public class Test {
    public static void main(String[] args) throws Exception{
        String str = "あ";
        System.out.println(str.getBytes("UTF-8").length);
        System.out.println(str.getBytes("Shift-JIS").length);
        System.out.println(str.getBytes("MS932").length);
        System.out.println(str.getBytes("EUC-JP").length);
    }
}

3
2
2
2