boolean valid = false;
while (!valid) {
	String s = in.nextString();
	try {
		a = Double.parse(s);
		if (a >= 0) {
			valid = true;
		}
	} catch (NumberFormatException e) {
	}
}

//OR

while (a < 0) {
	try {
		a = Double.parse(in.nextString());
	} catch (NumberFormatException e) {
	}
}