Cannot return modified String from static method

Question

Im very new to coding and cant seem to be able to return anything. I need to convert upper case characters to lower case and vice versa. Here's my code:

public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String str = sc.nextLine();
    invString(str);
    sc.close();

}

private static String invString(String str) {
    for (int i = 0; i < str.length(); i++) {
        char ch = str.charAt(i);
        if (ch > 97) {
            ch = Character.toUpperCase(ch);
            return str;
        } else {
            ch = Character.toLowerCase(ch);
            return str;
        }
    }
    return null;
}

What am i doing wrong? ( in terms of returning, the code isnt complete yet)

EDIT****************

thanks for the helpful remarks, as i understood i do not have a place where my modifications are stored, so i added String res = ""; and kept adding the values into String res but in the end, when returning res, i still dont get any output...Here is the whole thing:

public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    String str = sc.nextLine();
    String st = invString(str);
    sc.close();

}

private static String invString(String str) {
    String res = "";
    for (int i = 0; i < str.length(); i++) {
        char ch = str.charAt(i);

        if (65 <= ch && ch <= 90) {
            ch += 32;
            res += ch;

        } else if (97 <= ch && ch <= 122) {
            ch -= 32;
            res += ch;
        }


    }

    return res;
}

ps. Im not using the ready methods because my task asks for it.

Answer 1

There are a number of flaws in your code. Firstly, you are attempting to return the original string str in every if statement. So what happens is the method invString( String ) simply returns the original string that is passed as argument. Instead, you should keep adding the characters to a new String variable (as @Massimo said). Also, you are returning null in the end. Why even do that? You would want to return the new String variable instead.

private static String invString(String str) {

    String s=""; //new String variable
    for (int i = 0; i < str.length(); i++) {
        char ch = str.charAt(i);
        if (ch > 97) {
            ch = Character.toUpperCase(ch);
            s+=ch; //do this instead of returning str
        } else {
            ch = Character.toLowerCase(ch);
            s+=ch; //same here
        }
    }
    return s; //return the new String
}

Secondly, in your main method, simply calling the method is wrong as it returns a value. You should assign the value returned by invString() to a String variable.

public static void main(String[] args){
    ...
    String st = invString(str); //st stores the value of str with 
                                //letters' cases changed
}

Answer 2

You return you str object without updating it at all. You should generate a new string in which put the characters for which you reverse the case. Check the last answers in How can I invert the case of a String in Java?

Answer 3

If you want to use the inverted string, you need to actually use the returned value, e.g:

str = invString (str)

Strings are immutable so you can't change the characters within them. You can only replace the string variable with a new string.

Modifying the characters you are accessing doesn't affect the string. You need to build a new string (look up StringBuilder) and return it.