Aigle_Notes

Files

Files are collections of data stored in the hard drive of a computer. Their format determines what data can be stored in it as well as the program required to read it.

• Information is made of bits [0s and 1s].
• Modern hard drives hold about 8 trillion bits.

.psd files are read using Adobe Photoshop, .txt can be read using any text editor

Filesystem of a computer is responsible for keeping track of file location and performing operations [such as creating new files and such]. It abstracts the structure of a hard drive [i.e. you don’t need to worry about how a file is actually stored].

Reading from a file

File class

We import the input/output utility File in our program. Then we create an object and assign it our file’s parameters.

• File class tends to be more object-oriented.

import java.io.File;
public class reading
{
    public static void main(String[] args)
    {
        File file = new File("C:\\Users\\Ambha\\Python\\Java\\Nothing\\one.txt");
        //program goes here
    }
}

Although it looks simple to use, there are a few drawbacks:

• Can’t throw exceptions if error occurs
• Doesn’t produce desired output at times
• Doesn’t support symbolic links

It is better to use Path class instead. Here’s how you convert a File class object to Path class :

Path varName = fileName.toPath();

Path class

We import the Path utility to our program and then create an object.

import java.nio.file.Path;
public class reading
{
    public static void main(String[] args)
    {
        Path file = Paths.get("fileName");
        //program
    }
}

Now, we’ve some technical terms to review here. Let’s check them out one-by-one…

• Absolute path : Complete location of a file; starts from root directory till the file name.
• Relative path : Location of a file described from the current directory.
• Canonical path : Shortest absolute path to a file.
• URI [Uniform Resource Identifier] : Sequence of characters that allows complete identification of a resource. Similar to URL.

[!example] Path : file.txt Absolute path : /home/User/test/file.txt

To read the file, we create a Scanner object and assign it the Path object. While doing this, we surround the code with try and catch blocks to handle errors.

• After that, we read each line using a while loop that will break when the scanner reaches the end of the file.

while (fileScan.hasNextLine())
{
    String line = fileScan.nextLine();
}

[!note] Path class often throws an IOException and returns a Boolean value when any operation is done.

Empty lines in file

Sometimes we may encounter an empty line while reading a file. To skip it, we use continue and isEmpty() method.

//some program
while (fileScan.hasNextLine())
{
    String line = fileScan.nextLine();
    if (line.isEmpty())
    {
        continue;
    }
}

Comma-separated values [CSV]

Note that if we’re reading from a CSV file, we’ve to split the string according to commas.

//some program before
while (true)
{

 System.out.print("Enter name and age separated by a comma: ");
 String line = scanner.nextLine();

 if (line.equals(""))
 {
  break;
 }

 String[] parts = line.split(",");
 String name = parts[0];
 int age = Integer.valueOf(parts[1]);

 //printing name and age
}

Reading objects from a file

We should create a method for creating objects from file data.

In that method, we create an ArrayList object and then read the file using a while loop [like we do with a CSV file] and assigning the variables created for each part of a line as object attributes. Then we’d use a for-each loop to print every item of the ArrayList object.