If you can use commons-lang3:
public static void main(String[] args) {
// Algorithm 1
String o = null;
for (int i = 0; i < 10; i++) {
String x = StringUtils.repeat("X", i+1);
for (int j = 0; j < 9; j++) {
o =StringUtils.repeat("0", j-i+1);
}
System.out.println(x+o);
}
}