Cannot return modified String from static method
<p>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: </p>

<pre><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 &lt; str.length(); i++) {
        char ch = str.charAt(i);
        if (ch &gt; 97) {
            ch = Character.toUpperCase(ch);
            return str;
        } else {
            ch = Character.toLowerCase(ch);
            return str;
        }
    }
    return null;
}
</code></pre>

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

<p>EDIT****************</p>

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

<pre><code>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 &lt; str.length(); i++) {
        char ch = str.charAt(i);

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

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


    }

    return res;
}
</code></pre>

<p>ps. Im not using the ready methods because my task asks for it. </p>