如何从文件中读取并将其存储在对象的 ArrayList 中？
How to read from files and store it in ArrayList of objects?
这是我的保存方式
public static void save() {
try {
PrintWriter myWriter = new PrintWriter("database.txt");
for(int i=0; i<people.size(); i++) {
myWriter.println(people.get(i).toString());
}
myWriter.close();
System.out.println("Successfully wrote to the file.");
menu();
} catch (IOException e) {
System.out.println("An error occurred.");
e.printStackTrace();
}
}
这是文件中的样子
Donald Trump 23323.00
这是数组列表的字段和名称
ArrayList<Person> people = new ArrayList<Person>();
public Person(String name, String password, double money) {
this.name = name;
this.password = password;
this.money = money;
}
constructors below.....
如何读取该文件并将其存储在对象数组列表中？需要帮助:D
逐行读取文件并使用您在 Person toString class.
中使用的相同分隔符
喜欢：假设您使用 " " 作为分隔符。
然后 read line by line 并使用该分隔符拆分数据并分别转换数据
String line = reader.readLine();
String[] array = line.split(" ")// use same delimiter used to write
if(array.lenght() ==3){ // to check if data has all three parameter
people.add(new Person(array[0], array[1], Double.parseDouble(array[2])));
// you have to handle case if Double.parseDouble(array[2]) throws exception
}
并不是说您写入数据文本文件的方式有任何问题，只是我认为最好遵循更传统的 CSV 样式文件格式，这种文件格式专门用于数据存储这种类型的。
例如，CSV 文件中的每一行都被视为一个记录行，通常使用逗号 (,) 来分隔该行中的字段数据列，而不是空格或制表符（如在您的文件中），并且这显然是有充分理由的。最终需要检索文件中的数据，如果列字段中包含空格怎么办？例如，一些姓氏包含两个词（Simone de Beauvoir、Herbert M. Turner III、Ashley M. St. John 等）。必须对此进行一些考虑，是的，肯定有解决方法，但总而言之，使用除空格之外的更具体的定界符更容易。您可能需要考虑将空白定界符更改为逗号或分号定界符。您甚至可以在 Person class toString() 方法中将其作为选项提供：
/* Example Person Class... */
import java.io.Serializable;
public class Person implements Serializable {
// Default serialVersion id
private static final long serialVersionUID = 1212L;
private String name;
private String password;
private double money;
public Person() { }
public Person(String name, String password, double money) {
this.name = name;
this.password = password;
this.money = money;
}
public String toString(String delimiterToUse) {
return new StringBuffer("").append(this.name).append(delimiterToUse)
.append(this.password).append(delimiterToUse)
.append(String.format("%.2f", this.money)).toString();
}
@Override
public String toString() {
return new StringBuffer("").append(this.name).append(" ")
.append(this.password).append(" ")
.append(String.format("%.2f", this.money)).toString();
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
public double getMoney() {
return money;
}
public void setMoney(double money) {
this.money = money;
}
}
并且在您的 save() 方法中，您的现有行可能会使用 class 默认分隔符空白 (" "):
myWriter.println(people.get(i).toString());
或使用不同的分隔符，如 comma/space 组合 (", "):
myWriter.println(people.get(i).toString(", "));
文件中的数据记录将类似于：
Donald Trump, myPassword, 23323.0
正上方的数据行现在可以更容易地使用类似 String#split() 方法的方法进行解析，例如：
public static List<Person> readInPeople(String databaseFile) {
/* Declare a List Interface to hold all the read in records
of people from the database.txt file. */
List<Person> people = new ArrayList<>();
// 'Try With Resouces' is used to so as to auto-close the reader.
try (BufferedReader reader = new BufferedReader(new FileReader("database.txt"))) {
String dataLine;
while ((dataLine = reader.readLine()) != null) {
dataLine = dataLine.trim();
// Skip past blank lines.
if (dataLine.equals("")) {
continue;
}
/* Split the read in dataline delimited field values into a
String Array. A Regular Expression is used within the split()
method that takes care of any comma/space delimiter combination
situation such as: "," or ", " or " ," or " , " */
String[] dataLineParts = dataLine.split("\s{0,},\s{0,}");
// Ensure defaults for people.
String name = "", password = "";
double money = 0.0d;
/* Place each split data line part into the appropriate variable
IF it exists otherwise the initialized default (above) is used. */
if (dataLineParts.length >= 1) {
name = dataLineParts[0];
if (dataLineParts.length >= 2) {
password = dataLineParts[1];
if (dataLineParts.length >= 3) {
/* Make sure the data read in is indeed a string
representation of a signed or unsigned Integer
or double/float type numerical value. The Regular
Expression within the String#matches() method
does this. */
if (dataLineParts[2].matches("-?\d+(\.\d+)?")) {
money = Double.parseDouble(dataLineParts[2]);
}
}
}
}
// Add the person from file into the people List.
people.add(new Person(name, password, money));
}
}
// Catch Exceptions...
catch (FileNotFoundException ex) {
System.err.println(ex.getMessage());
}
catch (IOException ex) {
System.err.println(ex.getMessage());
}
/* Return the list of people read in from the
database text file. */
return people;
}
要使用此方法，您可以这样做：
// Call the readInPeople() method to fill the people List.
List<Person> people = readInPeople("database.txt");
/* Display the people List in Console Window
using a for/each loop. */
// Create a header for the data display.
// Also taking advantage of the String#format() and String#join() methods.
// String#join() is used to create the "=" Header underline.
String header = String.format("%-20s %-15s %s\n", "Name", "Password", "Money");
header += String.join("", Collections.nCopies(header.length(), "="));
System.out.println(header);
// Display the list. Also taking advantage of the printf() method.
for (Person peeps : people) {
System.out.printf("%-20s %-15s %s\n", peeps.getName(), peeps.getPassword(),
String.format("%.2f", peeps.getMoney()));
}
控制台显示可能如下所示：
Name Password Money
===========================================
Donald Trump myPassword 23323.00
Tracey Johnson baseball 2233.00
Simone de Beauvoir IloveFrance 32000.00
这是我的保存方式
public static void save() {
try {
PrintWriter myWriter = new PrintWriter("database.txt");
for(int i=0; i<people.size(); i++) {
myWriter.println(people.get(i).toString());
}
myWriter.close();
System.out.println("Successfully wrote to the file.");
menu();
} catch (IOException e) {
System.out.println("An error occurred.");
e.printStackTrace();
}
}
这是文件中的样子
Donald Trump 23323.00
这是数组列表的字段和名称
ArrayList<Person> people = new ArrayList<Person>();
public Person(String name, String password, double money) {
this.name = name;
this.password = password;
this.money = money;
}
constructors below.....
如何读取该文件并将其存储在对象数组列表中？需要帮助:D
逐行读取文件并使用您在 Person toString class.
喜欢：假设您使用 " " 作为分隔符。
然后 read line by line 并使用该分隔符拆分数据并分别转换数据
String line = reader.readLine();
String[] array = line.split(" ")// use same delimiter used to write
if(array.lenght() ==3){ // to check if data has all three parameter
people.add(new Person(array[0], array[1], Double.parseDouble(array[2])));
// you have to handle case if Double.parseDouble(array[2]) throws exception
}
并不是说您写入数据文本文件的方式有任何问题，只是我认为最好遵循更传统的 CSV 样式文件格式，这种文件格式专门用于数据存储这种类型的。
例如，CSV 文件中的每一行都被视为一个记录行，通常使用逗号 (,) 来分隔该行中的字段数据列，而不是空格或制表符（如在您的文件中），并且这显然是有充分理由的。最终需要检索文件中的数据，如果列字段中包含空格怎么办？例如，一些姓氏包含两个词（Simone de Beauvoir、Herbert M. Turner III、Ashley M. St. John 等）。必须对此进行一些考虑，是的，肯定有解决方法，但总而言之，使用除空格之外的更具体的定界符更容易。您可能需要考虑将空白定界符更改为逗号或分号定界符。您甚至可以在 Person class toString() 方法中将其作为选项提供：
/* Example Person Class... */
import java.io.Serializable;
public class Person implements Serializable {
// Default serialVersion id
private static final long serialVersionUID = 1212L;
private String name;
private String password;
private double money;
public Person() { }
public Person(String name, String password, double money) {
this.name = name;
this.password = password;
this.money = money;
}
public String toString(String delimiterToUse) {
return new StringBuffer("").append(this.name).append(delimiterToUse)
.append(this.password).append(delimiterToUse)
.append(String.format("%.2f", this.money)).toString();
}
@Override
public String toString() {
return new StringBuffer("").append(this.name).append(" ")
.append(this.password).append(" ")
.append(String.format("%.2f", this.money)).toString();
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
public double getMoney() {
return money;
}
public void setMoney(double money) {
this.money = money;
}
}
并且在您的 save() 方法中，您的现有行可能会使用 class 默认分隔符空白 (" "):
myWriter.println(people.get(i).toString());
或使用不同的分隔符，如 comma/space 组合 (", "):
myWriter.println(people.get(i).toString(", "));
文件中的数据记录将类似于：
Donald Trump, myPassword, 23323.0
正上方的数据行现在可以更容易地使用类似 String#split() 方法的方法进行解析，例如：
public static List<Person> readInPeople(String databaseFile) {
/* Declare a List Interface to hold all the read in records
of people from the database.txt file. */
List<Person> people = new ArrayList<>();
// 'Try With Resouces' is used to so as to auto-close the reader.
try (BufferedReader reader = new BufferedReader(new FileReader("database.txt"))) {
String dataLine;
while ((dataLine = reader.readLine()) != null) {
dataLine = dataLine.trim();
// Skip past blank lines.
if (dataLine.equals("")) {
continue;
}
/* Split the read in dataline delimited field values into a
String Array. A Regular Expression is used within the split()
method that takes care of any comma/space delimiter combination
situation such as: "," or ", " or " ," or " , " */
String[] dataLineParts = dataLine.split("\s{0,},\s{0,}");
// Ensure defaults for people.
String name = "", password = "";
double money = 0.0d;
/* Place each split data line part into the appropriate variable
IF it exists otherwise the initialized default (above) is used. */
if (dataLineParts.length >= 1) {
name = dataLineParts[0];
if (dataLineParts.length >= 2) {
password = dataLineParts[1];
if (dataLineParts.length >= 3) {
/* Make sure the data read in is indeed a string
representation of a signed or unsigned Integer
or double/float type numerical value. The Regular
Expression within the String#matches() method
does this. */
if (dataLineParts[2].matches("-?\d+(\.\d+)?")) {
money = Double.parseDouble(dataLineParts[2]);
}
}
}
}
// Add the person from file into the people List.
people.add(new Person(name, password, money));
}
}
// Catch Exceptions...
catch (FileNotFoundException ex) {
System.err.println(ex.getMessage());
}
catch (IOException ex) {
System.err.println(ex.getMessage());
}
/* Return the list of people read in from the
database text file. */
return people;
}
要使用此方法，您可以这样做：
// Call the readInPeople() method to fill the people List.
List<Person> people = readInPeople("database.txt");
/* Display the people List in Console Window
using a for/each loop. */
// Create a header for the data display.
// Also taking advantage of the String#format() and String#join() methods.
// String#join() is used to create the "=" Header underline.
String header = String.format("%-20s %-15s %s\n", "Name", "Password", "Money");
header += String.join("", Collections.nCopies(header.length(), "="));
System.out.println(header);
// Display the list. Also taking advantage of the printf() method.
for (Person peeps : people) {
System.out.printf("%-20s %-15s %s\n", peeps.getName(), peeps.getPassword(),
String.format("%.2f", peeps.getMoney()));
}
控制台显示可能如下所示：
Name Password Money
===========================================
Donald Trump myPassword 23323.00
Tracey Johnson baseball 2233.00
Simone de Beauvoir IloveFrance 32000.00