Find the first reoccuring char from a String
The Character type already has everything you need. It is either a character or the special value null.

For this task you don't need a map, a simple set is enough.

When you move this code into a separate method, using break becomes bad style because you should directly return. In the currect code it's ok.

public static Character findFirstRecurring(String str, char ch) {
    // Left as an exercise
    …
    return null;
}