Java will be a string of capital letters to lowercase letters, lowercase letters become uppercase letters

Title Date: 2020/1/14

Title Function: a string of capital letters to lowercase letters, lowercase letters become uppercase letters

Title IDE: Intellij IDEA

package test.demo;

import java.util.Arrays;

public class ChangeLetter {
    public static void main(String[] args){
        String s1 = new String("Hello world");
        char[] s2 = s1.toCharArray();
        System.out.println(s2);
        for(int i=0;i<s2.length;i++){
            if(Character.isUpperCase(s2[i])){
                s2[i] = Character.toLowerCase(s2[i]);
            }
            if(Character.isLowerCase(s2[i])){
                s2[i] = Character.toUpperCase(s2[i]);
            }
        }
        System.out.println(s2);
    }
}