How to create a csv file from ConcurrentHashMap in Java?

Question

I need to create a single string named CSVOutput, and load all keys and values from the ConcurrentHashMap<String, Integer>, counts into this file separated by commas. So I'll have two columns in the resulting csv file, one is for the keys and the other one for the values.

I have written this code for my method OutputCountsAsCSV:

private void OutputCountsAsCSV(ConcurrentHashMap<String, Integer> counts, String filename) {
    String CSVOutput = new String("");
    for (Entry<String, Integer> entry : counts.entrySet()) {
        String rowText = String.format("%s,%d\n", entry.getKey(), entry.getValue());  # defines new lines/rows
        CSVOutput += rowText;
        try (FileWriter writer = new FileWriter(filename)) {
            writer.write(CSVOutput);
        }
        catch (Exception e) {
            System.out.println("Saving CSV to file failed...");
        }
    }
    System.out.println("CSV File saved successfully...");
}

However, I'm told I should have followed this order:

1. create and prepare file
2. for each entry, write data to file
3. close the file (and print that it happened successfully).

Indeed, my System.out.println("CSV File saved successfully...") seems to be inside the loop and is being printed many times.

How can I do it correctly?

I am a beginner in Java, so I'm very grateful for any help.

Answer 1

Here is a better approach (do your own research about the finally keywords)

private void OutputCountsAsCSV(ConcurrentHashMap<String, Integer> counts, String filename) {
            
    try (FileWriter writer = new FileWriter(filename)) {
        
        for (Entry<String, Integer> entry : counts.entrySet()) {
            
            String rowText = String.format("%s,%d\n", entry.getKey(), entry.getValue());  
            writer.append(rowText);
        }
    
    } catch (Exception e) {
        System.out.println("Saving CSV to file failed...");
    } finally {
        System.out.println("CSV File saved successfully...");      
    } 
}

Answer 2

The Answer by qdoot seems correct. I'll add a few points:

• Use the more general Map as the type of your parameter. See another Question.
• Use NIO.2 in modern Java for working with files. See Wikipedia.
• The RFC 4180 standard specification for CSV, Common Format and MIME Type for Comma-Separated Values (CSV) Files, requires CRLF (Carriage Return, Linefeed) as the line terminator.
• Note that a Map such as ConcurrentHashMap may iterate in any arbitrary order. If you care about order, use a NavigableMap such as TreeMap.

Core code:

try (
        final Writer writer = Files.newBufferedWriter( path , StandardCharsets.UTF_8 ) ;
)
{
    for ( Map.Entry < String, Integer > entry : nameToNumberMap.entrySet() )
    {
        String line = String.format( "%s,%d" , entry.getKey() , entry.getValue() );
        writer.write( line.concat( CRLF ) );
    }
}
catch ( IOException e )
{
    System.out.println( "ERROR - Failed when opening or writing to file. " + Instant.now() + " Message # da8fc3ac-1f6b-4722-bbf9-a86f2acbb77f." );
    throw new RuntimeException( e );
}
System.out.println( "INFO - Done writing CSV file. " + Instant.now() + " Message # fa26f37e-70b7-42e8-a8cf-bf72ad0b2da8." );

Full example app.

package work.basil.example.csv;

import java.io.IOException;
import java.io.Writer;
import java.nio.charset.StandardCharsets;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.time.Instant;
import java.util.Map;

public class App
{
    public static void main ( String[] args )
    {
        App app = new App();
        app.demo();
    }

    private void demo ( )
    {
        Map < String, Integer > nameToNumberMap =
                Map.of(
                        "Alice" , 1 ,
                        "Bob" , 2 ,
                        "Carol" , 3
                );
        Path path = Paths.get( "/Users/basil_dot_work" , "demo.csv" );
        System.out.println( path.toAbsolutePath() );
        this.writeCsv( nameToNumberMap , path );
    }

    private void writeCsv ( final Map < String, Integer > nameToNumberMap , final Path path )
    {
        // The RFC 4180 standard for CSV requires a CRLF (Carriage Return, Linefeed) as the line terminator. *Not* platform-specific, not Unix-style Linefeed.
        final String CRLF = Character.toString( 13 ) + Character.toString( 10 );

        // Open file.
        try (
                final Writer writer = Files.newBufferedWriter( path , StandardCharsets.UTF_8 ) ;
        )
        {
            for ( Map.Entry < String, Integer > entry : nameToNumberMap.entrySet() )
            {
                String line = String.format( "%s,%d" , entry.getKey() , entry.getValue() );
                writer.write( line.concat( CRLF ) );
            }
        }
        catch ( IOException e )
        {
            System.out.println( "ERROR - Failed when opening or writing to file. " + Instant.now() + " Message # da8fc3ac-1f6b-4722-bbf9-a86f2acbb77f." );
            throw new RuntimeException( e );
        }
        finally
        {
            System.out.println( "INFO - Done writing CSV file. " + Instant.now() + " Message # fa26f37e-70b7-42e8-a8cf-bf72ad0b2da8." );
        }
    }
}

When run.

/Users/some_user/demo.csv
INFO - Done writing CSV file. 2022-08-16T20:06:07.610196Z Message # fa26f37e-70b7-42e8-a8cf-bf72ad0b2da8.

Note to other readers: We used try-with-resources syntax to make sure the file is always closed gracefully, even when encountering exceptions. The code in the Question does the same. See The Java Tutorials by Oracle Corp.

This Question & Answer is for learning purposes. In real work, I would not generate CSV text. I would use one of the several good libraries available in the Java ecosystem for handling CSV text. Example: Apache Commons CSV.