java- how to obtain/store string variables which have different charsets

I have some data in a variable of type List. Different elements in the list are encoded in different charsets. I know the charset corresponding to each element.

I want to know: how do I extract each String in its respective charset?

Also, if I have some text being entered into my Java application by a user (along with its charset), then how can I store each value in a list variable? and how can I store/retrieve such a value in a String variable?

And after the data is stored/retrieved, will the changes of charset mean that now the default charset (UTF-8) is no longer being used? If this is the case, how can I restore the default charset?