"After all, the engineers only needed to refuse to fix anything, and modern industry would grind to a halt." -Michael Lewis

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Java IO: Paths and Files

Nov 2018

The sample code for this repository can be found on Github.

System paths and file manipulation, usually within the java.nio package, in Java allow you to forgo some of the details related to streaming of files--which, while they offer low level details and optimization opportunities, typically take longer to develop and get right.

You can find the absolute path:

public void visitingDirectories_walkSubDirectories() throws Exception {
    try (Stream<Path> entries  = Files.walk(Paths.get(Utils.pathToResources))) {
        System.out.println("counting via walk");
        long count = entries.peek(System.out::println).count();
        assertTrue(count > 0);

With the absolute path of your current working directory in place, you can then ask for the parent, filename, root, and many other metadata qualities:

public void someUsefulStuff() {
    Path absolutePath = Paths.get("").toAbsolutePath();

    Path parentOfAbsolutePath = absolutePath.getParent();

    Path fileNameOfPath = parentOfAbsolutePath.getFileName();

    Path root = absolutePath.getRoot();
    System.out.println(root); // different on windows vs unix

You can get a Scanner and scan through all the characters:

public void getScannerFromPath() throws Exception {
    Path pathToExampleFile = Paths.get(Utils.simpleExampleFilePath);

    try (Scanner scanner = new Scanner(pathToExampleFile)) {

Another way to read a file as a String (in this example, a file with UTF-8 character set) is to start by reading all the bytes from that file, and then constructing a String out of that byte array:

public void files_readAllBytes_thenStrings() throws Exception {
    Path pathToExampleFile = Paths.get(Utils.simpleExampleFilePath);

    byte[] bytes = Files.readAllBytes(pathToExampleFile);
    String content = new String(bytes, StandardCharsets.UTF_8);

    assertEquals(content, "this is some text");

You can, similarly, read in all the lines as a List<String>:

public void files_sequenceOfLines() throws Exception {
    Path pathToExampleFile = Paths.get(Utils.simpleExampleFilePath);

    List<String> allLines = Files.readAllLines(pathToExampleFile);

    assertEquals("this is some text", allLines.get(0));

For write operations, you can write a string by converting them to bytes, with the appropriate character set, then using Files.write(..):

public void files_writeStringToFile() throws Exception {
    String stuffToWrite = "some new text";

    Path newFilePath = Paths.get(Utils.pathToResources + "new-file-with-text.txt");

    Files.write(newFilePath, stuffToWrite.getBytes(StandardCharsets.UTF_8));

    assertEquals(stuffToWrite, Utils.readFileAsText(newFilePath));

The default behavior of Files.write(..) is to overwrite a file if it already exists. If you want to append a file, you can do so by passing the StandardOpenOption enumerated value into the third argument:

public void files_appendToFile() throws Exception {
    String beginnings = "some next text\n";

    Path newFilePath = Paths.get(Utils.pathToResources, "append-string-ex.txt");
    Files.write(newFilePath, beginnings.getBytes(StandardCharsets.UTF_8));

    // append
    for (int i = 0; i < 5; i++) {
        Files.write(newFilePath, beginnings.getBytes(StandardCharsets.UTF_8), StandardOpenOption.APPEND);

    String writtenText = Utils.readFileAsText(newFilePath);
    assertTrue(writtenText.startsWith(beginnings + beginnings));

Nick Fisher is a software engineer in the Pacific Northwest. He focuses on building highly scalable and maintainable backend systems.