你可以假设输入一定在小写字母 a ~ z 之间

public class Solution {
    
    
    
    public static char lowercaseToUppercase(char character) {
    
    
        character = (char)(character - 32);
        return character;
    }
    public static char uppercaseTolowercase(char character){
    
    
        character = (char)(character + 32);
        return character;
    }
    public static void main(String [] args){
    
    
        Scanner sc = new Scanner(System.in);
        String str = sc.next();
        char character = str.charAt(0);
        //Solution so = new Solution();
        if(character >= 'a' && character <= 'z'){
    
    
            System.out.print(lowercaseToUppercase(character));
        }
        if(character >= 'A' && character <= 'Z'){
    
    
            System.out.print(uppercaseTolowercase(character));
        }
    }
}

注意事项：如果方法为非静态方法，需要在主函数中创建类对象调用方法。