class Testing {
public static void main(String[] args) {
char c1 = 'a', c2;
c2 = (char) ( c1 - ('a' - 'A'));

String s1 = "abc", s2 = "";
for (int i=0; i<s1.length(); i++) {
s2 = s2 + (char) ( s1.charAt(i) - ('a' - 'A') );
}
System.out.println( c2 + " " + s2);
}
}
----------
do add other code  to check that the char you are converting
is between 'a' to 'z'.

Another approach would be to have an array of char ("char[65536]
Uppercase;") which contains all characters supported on your
platform; then add the following:
Uppercase ['a'] = 'A';
Uppercase ['b'] = 'B';
...
Uppercase ['z'] = 'Z';

Then, whenever you want to substitute a character by its uppercase
version, use
Uppercase [ch]
instead of ch.

I know, looks ugly, but according to OP I wanted to avoid calling any
functions, such as toupper().