将实例变量写入文本文件
将实例变量写入文本文件
提问于 2013-12-06 15:51:03
我需要获取对象的列表，并将它们的实例变量写入文本文件。看起来会是这样的：
Hot Dog,1.25,Grocery Store
Gas,42.15,Gas Station
etc.我有一些代码如下所示：
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，而没有其他任何东西。我搞不清到底怎么回事。我知道对象序列化是一个更快的选择，但是对于这个项目，由于我还在学习，我想使用这种方式。
其中一个答案所要求的主要方法：
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());
}
}回答 3
Stack Overflow用户
回答已采纳
发布于 2013-12-06 15:53:53
我认为问题在于每次迭代都要创建一个新的PrintWriter。您应该在循环之外声明它。所发生的情况是，当创建新的PrintWriter时，它会覆盖存储在文件中的以前的数据。
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 {
}Stack Overflow用户
发布于 2013-12-06 16:11:45
这是因为您正在为列表中的每个对象实例化一个新的PrintWriter对象(和一个新的FileWriter对象)。
您应该在for循环之前实例化它一次。替换
PrintWriter out = null;使用
PrintWriter out = new PrintWriter(new FileWriter(fileName));请注意:使用当前代码，如果您的NullPointerException是空的，那么您可能会在out.close();行中得到一个ArrayList。
Stack Overflow用户
发布于 2013-12-06 16:32:31
首先:您正在编写java，而不是C++。使用Java结构和技术。
正如MadConan所提到的，您的实现过度了。在每个对象类型(费用和收入)上使用toString() (或toBlammy() --不是字符串)来格式化输出。
提示:只要有一堆if (instanceof blammy)，就应该考虑多态性。
您的代码应该如下所示：
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.
}
}页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：
https://stackoverflow.com/questions/20428104
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