5. 대문자 ↔ 소문자

package _5;

import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		
		String str = null;
		Scanner sc = new Scanner(System.in);		
		str = sc.next();
		
		char[] arr = str.toCharArray();
									
		for (int i = 0; i < arr.length; i++) {
			if(Character.isUpperCase(arr[i])) {
				arr[i] = Character.toLowerCase(arr[i]);
			} else {
				arr[i] = Character.toUpperCase(arr[i]);
			}
		}
		
		//str = String.valueOf(arr);
		str = String.copyValueOf(arr);
		System.out.println(str);
		
	}
		
}

---

str.toCharArray(); // 문자열 -> 문자배열

Character.isUpperCase(문자);
Character.isLowerCase(문자);

Character.toUpperCase(문자);
Character.toLowerCase(문자);

String.valueOf(문자열);
String.copyValueOf(문자열);