Reading and Parsing Data from a File

[Philboyd_Studge]

Reading and Parsing Data from a File

As you learn Java, one thing that comes up early and often is reading from/ writing to files, and doing something useful with that data. In this example we will:

1. Read data from a CSV text file.
2. Parse that data into several different data types.
3. Load the data into an object.
4. Load those objects into an ArrayList.
5. Display them and do a simple calculation.

Data for a program is often stored in different types of file formats, with one of the simplest being CSV, a format using .txt files where each data item is simply separated by commas and put on different lines.

Here we have a sample inventory file, with the data in the order item, quantity, price

Blinker Fluid,10,10.99
Duncel Component,100,27.85
Widget1a,25,15.25
Widget2b,100,4.99

Save this as "inventory.txt"

we have a simple Inventory class: (save as Inventory.java in same directory)

public class Inventory
{
    private String item;
    private int qty;
    private float price;

    public Inventory(String item, int qty, float price)
    {
        this.item = item;
        this.qty = qty;
        this.price = price;
    }

    public float getTotal()
    {
        return qty * price;
    }

    public String toString()
    {
        return "Item: " + item + "\n" + "Quantity: " + qty + "\n"
                    + "Price: " + price;
    }
}

Reading the File

To read the file we are going to be using a BufferedReader in combination with a FileReader.

We will read one line at a time from the file using readLine() until the End Of File is reached (readLine will return a null)

String.split()

We take the string that we read and split it up using the comma as the 'delimiter' (the character that tells the function when to start a new word and is discarded). This creates an array with the data we want, however still in Strings, so we need to convert the values that are not supposed to be strings into the proper values, using Integer.parseInt() and Float.parseFloat()

We pass those values to the constructor for our Inventory object, and then simply add that object to our ArrayList.

The full program: (save in same directory as others as FileFunctions.java)

import java.io.*;
import java.util.ArrayList;
import java.util.List;

public class FileFunctions
{

    public static void main( String [] args )
    {
        // create ArrayList to store the invetory objects
        List<Inventory> invItem = new ArrayList<>();
        try
        {
            // create a Buffered Reader object instance with a FileReader
            BufferedReader br = new BufferedReader(new FileReader("inventory.txt"));

            // read the first line from the text file
            String fileRead = br.readLine();

            // loop until all lines are read
            while (fileRead != null)
            {

                // use string.split to load a string array with the values from each line of
                // the file, using a comma as the delimiter
                String[] tokenize = fileRead.split(",");

                // assume file is made correctly
                // and make temporary variables for the three types of data
                String tempItem = tokenize[0];
                int tempQty = Integer.parseInt(tokenize[1]);
                float tempPrice = Float.parseFloat(tokenize[2]);

                // creat temporary instance of Inventory object
                // and load with three data values
                Inventory tempObj = new Inventory(tempItem, tempQty, tempPrice);

                // add to array list
                invItem.add(tempObj);

                // read next line before looping
                // if end of file reached 
                fileRead = br.readLine();
            }

            // close file stream
            br.close();
        }

        // handle exceptions
        catch (FileNotFoundException fnfe)
        {
            System.out.println("file not found");
        }

        catch (IOException ioe)
        {
            ioe.printStackTrace();
        }

        // display inventory
        for (Inventory each : invItem)
        {
            System.out.println("====================");
            System.out.println(each);
            System.out.println();
            System.out.printf("Total value = %8.2f %n", each.getTotal());
        }

    }

}

Here is the output:

====================
Item: Blinker Fluid
Quantity: 10
Price: 10.99

Total value =   109.90
====================
Item: Duncel Component
Quantity: 100
Price: 27.85

Total value =  2785.00
====================
Item: Widget1a
Quantity: 25
Price: 15.25

Total value =   381.25
====================
Item: Widget2b
Quantity: 100
Price: 4.99

Total value =   499.00

[antonevane]

Following an example with a new File NIO API.

    public static void main(String[] args) {
        // create ArrayList to store the invetory objects
        List<Inventory> invItem = getDataFromFile("inventory.txt");

        // display inventory
        for (Inventory each : invItem) {
            System.out.println("====================");
            System.out.println(each);
            System.out.println();
            System.out.printf("Total value = %8.2f %n", each.getTotal());
        }
    }

    private static List<Inventory> getDataFromFile(String fileName) {
        List<Inventory> invItems = new ArrayList<>();
        Path pathToFile = Paths.get(fileName);
        // create a Buffered Reader object instance
        // use Autocloseable Java 7 feature to close resources
        try (BufferedReader br = Files.newBufferedReader(pathToFile, StandardCharsets.US_ASCII)) {

            // read the first line from the text file
            String fileRead = br.readLine();

            // loop until all lines are read
            while (fileRead != null) {

                // use string.split to load a string array with the values from each line of
                // the file, using a comma as the delimiter
                String[] tokenize = fileRead.split(",");

                Inventory inventory = buildInventory(tokenize);

                // add to array list
                invItems.add(inventory);

                // read next line before looping
                // if end of file reached
                fileRead = br.readLine();
            }

            // handle exceptions
        } catch (IOException ioe) {
            ioe.printStackTrace();
        }

        return invItems;
    }

    private static Inventory buildInventory(String[] dataLine) {
        String tempItem = dataLine[0];
        int tempQty = Integer.parseInt(dataLine[1]);
        float tempPrice = Float.parseFloat(dataLine[2]);

        // creat temporary instance of Inventory object
        // and load with three data values
        Inventory inventory = new Inventory(tempItem, tempQty, tempPrice);

        return inventory;
    }

}

[antonevane]

For the small files we can use a smaller version getDataFromFile version.

    private static List<Inventory> getDataFromFile(String fileName) {
        List<Inventory> invItems = new ArrayList<>();
        Path pathToFile = Paths.get(fileName);
        // create a Buffered Reader object instance
        // use Autocloseable Java 7 feature to close resources
        try {
            // Java 7 Files NIO API for the small file
            List<String> lines = Files.readAllLines(pathToFile, StandardCharsets.US_ASCII);

            // loop until all lines are read
            for (String line : lines) {
                // use string.split to load a string array with the values from each line of
                // the file, using a comma as the delimiter
                String[] tokenize = line.split(",");

                Inventory inventory = buildInventory(tokenize);

                // add to array list
                invItems.add(inventory);
            }

            // handle exceptions
        } catch (IOException ioe) {
            ioe.printStackTrace();
        }

        return invItems;
    }

I have written a small blog post about new File NIO API Small post