如何从文件中读取并将其存储在对象的 ArrayList 中？

Question

This is my save method

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();
        }
    }

This is what it looks like in the file

    Donald     Trump  23323.00

This is the fields and the name of the arraylist

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.....

How do i read that file and store it in the arraylist of objects? Need help :D

Answer 1

Not that there is anything wrong with the way you have written to your data text file, it's just that I think it is better to follow a more conventional CSV style file format which is specific for data storage of this type.

For example, each line within a CSV file is considered a record row and typically a comma (,) is used to separate columns of field data within that row instead of a whitespace or tab (like in your file) and there is obviously good reason for that. Eventually that data within the file will need to be retrieved, what if a column field contains a whitespace in it? Some last names for example contain two words (Simone de Beauvoir, Herbert M. Turner III, Ashley M. St. John, etc). Some consideration must be given for this and yes, there is definitely a work-around for this but all in all, it's just easier to utilize a more specific delimiter other that whitespace. You may want to consider changing your whitespace delimiter for perhaps a comma or semicolon delimiter. You could even provide this as an option within your Person class toString() method:

/* 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;
    }
}

And in your save() method you might have your existing line to utilize the class default delimiter of whitespace ( " " ):

myWriter.println(people.get(i).toString());

or utilize a different delimiter like a comma/space combination ( ", " ):

    myWriter.println(people.get(i).toString(", "));

The data records in file would then look something like:

Donald Trump, myPassword, 23323.0

This data line directly above would now be easier to parse using something like the String#split() method, for example:

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;
}

To use this method you might do it something like this:

// 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()));
}

The Console display could look something like this:

Name                 Password        Money
===========================================
Donald Trump         myPassword      23323.00
Tracey Johnson       baseball        2233.00
Simone de Beauvoir   IloveFrance     32000.00

Answer 2

Read the file line by line and use same delimiter you have used in toString of Person class.

Like: let se you have used " " as delimiter.

then read line by line and split the data using that delimiter and convert the data respectively

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
}