All-in-One Guide: How to work with a CSV file in Java

Posted by Marta on December 18, 2020 Viewed 4558 times

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This article will show you all you need to know to work with a CSV file in Java. You will see how to read data from a file and how to write to a file.

Writing and reading a file is also known as deserializing and serialize. Both are widespread tasks, necessary when you need to persist your data. Let’s say, for instance, you are looping through the products of a website. You could grab this data and save it in a CSV file. Afterward, you could open this file with Excel to analyze the prices. 

A CSV file is a file where the data are separated by commas, as follows:

employees.csv

Robert Smith, 30, Sales Assistant
Kate Court, 26, Receptionist
Steve Roberts, 32, Driver

Let’s see how we can read and write the data of this CSV files in Java.

How to read a CSV file in java with examples

A simple solution to read data of a file is using the BufferedReader.readLine() method. First, you will need to open the file for reading using the method Files.newBufferedReader(). This method returns a BufferedReader to read text from the file. Next, you can utilize the BufferedReader.readLine() and read each line of the file. Let’s see this in action below:

import java.io.BufferedReader;
import java.io.IOException;
import java.nio.charset.Charset;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.util.ArrayList;
import java.util.List;

public class CSVFileHelper {
    public List<String> readLinesFromCSVFile(String path) throws IOException {
        Path filePath = Paths.get(path);
        List<String> results = new ArrayList<>();

        try(BufferedReader buffer = Files.newBufferedReader(filePath, Charset.defaultCharset())){
            String line = "";
            while((line = buffer.readLine()) != null){
                results.add(line);
            }

        }
        return results;
    }
}

You check if the code above works, add the following method to the class CSVFileHelper. This code will open the employees.csv, read all lines from the file, and print out the first line. See how this will work in Java :

public static void main(String[] args) throws IOException {

        List<String> lines = new CSVFileHelper().readLinesFromCSVFile("employees.csv");

        System.out.println(lines.get(0));
    }

Output:

Robert Smith, 30, Sales Assistant

Problem: NoSuchFileException error!

After running the above code, you might encounter the following error:

Exception in thread "main" java.nio.file.NoSuchFileException: employee.csv
	at sun.nio.fs.UnixException.translateToIOException(UnixException.java:86)
	at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:102)
	at sun.nio.fs.UnixException.rethrowAsIOException(UnixException.java:107)
	at sun.nio.fs.UnixFileSystemProvider.newByteChannel(UnixFileSystemProvider.java:214)
	at java.nio.file.Files.newByteChannel(Files.java:361)
	at java.nio.file.Files.newByteChannel(Files.java:407)
	at java.nio.file.spi.FileSystemProvider.newInputStream(FileSystemProvider.java:384)
	at java.nio.file.Files.newInputStream(Files.java:152)
	at java.nio.file.Files.newBufferedReader(Files.java:2784)
	at cvsfile.CSVFileHelper.readLinesFromCSVFile(CSVFileHelper.java:17)
	at cvsfile.CSVFileHelper.main(CSVFileHelper.java:29)

This error means that Java couldn’t find the employees.csv file. Usually, this happens because the indicated path doesn’t correspond with the real file location. By default, Java will look for the file within your project folder, using the root project as the origin. As a result, if the path is “employees.csv”, you should place this file at the project root. See the image below:

Write CSV file in java

Let’s suppose that as new employees join your company, you would like to add them to your employees.csv to keep track. Therefore you need to write to the CVS file. In Java, you can export data to a file using the BufferedWriter class and the method BufferedWriter.append(). The append() method will add any text you pass in at the end of the file.

You can create a BufferedWriter using the method Files.newBufferedWriter(), indicating the mode you want to use: WRITE, APPEND, CREATE, CREATE_NEW … in this case, since I want to add text at the end of the file, APPEND will work correctly. Note the method Files.newBufferedWriter() was included in Java 8; it won’t be available in earlier Java versions.

 public void writeLinesToCSVFile(String path, String... lines) throws IOException {
        Path filePath = Paths.get(path);
        try(BufferedWriter buffer =
                    Files.newBufferedWriter(filePath,
                                            Charset.defaultCharset(),
                                            StandardOpenOption.APPEND)){
            buffer.newLine();
            for(String line:lines){
               buffer.append(line);
               buffer.newLine();
            }
        }
    }

Note the code above won’t start adding the content on a new line; instead it will start writing right after the last character. Therefore; if you want to add a line, you need to add it yourself using the .newline() method.

To check the above code works correctly, use the following piece of code:

public static void main(String[] args) throws IOException {

        new CSVFileHelper().writeLinesToCSVFile("employees.csv",
                "Marta,32,Assistant",
                        "Jessica,35,Actor");

        List<String> lines = new CSVFileHelper().readLinesFromCSVFile("employees.csv");

        System.out.println(lines.get(0));
    }

After executing the code above, your employees.csv file should have two new lines at the end:

Robert Smith, 30, Sales Assistant
Kate Court, 26, Receptionist
Steve Roberts, 32, Driver
Marta,32,Assistant
Jessica,35,Actor

How to write Object into CSV file in java

Writing objects into a CSV file in java is similar to adding a plain string. The only difference is that you should convert the object to a string before passing the object for writing. To continue with the same example, I will create a class Person and add a new method to write Person object to the file. See this in action below:

    private static class Person{
        String name;
        int age;
        String jobTitle;

        Person(String name, int age, String jobTitle){
            this.name = name;
            this.age = age;
            this.jobTitle = jobTitle;
        }

        String toCsv(){
            return name + "," + age + "," + jobTitle;
        }
    }
    public void writeObjectsToCSVFile(String path, Person... persons) throws IOException {
        Path filePath = Paths.get(path);
        try(BufferedWriter buffer =
                    Files.newBufferedWriter(filePath,
                            Charset.defaultCharset(),
                            StandardOpenOption.APPEND)){
            buffer.newLine();
            for(Person person:persons){
                buffer.append(person.toCsv());
                buffer.newLine();
            }
        }
    }

And here is some code that will call the code above:

public static void main(String[] args) throws IOException {

        new CSVFileHelper().writeObjectsToCSVFile("employees.csv",
                new Person("Marta",32,"Assistant"),
                new Person( "Jessica",35,"Actor"));

        List<String> lines = new CSVFileHelper().readLinesFromCSVFile("employees.csv");

        System.out.println(lines.get(0));
    }

After running this code, just as before, the code added two new lines with the employee information into the CSV file.

Check out the source code here.

Conclusion

To summarise, in this article, you have learned how to read and write data into a CSV file. This task is relatively easy to achieve using BufferedReader and BufferedWriter, enabling read or write access to the file.

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