How to write array of integers into file in Java

Question

I have an array of integers:

[56, 6090, 1510, 256, 17]

How do I write this into a file in append mode? I'll have many more arrays just like to write to the same file.. how do I append them line by line?

I want my final file to look like this:

A B C D E
56 6090 1510 256 17
60 42 3400 5499 12

How do I get this to work? How do I write the headers?

I have this but my resulting file looks strange. What are those characters?

    public static void main(String args[]) throws IOException {
            // input files
            String fileName = "SmallAreaIncomePovertyEstData.txt";
        File f = new File(fileName);

        // output files
        String tempFileName = "tempFile.txt";
        File outputf = new File(tempFileName);

        //writer to the output file
        BufferedWriter outputWriter = new BufferedWriter(new FileWriter(outputf, true));
        BufferedReader br = new BufferedReader(new FileReader(f));

          ...

            for(int i = 0; i < rowDataOnlyIntegers.length; i++) {
                outputWriter.write(rowDataOnlyIntegers[i]);
            }
            outputWriter.newLine();
        }
        br.close();
        outputWriter.close();

Possible duplicate of [How to append text to an existing file in Java](https://stackoverflow.com/questions/1625234/how-to-append-text-to-an-existing-file-in-java) — Jonas, Aug 06 '19 at 05:48
There is an `String.join()` method for creating an string out of an array: https://docs.oracle.com/javase/8/docs/api/java/lang/String.html#join-java.lang.CharSequence-java.lang.CharSequence...- — Jonas, Aug 06 '19 at 05:51

score 2 · Answer 1 · answered Aug 06 '19 at 05:52

I suggest that you use a PrintWriter to write to a file instead. The method write that you are calling writes a character encoded as the integer argument, not the string representation of the integer, which you have expected. So write(32) would write a space character, because a space is encoded as 32 in UTF-16.

PrintWriter allows you to directly write strings instead.

Here's how to use a PrintWriter.

String tempFileName = "tempFile.txt";
File outputf = new File(tempFileName);

//writer to the output file
PrintWriter outputWriter = new PrintWriter(outputf);

  ...

    for(int i = 0; i < rowDataOnlyIntegers.length; i++) {
        outputWriter.print(rowDataOnlyIntegers[i]);
        outputWriter.print(" ");
    }
    outputWriter.println();

ahoxha · Answer 2 · 2019-08-19T11:58:08.637

You can use the Java 8 API to manipulate integer arrays and write lines into a file. In my opinion, it's easier and cleaner.

The process of writing all the lines into a file is just one line of code (assuming that you don't load super large amount of data into the memory all at once):

void writeLines(Path file, List<String> lines) throws IOException {
    Files.write(file, lines, StandardOpenOption.APPEND);
}

Explanation and an example

Let's suppose you have a method that provides you with a list of integer arrays:

List<int[]> getIntArrays() {
        int[] a1 = { 23, 54, 656, 332, 556 };
        int[] a2 = { 12, 45, 6556, 232, 323 };
        int[] a3 = { 898, 787, 23, 4545, 233 };
        return Arrays.asList(a1, a2, a3);
}

You can convert each array into a one-line string this way:

String toString(int[] array) {
    String DELIMITER = " ";
    return String.join(DELIMITER, IntStream.of(array)
                                  .mapToObj(a -> String.valueOf(a))
                                  .collect(Collectors.toList()));
}

The next method converts a list of integer arrays into a list of strings:

List<String> toString(List<int[]> arrays) {
    return arrays.stream().map(a -> toString(a)).collect(Collectors.toList());
}

This list can be fed to the writeLines method above and the lines will be written to a file.

How do we write the header?

Here's a full example. It writes the header add all the lines of integers.

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.nio.file.StandardOpenOption;
import java.util.Arrays;
import java.util.List;
import java.util.stream.Collectors;
import java.util.stream.IntStream;

public class ArrayWriter {

    private final static Charset UTF8 = Charset.forName("UTF-8");
    private final static String DELIMITER = " ";

    public static void main(String[] args) throws IOException {
        Path file = Paths.get("test.txt");
        writeHeader(file);
        writeLines(file, toString(getIntArrays()));
    }

    private static void writeHeader(Path file) throws IOException {
        String line = String.join(DELIMITER, getHeader());
        Files.write(file, Arrays.asList(line), UTF8, StandardOpenOption.CREATE);
    }

    private static void writeLines(Path file, List<String> lines) throws IOException {
        Files.write(file, lines, UTF8, StandardOpenOption.APPEND);
    }

    private static List<String> toString(List<int[]> arrays) {
        return arrays.stream().map(a -> toString(a)).collect(Collectors.toList());
    }

    private static String toString(int[] array) {
        return String.join(DELIMITER,
                IntStream.of(array).mapToObj(a -> String.valueOf(a)).collect(Collectors.toList()));
    }

    public static String[] getHeader() {
        return new String[] { "A", "B", "C", "D", "E" };
    }

    public static List<int[]> getIntArrays() {
        int[] a1 = { 23, 54, 656, 332, 556 };
        int[] a2 = { 12, 45, 6556, 232, 323 };
        int[] a3 = { 898, 787, 23, 4545, 233 };
        return Arrays.asList(a1, a2, a3);
    }
}

How to write array of integers into file in Java

2 Answers2

Explanation and an example