import?java.util.ArrayList;
public?class?Main?{
????public?static?void?main(String[]?args)?{
????????String?s?=?"abCD";
????????for?(char?c?:?s.toCharArray())?{
????????????if?(Character.isLowerCase(c))?{
????????????????System.out.println(Character.toUpperCase(c));
????????????}?else?{
????????????????System.out.println(Character.toLowerCase(c));
????????????}
????????}
????????String?test?=?"abcd";
????????char[]?chars?=?test.toCharArray();
????????ArrayList<Character>?list?=?new?ArrayList<>();
????????for?(char?c?:?s.toCharArray())?{
????????????for?(char?aChar?:?chars)?{
????????????????if?(c?==?aChar)?{
????????????????????list.add(c);
????????????????}
????????????}
????????}
????????System.out.println(list);
????}
}
望采納!