15. File Intro

Sample code: class github.

You should be familiar with the basic methods listed in the JavaMethods Chapter 15 PowerPoint.

File

The official File documentation has all of the details. This is a brief summary of methods you are likely to use.

• new File("filename"). I recommend you only files in the same directory as your Java program, until you become very comfortable with Files.
• String getName()
• long lastModified(): Return modification time, in unusual units.
• boolean isDirectory()
• File[] listFiles()

Scanner

Consult the Chapter 15 PowerPoint for common functions or the official documentation for full information.

Miscellaneous:

• NoSuchElementException is returned when a Scanner is asked for something that it cannot provide - for example, nextInt() when the next item is not an integer.

PrintWriter

• PrintWriter out = PrintWriter (file): Construct based on a File object.
• out.println(): Print, like usual.
• out.close(): Write all of the data to the disk.

Common Problems and Advice

• Read error messages closely.

  • Know what the error is. For example, this is a FileNotFound error caused by trying to read myfolder with Scanner, like a file.

      java.io.FileNotFoundException: myfolder (Is a directory)
          at java.io.FileInputStream.open0(Native Method)
          at java.io.FileInputStream.open(FileInputStream.java:195)
          at java.io.FileInputStream.<init>(FileInputStream.java:138)
          at java.util.Scanner.<init>(Scanner.java:611)
    

  • from a File? Know what File causes it.

  • from a Scanner? Know exactly what input causes the problem. Print lots! For example, this error comes from trying to get the next() item when there is nothing more available.

      java.util.NoSuchElementException
          at java.util.Scanner.throwFor(Scanner.java:862)
          at java.util.Scanner.next(Scanner.java:1371)
    

• Make sure you only call listFiles on a File that is a directory.

• Reading a directory like a file will give you a FileNotFound or permissions error.

• Listing the files in a file that is not a directory will return null but not give an error. This is dangerous because you cannot loop through the returned array (since nothing is created).

• You can get very unusual error messages if you start recursively examining a folder that contains your system files. Make folders especially for testing.