Question

我需要获取一个对象列表并将它们的实例变量写入文本文件。它看起来像这样：

Hot Dog,1.25,Grocery Store
Gas,42.15,Gas Station
etc.

我有一些看起来像这样的代码：

public void writeListToFile(String fileName, ArrayList<BudgetItem> writeList) throws Exception {
    PrintWriter out = null;
    for(int i = 0; i<writeList.size(); i++) {   
        if(writeList.get(i) instanceof Expense) {
            Expense writeExpense = (Expense) writeList.get(i);
            try {
                out = new PrintWriter(new FileWriter(fileName));
                dump(out, writeExpense);
            }
            finally {
            }
        }
        else if(writeList.get(i) instanceof Income) {
            Income writeIncome = (Income) writeList.get(i);
            try {
                out = new PrintWriter(new FileWriter(fileName));
                dump(out, writeIncome);
            }
            finally {
            }
        }
    }
    out.close();
}

public void dump(PrintWriter out, Expense writeExpense) {
    out.print(writeExpense.getDateOfTransaction().get(GregorianCalendar.YEAR));
    out.print(",");
    out.print(writeExpense.getDateOfTransaction().get(GregorianCalendar.MONTH));
    out.print(",");
    out.print(writeExpense.getDateOfTransaction().get(GregorianCalendar.DATE));
    out.print(",");
    out.print(writeExpense.getItemName());
    out.print(",");
    out.print(writeExpense.getMethodOfPay());
    out.print(",");
    out.print(writeExpense.getPlaceOfPurchase());
    out.print(",");
    out.print(writeExpense.getQuantity());
    out.print(",");
    out.print(writeExpense.getPrice());
    out.print("\n");
}

and one other method similar to the 2nd one.

当我运行它时，它只写出一行，列表中的第一个对象，没有别的。我无法弄清楚发生了什么。我知道对象序列化是一个更快的选择，但对于这个项目，因为我还在学习，我想用这种方式。

其中一个答案要求的主要方法：

public static void main(String[] args) throws Exception {
    String itemName = "Hot Dog";
    int quantity = 1;
    String placeOfPurchase = "Weiner Stand";
    String methodOfPay = "Credit";
    BigDecimal price = new BigDecimal(1.25);
    GregorianCalendar g = new GregorianCalendar(2013,11,1);
    Expense e = new Expense(g, price, itemName, quantity, placeOfPurchase, methodOfPay);
    BudgetItem bi = (BudgetItem) e;
    String itemName2 = "Gun";
    int quantity2 = 1;
    String placeOfPurchase2 = "Weiner Stand";
    String methodOfPay2 = "Credit";
    BigDecimal price2 = new BigDecimal(1.25);
    GregorianCalendar g2 = new GregorianCalendar(2013,11,1);
    Expense e2 = new Expense(g, price, itemName, quantity, placeOfPurchase, methodOfPay);
    BudgetItem bi2 = (BudgetItem) e2;
    ArrayList<BudgetItem> abi = new ArrayList<BudgetItem>();
    abi.add(bi);
    abi.add(bi2);
    RegisterFileIO rfio = new RegisterFileIO();
    rfio.writeListToFile(System.getProperty("user.dir") + "/data.out", abi);


    BufferedReader in = new BufferedReader(new FileReader(System.getProperty("user.dir") + "/data.out"));
    Scanner lineScanner = new Scanner(in);
    lineScanner.useDelimiter(",");
    while(lineScanner.hasNext()) {
        System.out.println(lineScanner.next());
    }
}

Answer 1

我认为问题是你每次迭代都会创建一个新的PrintWriter。你应该在循环之外声明它。发生的事情是，当创建新的PrintWriter时，它会覆盖存储在文件中的先前数据。

PrintWriter out = null;
try {
    out = new PrintWriter(new FileWriter(fileName));
    for(int i = 0; i<writeList.size(); i++) {   
        if(writeList.get(i) instanceof Expense) {
            Expense writeExpense = (Expense) writeList.get(i);

            dump(out, writeExpense);
    }
 } finally {
 }

Answer 2

这是因为您为列表中的每个对象实例化了一个新的PrintWriter对象（和一个新的FileWriter对象）。

您应该在for循环之前仅实例化一次。取代

PrintWriter out = null;

与

PrintWriter out = new PrintWriter(new FileWriter(fileName));

请注意：如果您的NullPointerException为空，那么对于您当前的代码，最终可能会在行out.close();处ArrayList结束。

Answer 3

首先：你正在编写java，而不是C ++。使用Java结构和技术。

如MadConan所述，您的实施过度。在每个对象类型（费用和收入）上使用toString（）（或toBlammy（） - blammy不是字符串）来格式化输出。

提示：只要你有一堆if (instanceof blammy)，就应该考虑多态性。

你的代码应该是这样的：

public void writeListToFile(
    final String fileName,
    final List<BudgetItem> listBudgetItem)
throws Exception
{
    PrintWriter out = null;

    try
    {
        out = new PrintWriter(new FileWriter(fileName));
        for(BudgetItem current : listBudgetItem)
        {
            out.println(current.toBlammy());
        }
    }
    catch (... exceptions)
    {
    }
    finally
    {
        // close the PrintWriter.
    }
}

将实例变量写入文本文件

3 个答案: