There is a chapter in
JLS 5 about conversions and promotions.
Let's say 'A' char is 0041 in hex (two bytes)
To convert to a bigger type add enough zeros to its left up to the number of digits in the target type:
0000 0041 This is an int (4 bytes) with the same value as the char 'A'.
If the number is negative you will have to add 1s instead.
Review the binary, hexadecimal and two's complement
formats if needed.