> (1) Finding a substring (say, aString ='efg') within the main string
> (eg, mainString = 'abcdefghijklm'). The mainString will have NOT have
> a fixed range of characters.

Do a Google search on "string matching" or "pattern matching". There
are many algorithms for doing this. Some simpler than others.

> (2) I know how to get length of a string, but is there a way to
> answer in an integer format (which I'd use in a FOR statement later
> on)? Eg. I get a length of 5 - how can I convert that into a integer
> variable. Note: I don't want to alter the String! Just get a integer
> output for length.

String.length() returns an int.

>
> (3) Convert a String into separate characters. Eg a string
> of 'abcde'. would be converted into five DISTINCT characters A B C D
> E that I could then perform calculations against. Again it's not
> fixed.

Here's the code to turn a String into a char array. char may be
replaced with the Character wrapper class.

char cArr[] = new char[s.length];
for(int i = 0; i < s.length(); i++){
cArr[i] = s.charAt[i];
}