大型数据类型字符串数组答案

【问题标题】：Large Arrays of dataType String大型数据类型字符串数组
【发布时间】：2015-02-08 00:20:31
【问题描述】：

我希望每个人都做得很好。我对这个开发人员的东西还很陌生，虽然我觉得它真的很有趣，但我开始明白如果你不跟踪自己在做什么，它会变得多么乏味。我正在为模拟 ATM 程序进行更新。新版本从 .txt 文件中读取以验证帐户信息。验证通过后，您可以查看余额、存款、取款或注销。完成交易后，程序会使用该帐户的更新余额更新旧的 .txt 文件。我终于让一切都按预期工作了，但是当我返回并检查更新的 .txt 文件时，该特定帐户已更新，但文件中的其余字符串已更新为 null，我无法弄清楚为什么以及是什么导致我的程序这样做。我感觉问题出在 upDate 方法中，我忽略了一些东西。您可以在下面看到我的源代码，在我得到的输出下方。任何有关此错误背后逻辑的解释将不胜感激。

import java.util.*;
import java.io.*;

public class ATM2 {

    public static Scanner kbd = new Scanner(System.in);

    /*
     * This method reads the number of lines in the .txt file
     * and returns the array size.
     */
    public static int arraySize () {
        int result = 0;
        try 
        {
            Scanner lineReader = new Scanner(new FileReader("ATMData.txt"));
            int lineCount=0;

            while(lineReader.hasNextLine())
            {
                String size;
                size = lineReader.nextLine();
                lineCount++;
            }
            lineReader.close();
            return lineCount;

        }
        catch(FileNotFoundException e)
        {
            System.out.println("ERROR:  FILE NOT FOUND");
            return result;
        }
    }

    //Method searches .txt file and returns String
    //array with all account info.
    public static String[] acctLine(int size){

        String[] validAccounts = null;
        try 
        {
            Scanner lineReader = new Scanner(new FileReader("ATMData.txt"));
            int lineCount=0;
            String [] acctNums= new String[size];

            while(lineReader.hasNextLine())
            {
                String lineInfo;
                lineInfo = lineReader.nextLine();
                acctNums[lineCount]=lineInfo;
                lineCount++;
            }
            lineReader.close();
            return acctNums;
        }
        catch(FileNotFoundException e)
        {
            System.out.println("ERROR: FILE NOT FOUND");
            return validAccounts;
        }
    }

    //Method updates the txt file with new acct information.
    public static void acctUpdate (String[] validAccount, String fileName){

        try
        {
            PrintWriter newFile = new PrintWriter(new FileWriter(fileName));
            for(int i=0; i<validAccount.length; i++){
                newFile.println(validAccount[i]);
            }
            newFile.close();
        }
        catch(Exception e){
            System.out.println("ERROR:  FILE NOT PRINTED");
        }

    }

    /*The checkID method determines if acctNum is a valid account number
     * and pwd is the correct password for the account.  If the account information
     * is valid, the method returns the current account balance, as a string.
     * If the account information is invalid, the method returns the string "error"
     */
    public static String checkID(String acctNum, String pwd, String [] validAccounts)
    {

        String result = "ERROR";
        String acctString = "ERROR";

        boolean foundIt = false;

        for(int i=0; i<validAccounts.length; i++)  //Loop compares the input data with array data.
        {   
            acctString = validAccounts[i];
            if (acctNum.equals(acctString.substring(0, acctString.indexOf(" "))) && 
                    pwd.equals(acctString.substring(acctString.indexOf(" ")+1,acctString.lastIndexOf(" ")))){
                foundIt = true;
                result = acctString.substring(acctString.lastIndexOf(" ") + 1);
            }
        }

        if (foundIt) //If it's true it returns the acct. balance.
        {
            return result;
        }
        else 
        {
            System.out.println("ERROR:  INVALID ACCOUNT TYPE"); 
        }
        return result;
    }

    /* This method finds the array index of the desired account string.
     *  and returns the the index of the line
     */
    public static int arrayIndex(String acctNum, String pwd, String [] validAccounts)
    {
        int accountPos=0;

        Boolean foundIt = false;
        //loop searches for the index of the array.
        for(accountPos=0; accountPos<validAccounts.length && !foundIt; accountPos++)
        {   
            if (acctNum.equals(validAccounts[accountPos].substring(0, validAccounts[accountPos].indexOf(" "))) && 
                    pwd.equals(validAccounts[accountPos].substring(validAccounts[accountPos].indexOf(" ")+1, validAccounts[accountPos].lastIndexOf(" "))))
            {               
                foundIt = true;
            }
        }
        return accountPos;
    }

    /*Method creates menu options and returns
     *the choice selected by the user.
     */
    public static int menu()
    {
        int menuChoice;
        do
        { 
            System.out.print("\nENTER TRANSACTION TYPE: CHOOSE 1-4."
                    + " \n 1. Display Balance"
                    + "\n 2. Deposit\n 3. Withdraw\n 4. Log Out\n");
            menuChoice = kbd.nextInt();
            if (menuChoice < 1 || menuChoice > 4){
                System.out.println("error");
            }
        }while (menuChoice < 1 || menuChoice > 4);

        return menuChoice;
    }

    /*Method displays the balance to the screen.
     */
    public static void displayBalance(double balance)
    {
        System.out.printf("\nYOUR NEW BALANCE IS: $%.2f\n", balance);
    }

    /*Method calculates the deposit going in to account.
    Returns the total deposit.
     */
    public static double deposit(double orgBal, double depAmt)
    {
        return orgBal + depAmt;
    }

    /*Method that calculates the balance after the withdraw.
     * returns a double with the amount withdrawn.
     */
    public static double withdraw(double orgBal, double wdrlAmt)
    {
        if (wdrlAmt > orgBal)
        {
            return orgBal;
        }
        else
        {
            return orgBal-wdrlAmt;
        }
    }

    public static void main(String[] args) {

        String accNum, pass, fileName="ATMData.txt", origBal = "ERROR";
        String acctInfo;
        int count = 0, menuOption = 0, acctPos;
        double depositAmt, withdrawAmt, currentBal; 
        String [] validAccounts =acctLine(arraySize());

        /*loop that will count the number of login attempts
         *you make and will exit program if it is more than 3.
         *as long as oriBal equals an error.  
         */
        do{

            System.out.println("PLEASE ENTER YOUR ACCOUNT NUMBER: ");
            accNum = kbd.next();

            System.out.println("ENTER YOUR PASSWORD: ");
            pass = kbd.next();

            origBal = checkID(accNum, pass, validAccounts);
            count++;

            acctPos = arrayIndex(accNum, pass, validAccounts)-1;

            if (count >= 3 && origBal.equals("ERROR:")){
                System.out.print("MAX LOGIN ATTEMPTS REACHED.");
                System.exit(0);
            }
            else if (!(origBal.equals("ERROR"))){
                System.out.println("\nYOUR CURRENT BALANCE IS: $ "+ origBal);
            }
            else
                System.out.println(origBal);

        }while(origBal.equals("ERROR"));

        currentBal=Double.parseDouble(origBal);

        /*this loop will keep track of the options that 
         *the user inputs in for the menu. and will 
         *give the option of deposit, withdraw, or logout.
         */
        while (menuOption != 4)
        { 
            menuOption=menu();
            switch (menuOption)
            {
            case 1:
                displayBalance(currentBal);
                break;
            case 2:
                System.out.print("HOW MUCH DO WISH TO DEPOSIT: $ ");
                depositAmt = kbd.nextDouble();
                currentBal = deposit(depositAmt, currentBal);
                displayBalance(currentBal);
                break;
            case 3:
                System.out.print("HOW MUCH DO YOU WISH TO WITHDRAW: $ ");
                withdrawAmt = kbd.nextDouble();
                if (withdrawAmt > currentBal)
                    System.out.println("ERROR:  YOU DON'T HAVE ENOUGH FUNDS TO COMPLETE"
                            + " TRANSACTION."
                            + " ENTER TRANSACTION TYPE: CHOOSE 1-4");
                currentBal = withdraw(currentBal, withdrawAmt);
                displayBalance(currentBal);
                break;
            case 4://Last option logs user out after it updates the ATMData.txt
                acctInfo = accNum + " " + pass + " " + String.valueOf(currentBal);
                validAccounts[acctPos]=acctInfo;
                acctUpdate(validAccounts, fileName);
                System.out.println("LOGOUT SUCCESSFULL.");
                System.exit(0);
                break;
                default:
            }
        }   
    }
}

这就是 .txt 在源代码更新之前的样子。唯一更新的是一行。

null
null
null
null
null
null
null
null
null
null
417927608 ;6FYE5l`LPX@ 4101.13
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null
null

因为我们是通过引用传递的，所以我们只需要在主程序中声明一次原始数组。 .

7560640 6Z[NVoee1S6 4424.43
04569505 f:H>wAr@<URH 4211.42
528859 ][[79_W?]=:< 724.26
973335820 Si>G7QMp?>e 3207.28
79154675 Y>vfTYwHm0J 3327.47
5554284 6n[^PwZUZlH 1028.66
58773836 28vcgSpwA01 3131.02
95873039 @4XwrN`T9 2598.28
1034703 n@HKk`mTar2Z 1176.75
10192851 X>ATvaR:qT 3857.89
417927608 ;6FYE5l`LPX@ 3401.13
143260 s2UgJtSHYFmn 378.68
34729802 rh\<3rmk7Dm< 255.48
277791753 BR2=siCDi 345.75
1014748 PK?KTMjA63e 1196.16
496714922 M1=I`ve;Wjw 501.26
654156 AB`6cvO0ql: 4310.51
898250859 t^D90Nw?Ii 3876.05
971750 [9q_5AuLlJ 135.97
796678134 KwV^03p:WW=< 3453.65
39482807 C5?H?b9UplL 4109.48
401599 MvHL7Yv07bW 4356.96
8321377 Jc:<_ke9B: 930.90
152773 VQ6\[2k:uoj 3265.93
624426519 kralkKKEia3 4858.39
650002 Km2t\R3>3@G 1470.78
004509766 _0S`1qb^V 1332.24
0274647 WDhg3\p]w 3147.50
974502771 Q4GT<_s3D9CK 4464.22
281866 c_1F]@RpV^ 2952.03
381121 rOo?CiA[kwTs 4555.21
1549064 F7^eh=njp\r 430.24
841319 IBe^GTLGv 4050.76
6499423 tW9>r[s_<\ 1418.09
8579740 cdB12UrLU 2905.74
33581232 <os4;WcEqb 2455.32
725175174 nWjIeFLK3E 3118.08
592410029 RqgPT382p2Lk 3863.32
08274516 DDr?^vuNQ 1630.37
808471 Ow:8udlBrYP 2590.05
769854 0`lpWGrMkrG 2643.59
3591827 MmThLS1DdL5 2715.30
9001605 \n;rMknh7C 4074.83
10345766 N>^qBqhk?39J 539.43
504385 j0Rq=li0c^ 253.77
362335041 >^VN5oh?OSR 324.19
5728008 QZ>BZhGEv 4441.67
26102273 1XCWR;W?^b\ 4203.51
3317249 dmjLKbgZ? 1958.44
793249 5J>ApQZAB 1906.46
536461203 CP5U:f5\d[_p 3742.25
030298042 VBtHo909;QM 986.28
6111228 mmPgiDMALt 1348.95
621992 r7UmSJ\9j 1440.22
310072768 B1ccagCH3 4755.56
819894803 IXfcY9PZG1 4399.39
362945795 5LAdX_s^G 3624.28
99929088 `oHijc^J0u 4507.69
821671 s@OV>NYlacV 4125.72
72368663 Mi?wTcZ_@I 2845.24
22692406 uT05^2plu 2530.73
343817 pB;Glu5sVb7P 2168.28
5138360 877<cT4T;t 1562.15

【问题讨论】：

标签： java arrays iostream

【解决方案1】：

您永远不会将以前的帐户数据写入validAccounts。当您运行 String [] validAccounts = new String[arraySize()]; 时，所有字符串实例都设置为 null，并且仅在您更新帐户时才会更改。您需要做的是，当您启动程序时，将现有文本文件读入该字符串数组，然后只需更改您需要的条目。完成后，将 String 数组写入文件。

【讨论】：

【解决方案2】：

在你的主要方法中字符串[] 有效帐户 = 新字符串[arraySize()]; 这意味着您有一个 NULL 值数组，您需要在此处而不是在其他方法中分配值数据，因为当您传递数组时，您传递的是对该数组的引用而不是数据本身所以在arrayIndex validAccounts = acctLine(arraySize());会将数据放入此方法中，并在返回时丢弃此数据。您需要分配一次数据，然后传递它而不在方法中写入它

例如主要是字符串[] 有效帐户 = 新字符串[arraySize()]; 应该 String[] 有效账户 = acctLine(arraySize()); 您不需要在arrayIndex 或checkID 中再次读取它。所以读取它一次并存储它，将这个数据传递给方法并从这个数据中读取而不是读取一个新文件

一个简单的例子就是这样 - 你会看到 a = new int.... 不会改变原始方法中的 a，但赋值会在 dosSomething 中发生

public static void main(String[] args) {

    int[] a = {1,2,3};
    doesNothing(a);
    for (int i: a)
        System.out.print(i);
    doesSomething(a);
    for (int i: a)
        System.out.print(i);

}

static void doesNothing(int[] a) {
    a = new int[]{4,5,6};
}

static void doesSomething(int[] a) {
    a[0] = 7;
    a[1] = 8;
    a[2] = 9;
}

【讨论】：

非常感谢，它成功了，而且你解释逻辑错误的方式使调整变得相当简单。我希望我能找到一位解释和教授它的 CS 教授。我已经用更改更新了决赛。虽然这适用于我正在做的事情，但你会建议什么可以提高更新代码的效率？再说一次，我是新手，我觉得我从你们这些热心的贡献者那里学到了很多。