程式碼:

 public static void main(String[] args) {
        System.err.println("小寫轉換大寫");
        String string = "abcdef";
        System.err.println("abcdef" + "->" + string.toUpperCase());
        String string1 = "abc123DEF";
        System.err.println("abcDEF" + "->" + string1.toUpperCase());
        String string2 = "12ABCdef3";
        System.err.println("ABCdef" + "->" + string2.toUpperCase());
        System.err.println("大寫轉換小寫");
        String string3 = "ABCDEF";
        System.err.println("ABCDEF" + "->" + string3.toLowerCase());
        String string4 = "ABC123DEF";
        System.err.println("ABCDEF" + "->" + string4.toLowerCase());
        String string5 = "12ABCDEF3";
        System.err.println("ABCDEF" + "->" + string5.toLowerCase());
    }
 

效果：