hello, can i get help with 12 and 13 using java and following the instructions. thanks!

12." Write your own implementation of indexof (ch, fromPos). 13. The String class has boolean methods startsWith(string prefix) and endsWith(String suffix), startsWith tests whether this string starts with a given substring; endsWith tests whether this string ends with a given substring. Pretending that these methods do not exist, write them using other string methods (but no iterations or recursion)