looping with string

say for e.g, i have this string

"Hello, how are you"

what i basically want to do is convert the string to this

hELLO, HOW ARE YOU"

so it converts the uppercase letters to low, and vice versa
it should only change the normal letters, i.e a-z and A-Z, and not anything else

can anyone help me please?

Look at java.lang.Character - it's got a lot of useful static methods.