Skipping lines before the CSV data

Some CSV files contain one or more lines of text before the actual CSV data starts. For example, it could look like this:

example.csv

This is an example of a CSV file that contains
three lines before the actual CSV records.

header 1,header 2
value 1,value 2

Strictly speaking, such a file is not a valid CSV file as defined by the CSV specification (RFC 4180).

The main problem with those files is:

• When working with named fields, the very first line (This is an example of a CSV file that contains) would be interpreted as the actual header line.
• An exception would be thrown with the default extraFieldStrategy(FieldMismatchStrategy.STRICT), as some lines have more fields than the first line.

FastCSV comes with two features to handle such files:

• skipLines(int lineCount): Skip a specific number of lines (lineCount) regardless of their content.
• skipLines(Predicate<String> predicate, int maxLines): Skip lines until a specific line (e.g., the header) is found. Throws a CsvParseException if none of the first maxLines lines matches.

Note

Do not mix this with comments in CSV files. Comments are lines that start with a specific character (e.g., #) and can be skipped by FastCSV using a CommentStrategy. See the example on how to handle comments for more information.

Example

This example demonstrates how to skip non-CSV head lines when reading such a CSV file with FastCSV.

ExampleCsvReaderWithNonCsvAtStart.java

import java.io.IOException;
import java.util.function.Predicate;

import de.siegmar.fastcsv.reader.CsvReader;

/// Example for reading CSV data with non-CSV data before the actual CSV header.
///
/// FastCSV supports Java 17 and later, but this code uses Java 25
/// for brevity, leveraging newer language features.
void main() throws IOException {
    final String data = """
        Your CSV file contains some non-CSV data before the actual CSV header?
        And you don't want to (mis)interpret them as CSV header? No problem!

        header1,header2
        foo,bar
        """;

    alternative1(data);
    alternative2(data);
}

void alternative1(final String data) throws IOException {
    IO.println("Alternative 1 - ignore specific number of lines");

    try (var csv = CsvReader.builder().ofNamedCsvRecord(data)) {
        // Skip the first 3 lines
        IO.println("Skipping the first 3 lines");
        csv.skipLines(3);

        // Read the CSV data
        csv.forEach(IO::println);
    }
}

void alternative2(final String data) throws IOException {
    IO.println("Alternative 2 - wait for a specific line");

    final Predicate<String> isHeader = line ->
        line.contains("header1");

    try (var csv = CsvReader.builder().ofNamedCsvRecord(data)) {
        // Skip until the header line is found, but not more than 10 lines
        final int actualSkipped = csv.skipLines(isHeader, 10);
        IO.println("Found header line after skipping " + actualSkipped + " lines");

        // Read the CSV data
        csv.forEach(IO::println);
    }
}

You also find this source code example in the FastCSV GitHub repository.