java: 'string index out of range: -1' 异常使用 indexOf()

Question

奇怪的问题。我运行这个(非常初级的)程序来查找文件中的用户名和密码，程序应该将输入的密码与保存的密码进行比较。然而，每次我都会收到一个奇怪的 String index out of range: -1 异常。我以前遇到过类似的问题，但是这次 indexOf('.') 调用返回 -1；它不喜欢。如果 indexOf() 导致错误，为什么会返回 -1？这是来源:

public String loginToClient() throws FileNotFoundException, IOException {
        //decryptUsers();
        int tries;
        tries = 5;
        while (tries > 0) {
            System.out.println("LOGIN");
            String usnm = c.readLine("Username: ");
            char [] passwd = c.readPassword("Password: ");
            users = new FileInputStream("users.fra");
            DataInputStream dis = new DataInputStream(users);
            BufferedReader br = new BufferedReader(new InputStreamReader(dis));
            String logindat = br.readLine();
            System.out.println(logindat);
            if (logindat.contains(usnm) == null) {
                System.err.println("Username not recognised, please try another or create user.");
                usnm = "INV";
                return usnm;
            }
            else {
                int startUsnm = logindat.indexOf(usnm);
                System.out.println("startUsnm: " + startUsnm);
                String logdat = logindat.substring(startUsnm, logindat.indexOf("."));
                System.out.println("logdat: " + logdat);
                int endUsnm = logdat.indexOf(':'); 
                System.out.println("endUsnm: " + endUsnm);
                int usnmend = endUsnm - 1;
                System.out.println("usnmend: " + usnmend);
                int startPass = endUsnm + 1;
                System.out.println("startPass: " + startPass);
                int endPass = logdat.indexOf('.');
                System.out.println("endPass: " + endPass);
                String Usnm = logdat.substring(0, usnmend);
                System.out.println("Usnm: " + Usnm);
                int passend = endPass - 1;
                System.out.println("passend: " + passend);
                String Pass = logdat.substring(startPass, passend);
                System.out.println("Pass: " + Pass);
                char [] Passwd = Pass.toCharArray();
                if (usnm.equals(Usnm)) {
                    if (Arrays.equals(passwd,Passwd)) {
                        System.out.println ("Logged in. Welcome, " + usnm + ".");
                        String data = "LOGIN: " + usnm;
                        printLog(data);
                        //encryptUsers();
                        return usnm;
                    }
                    else {
                        System.out.println ("Incorrect password, please try again.");
                        String data = "PASWFAIL: " + usnm;
                        printLog(data);
                        tries -= 1;
                    }
                }
                else {
                    System.out.println ("Username not recognised.");
                    printLog("USNAMFAIL");
                    usnm = "INV";
                    return usnm;
                    //encrytUsers();
                }
            }
        }
        //encryptUsers();
        System.exit(2);
        return usnm;
    }

这是一些输入/输出:

Startup initiated.
Logfile exists.
Users file exists.
New user? n
ELSE
LOGIN
Username: rik
Password: 
rik:55.
startUsnm: 0
endUsnm: 3
startPass: 4
endPass: -1
Usnm: rik
Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: -5
    at java.lang.String.substring(String.java:1949)
    at client0_0_2.loginToClient(client0_0_2.java:103)
    at client0_0_2.general(client0_0_2.java:209)
    at client0_0_2.<init>(client0_0_2.java:221)
    at client0_0_2.main(client0_0_2.java:228)

编辑:找到解决方案!

出于某种原因，indexOf() 不想找到 '.'- 当用连字符 ('-') 替换时，然而，它运行完美，看起来!

Answer 1

我认为错误在这一行:

String Pass = logdat.substring(startPass, passend);

出于某种原因(您必须确定原因)，您通过在字符串中搜索 . 来计算 passend。如果 . 不存在，indexOf 返回 -1 作为标记。不过，这不是导致异常的行。我认为是上面这行，因为如果您在 passend 为 -1 时尝试计算以 passend 结尾的子字符串，您会得到上述错误。

尝试确定为什么您的字符串中不包含 .。

希望这对您有所帮助!