好的，输入文本文件格式如下：
product sugar
boughton 13-3-2012
useby 12-12-2012
boughtat $3.56
quantity 5
product sauce pan
quantity 10
boughtat $19.99
boughton 13-3-2012
product sugar
boughton 23-3-2012
quantity 15
boughtat $2.99
useby 1-10-2012
product plate
quantity 10
boughtat $-19.99
boughton 13-3-2012
product orange juice
boughton 33-3-2012
quantity 15
boughtat $2.99
useby 1-10-2012
我的代码： DataIn.java
class DataIn {
public static ArrayList<Product> getData(String filename) {
ArrayList<Product> recordlist = new ArrayList<Product>();
int index = -1;
try {
File inFile = new File(filename);
Scanner reader = new Scanner(inFile);
String s;
Product p = null;
while (reader.hasNext()) {
s = reader.nextLine();
Scanner line = new Scanner(s);
String cmd;
if (line.hasNext()) {
cmd = line.next();
if (cmd.equalsIgnoreCase("product")) {
index++;
p = new Product();
p.setName(line.nextLine());
recordlist.add(index, p);
} else if (cmd.equalsIgnoreCase("boughton")) {
Calendar d;
d = ConvertToCal(line.nextLine());
p.setBoughtOn(d);
recordlist.set(index, p);
} else if (cmd.equalsIgnoreCase("useby")) {
Calendar d;
d = ConvertToCal(line.nextLine());
p.setUseBy(d);
recordlist.set(index, p);
} else if (cmd.equalsIgnoreCase("boughtat")) {
if (line.hasNextDouble()) {
p.setBoughtAt(line.nextDouble());
recordlist.set(index, p);
}
} else if (cmd.equalsIgnoreCase("quantity")) {
if (line.hasNextInt()) {
p.setQuantity(line.nextInt());
recordlist.set(index, p);
}
} else {
System.out.println("Error: no command");
}
}
}
reader.close();
return recordlist;
} catch (Exception e) {
System.out.println("Error" + e.getMessage());
return null;
}
}
public static Calendar ConvertToCal(String s) {
try {
Date d;
DateFormat f = new SimpleDateFormat("dd-MM-yyyy");
d = (Date) f.parse(s);
Calendar cal = Calendar.getInstance();
cal.setTime(d);
return cal;
} catch (ParseException e) {
System.out.println("Exception: " + e);
}
return null;
}
// method to print the output in the file
public static void writeData(ArrayList<Product> productlist, String fileName) {
if (productlist.size() == 0) {
System.out.println(" no product");
return;
}
try {
File fileOut = new File(fileName);
PrintWriter out = new PrintWriter(fileOut);
for (Product p : productlist) {
out.println("product " + p.getName());
out.println("boughtOn " + (p.getBoughtOn().get(Calendar.DATE)));
out.println("useBy " + (p.getUseBy()).get(Calendar.DATE)); //ERROR in here
out.println("boughtAt " + p.getBoughtAt());
out.println("quantity " + p.getQuantity());
}
out.close();
} catch (FileNotFoundException e) {
System.out.println("file not found");
}
}
}
Product.java
//products class
public class Product {
private String name;
private Calendar boughtOn;
private Calendar useBy;
private double boughtAt;
private int quantity;
// first product class constructor
public Product (){
name=null;
boughtOn=null;
useBy=null;
boughtAt=0;
quantity=0;
}
//second product class constructor
public Product( String pName, Calendar bOn, Calendar uBy, double bAt, int qt){
name= pName;
boughtOn=bOn;
useBy=uBy;
boughtAt= bAt;
quantity=qt;
}
//method to get the name of product
public String getName(){
return name;
}
//method to get the date that product bought on
public Calendar getBoughtOn() {
return boughtOn;
}
//method to get the date that product use by
public Calendar getUseBy() {
return useBy;
}
//method to get the purchase price of product
public double getBoughtAt() {
return boughtAt;
}
//method to get the quantity of the product
public int getQuantity() {
return quantity;
}
//method to change the product name
public void setName (String n) {
name=n;
}
//method to change the date that product bought on
public void setBoughtOn( Calendar bo) {
boughtOn=bo;
}
//method to set the date that product will expired at
public void setUseBy( Calendar ub) {
useBy=ub;
}
//method to set the purchase price of product
public void setBoughtAt( double ba) {
boughtAt= ba;
}
//method to set the product quantity
public void setQuantity( int q) {
quantity=q;
}
public String toString (){
return name +"\t" + boughtOn + "\t" + useBy + "\t" + boughtAt +"\t" + quantity;
}
public static void main(String []args){
ArrayList<Product> list;
list=DataIn.getData("products_1.txt");
DataIn.writeData(list,"save1.txt");
}
}
错误线程“main”中的异常java.lang.NullPointerException
在这行out.println（“useBy”+（p.getUseBy（））。get（Calendar.DATE））;在DataIn.java中
我想不通所以我在这里发帖全部，Mod请在java代码中重新格式化代码，我不知道该怎么做
此外，当我通过使用//避免该行时，程序可以运行，但我的输出文件只显示（买了和买了有问题）
product sugar
boughtOn 13
boughtAt 0.0
quantity 5
product sauce pan
boughtOn 13
boughtAt 0.0
quantity 10
product sugar
boughtOn 23
boughtAt 0.0
quantity 15
product plate
boughtOn 13
boughtAt 0.0
quantity 10
product orange juice
boughtOn 2
boughtAt 0.0
quantity 15
很抱歉，如果代码太长但很难解释我的问题。
答案 0 :(得分：0)
输入列表中的某些产品缺少“useby”字段，因此您的错误是正常的，因为您的某些Product对象具有NULL useby字段。