How to Read a File in Java: A Comprehensive Tutorial

In this Tutorial, we will learn about how to read a file in Java. File manipulation is a fundamental aspect of programming, especially when dealing with data processing and storage. Java provides robust libraries and classes to handle file operations efficiently. In this in-depth tutorial, we will explore the various techniques and best practices for reading files in Java.

Understanding File Processing in Java

Before delving into file reading techniques, it’s crucial to understand the basics of file processing in Java. Files are represented by the java.io.File class, which encapsulates the path to a file or directory. Java offers multiple classes like FileReader, BufferedReader, and Scanner to facilitate reading operations.

Reading Text Files Using FileReader and BufferedReader

Using FileReader and BufferedReader Classes

The FileReader class is used for reading character files. It works at the byte level, reading streams of characters. BufferedReader class, on the other hand, reads text from a character-input stream, buffering characters to provide efficient reading.

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.IOException;

public class TextFileReader {
    public static void main(String[] args) {
        String filePath = "example.txt";
        try (BufferedReader reader = new BufferedReader(new FileReader(filePath))) {
            String line;
            while ((line = reader.readLine()) != null) {
                System.out.println(line);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

In this example, we read a text file line by line using FileReader wrapped in a BufferedReader.

Reading CSV Files Using Scanner Class

CSV files are widely used for storing tabular data. Java’s Scanner class simplifies the process of reading from various sources, including files. Let’s see how we can read data from a CSV file.

Reading CSV File Using Scanner

import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;

public class CSVFileReader {
    public static void main(String[] args) {
        String filePath = "data.csv";

        try (Scanner scanner = new Scanner(new File(filePath))) {
            scanner.useDelimiter(",");

            while (scanner.hasNext()) {
                System.out.print(scanner.next() + " ");
            }
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        }
    }
}

In this example, the Scanner reads the CSV file and separates values using a comma (,).

Best Practices and Error Handling

Handling Exceptions

When dealing with file operations, exceptions such as FileNotFoundException and IOException must be handled properly to ensure graceful error recovery and prevent application crashes.

Using Try-With-Resources

Java 7 introduced the try-with-resources statement, which ensures that each resource is closed at the end of the statement. It simplifies resource management and reduces the chance of resource leaks and related issues.

try (BufferedReader reader = new BufferedReader(new FileReader(filePath))) {
    // Read file content here
} catch (IOException e) {
    e.printStackTrace();
}

Conclusion

In this extensive tutorial, we explored various techniques for reading files in Java, ranging from basic text files to more complex CSV files. Understanding the classes and methods provided by Java’s I/O packages is essential for effective file processing.

Remember to handle exceptions diligently and use try-with-resources to manage resources efficiently. With the knowledge gained from this tutorial, you can confidently read and manipulate files in your Java applications, ensuring smooth and reliable data processing.

By incorporating these practices and techniques into your Java projects, you are well-equipped to handle a wide array of file-reading scenarios, making your applications more versatile and robust. If you face any problem to read a file using java programming then you can search for Java assignment help. Happy coding

How do I created tab delimited data file in Java?

The following code snippet show you how to create a tab delimited data file in Java. The tab character is represented using the \t sequence of characters, a backslash (\) character followed by the t letter. In the code below we start by defining some data that we are going to write to the file.

We create a PrintWriter object, passes a BufferedWritter created using the Files.newBufferedWriter() method. The countries.dat is the file name where the data will be written. Because we are using the try-with-resources the PrintWriter and the related object will be closed automatically when the file operation finishes.

package org.kodejava.io;

import java.io.IOException;
import java.io.PrintWriter;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.List;

public class TabDelimitedDataFile {
    public static void main(String[] args) throws IOException {
        List<String[]> data = new ArrayList<>();
        data.add(new String[]{"Afghanistan", "AF", "AFG", "004", "Asia"});
        data.add(new String[]{"Åland Islands", "AX", "ALA", "248", "Europe"});
        data.add(new String[]{"Albania", "AL", "ALB", "008", "Europe"});
        data.add(new String[]{"Algeria", "DZ", "DZA", "012", "Africa"});
        data.add(new String[]{"American Samoa", "AS", "ASM", "016", "Polynesia"});
        data.add(new String[]{"Andorra", "AD", "AND", "020", "South Europe"});
        data.add(new String[]{"Angola", "AO", "AGO", "024", "Africa"});
        data.add(new String[]{"Anguilla", "AI", "AIA", "660", "Americas"});
        data.add(new String[]{"Antarctica", "AQ", "ATA", "010", ""});
        data.add(new String[]{"Argentina", "AR", "ARG", "032", "Americas"});

        try (PrintWriter writer = new PrintWriter(
                Files.newBufferedWriter(Paths.get("countries.dat")))) {
            for (String[] row : data) {
                writer.printf("%1$20s\t%2$3s\t\t%3$3s\t\t%4$3s\t\t%5$s",
                        row[0], row[1], row[2], row[3], row[4]);
                writer.println();
            }
        }
    }
}

The output of the code snippet above are:

         Afghanistan     AF     AFG     004     Asia
       Åland Islands     AX     ALA     248     Europe
             Albania     AL     ALB     008     Europe
             Algeria     DZ     DZA     012     Africa
      American Samoa     AS     ASM     016     Polynesia
             Andorra     AD     AND     020     South Europe
              Angola     AO     AGO     024     Africa
            Anguilla     AI     AIA     660     Americas
          Antarctica     AQ     ATA     010     
           Argentina     AR     ARG     032     Americas

How do I read text file content line by line to a List of Strings using Commons IO?

The following example show how to use the Apache Commons IO library to read a text file line by line to a List of String. In the code snippet below we will read the contents of a file called sample.txt using FileUtils class. We use FileUtils.readLines() method to read the contents line by line and return the result as a List of Strings.

package org.kodejava.commons.io;

import org.apache.commons.io.FileUtils;

import java.io.File;
import java.io.IOException;
import java.util.List;

public class ReadFileToListSample {
    public static void main(String[] args) {
        // Create a file object of sample.txt
        File file = new File("README.md");

        try {
            List<String> contents = FileUtils.readLines(file, "UTF-8");

            // Iterate the result to print each line of the file.
            for (String line : contents) {
                System.out.println(line);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

Maven Dependencies

<dependency>
    <groupId>commons-io</groupId>
    <artifactId>commons-io</artifactId>
    <version>2.14.0</version>
</dependency>

Maven Central

How do I copy a file in JDK 7?

In this example you’ll see how to copy a file using the new API provided in the JDK 7. The first step is to define the source and the target of the file to be copied. For this we can use the Path class. To create an instance of Path we use the Paths.get() method by passing the path information as the arguments.

Next we can configure the file copy operation option. For this we can define it as an array of CopyOtion. We can use copy option such as StandardCopyOption.REPLACE_EXISTING and StandardCopyOption.COPY_ATTRIBUTES.

Finally, to copy the file we use the Files.copy() method. We give three arguments to this method, they are the source file, the target file and the copy options information.

Let’s see the code snippet below:

package org.kodejava.io;

import java.io.IOException;
import java.nio.file.*;

public class NioFileCopyDemo {
    public static void main(String[] args) {
        // Define the source and target of the file to be copied.
        Path source = Paths.get("D:/resources/data.txt");
        Path target = Paths.get("D:/resources/data.bak");

        // Define the options used in the file copy process.
        CopyOption[] options = new CopyOption[]{
                StandardCopyOption.REPLACE_EXISTING,
                StandardCopyOption.COPY_ATTRIBUTES
        };

        try {
            // Copy file from source to target using the defined 
            // configuration.
            Files.copy(source, target, options);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

How do I set the value of file attributes?

This code snippet show you an example on how to set the value of file attributes. Here we will set the DosFileAttributes. To set the value of file attributes we use the Files.setAttributes() method. To set DosFileAttributes we can use the following attributes: "dos:archive", "dos:hidden", "dos:readonly" and "dos:system".

For details let’s see the code snippet below:

package org.kodejava.io;

import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.attribute.DosFileAttributes;

public class UpdateDosFileAttributesExample {
    public static void main(String[] args) throws Exception {
        String path = "D:/resources/data.txt";
        Path file = Paths.get(path);

        // Get current Dos file attributes and print it.
        DosFileAttributes attr = Files.readAttributes(file, DosFileAttributes.class);
        printAttributes(attr);

        // Set a new file attributes.
        Files.setAttribute(file, "dos:archive", false);
        Files.setAttribute(file, "dos:hidden", false);
        Files.setAttribute(file, "dos:readonly", false);
        Files.setAttribute(file, "dos:system", false);

        // Read the newly set file attributes and print it.
        attr = Files.readAttributes(file, DosFileAttributes.class);
        printAttributes(attr);
    }

    /**
     * Print the DosFileAttributes information.
     *
     * @param attr DosFileAttributes.
     */
    private static void printAttributes(DosFileAttributes attr) {
        System.out.println("isArchive()  = " + attr.isArchive());
        System.out.println("isHidden()   = " + attr.isHidden());
        System.out.println("isReadOnly() = " + attr.isReadOnly());
        System.out.println("isSystem()   = " + attr.isSystem());
        System.out.println("----------------------------------------");
    }
}

The output of the code snippet:

isArchive()  = true
isHidden()   = true
isReadOnly() = true
isSystem()   = true
----------------------------------------
isArchive()  = false
isHidden()   = false
isReadOnly() = false
isSystem()   = false
----------------------------------------