Quiz yourself: Use the Files class to copy or move files and directories

Test your knowledge of the java.nio.file.Files class.

October 19, 2020 | Download a PDF of this article
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The objective of this quiz is to determine what you know about using the java.nio.file.Files class and how it works with files and directories.

Which of the following statements is true? Choose one.

A. It cannot move a nonempty directory.
B. It cannot copy a file if a target file with the same name already exists.
C. It can copy a file to a nonexistent folder and the required target folder tree will be created automatically.
D. It can move a file atomically.

Answer. The java.nio.file.Files class in Java 11 provides a wide variety of static utility methods for working with files and directories. Two particular methods from this group are discussed in this question. The following methods copy and move a file in the file system:

Files.copy (Path source, Path target, CopyOption... options)
Files.move (Path source, Path target, CopyOption... options)

By the way, the Files class contains three overloaded methods called copy, but the other two work with input/output stream data.

The Files.move() method has some restrictions related to directories, which are described in its documentation as follows: “This method may be invoked to move an empty directory.”

The documentation then continues to constraints on moving nonempty directories, which can sometimes be moved. The ability to move a nonempty directory is based on whether the source and target are in the same java.nio.file.FileStore, which might correspond, for example, to being on the same disk. Given these observations, option A is incorrect.

The documentation for the copy method notes that, by default, the Files.copy() method throws a java.nio.file.FileAlreadyExistsException exception if a target file already exists. However, the developer has the option to alter this behavior by adding StandardCopyOption.REPLACE_EXISTING as one of the varargs parameters to the method invocation. In this case, if the target file exists, it will be overwritten. From this, you can see that option B is incorrect.

Option C suggests that missing directories leading to the target of a copy operation will be created automatically. This is not obvious from the documentation, but if you try it, you’ll find that it will fail. In this situation, the method will throw a java.nio.file.NoSuchFileException exception. In view of this, you can see that option C is incorrect.

Note: The documentation for the Files utility class describes the createDirectories method by noting that it “Creates a directory by creating all nonexistent parent directories first.” Clearly this would be a helpful method for addressing the goal implicit in option C, that is, copying a file to a nonexistent folder and creating the needed directory tree automatically.

What about option D? The documentation for the move method describes the StandardCopyOption.ATOMIC_MOVE option, which can be used to move a file atomically. However, as with option A, this will work only within the same FileStore. If you try to move atomically from one FileStore to another, the method will throw an exception: java.nio.file.AtomicMoveNotSupportedException. In that case, the move operation can succeed if you don’t require the atomic behavior. Of course, such a move will be slightly less reliable since it might copy some of the files and then fail, resulting in an unpredictable final state on the file system. So, don’t do that.

Conclusion: The correct answer is option D.

Simon Roberts

Simon Roberts joined Sun Microsystems in time to teach Sun’s first Java classes in the UK. He created the Sun Certified Java Programmer and Sun Certified Java Developer exams. He wrote several Java certification guides and is currently a freelance educator who publishes recorded and live video training through Pearson InformIT (available direct and through the O’Reilly Safari Books Online service). He remains involved with Oracle’s Java certification projects.

Mikalai Zaikin

Mikalai Zaikin is a lead Java developer at IBA IT Park in Minsk, Belarus. During his career, he has helped Oracle with development of Java certification exams, and he has been a technical reviewer of several Java certification books, including three editions of the famous Sun Certified Programmer for Java study guides by Kathy Sierra and Bert Bates.

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