 1 && Math.abs(ymax - y0) > 1) {
    int rx = r.nextInt(xmax - 1) + 1;    // line A
    int ry = r.nextInt(ymax - 1) + 1;    // line B

    StdDraw.line(rx, y0, rx, ymax);
    StdDraw.line(x0, ry, xmax, ry);

    draw(x0, y0, rx, ry);        // line C
    draw(x0, ry, rx, ymax);      // line D
    draw(rx, ry, xmax, ymax);    // line E
    draw(rx, y0, xmax, ry);      // line F
}
else ;

That is, I'm passing the lower left corner coordinates (x0, y0) and the upper right corner coordinates (xmax, ymax). Whenever I comment out lines D, E, and F, everything works fine. But when I uncomment any of D, E, or F, I get this error java.lang.IllegalArgumentException: n must be positive , usually pointed at line A or line B. Can someone please tell me what this error means and how I can fix it?

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Considering that nextInt(int n) seems to contain the only n variable in that piece of code it is likely the one that is throwing that error.

Make sure that xmax-1 and ymax-1 don't become negative as the documentation for nextInt specifically says it throws this error on negative numbers:

http://docs.oracle.com/javase/1.4.2/docs/api/java/util/Random.html#nextInt(int)

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This is because somewhere during your recursion the value passed in nextInt -> (xmax - 1) is becoming negative.

r.nextInt(xmax - 1);

If (xmax - 1) is negative you will get exception. Because, nextInt() method takes only positive values.

You can add an additional check before this line to check whether your difference is negative or positive. And accordingly, you can skip this part of code.

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