Files

• The memory triangle

• File data
  • The Java view of the disk
  • Java supports two types of file data: byte and char
  • Files may be read/written directly at the byte or char level.
  • Byte data is transferred using classes that build on the FileInputStream and FileOutputStream classes.
  • Character data is transferred using classes that build on the FileReader and FileWriter classes.
  • The java.io package provides classes and methods for reading and writing files.

• An object of the File class makes a connection to a file name. It does not open the file, or check the disk to see if the file exists.
  • The exists method checks if the name exists on the disk
  • The isFile method checks if the name is a file
  • The isDirectory method checks if the name is a directory
  • The canRead method checks if the name is a file that can be read, or a directory that can be listed
  • The canWrite method checks if the name is a file that can be written, or a directory that can have files added or removed
  • The length method returns the length of the named file
  • The list method returns an array of names in a directory
  • Example: FileExamine.java

• Files and exceptions
  • Many of the file handling methods throw exceptions.
  • The most common is an IOException, and indeed the others are all "types of" IOExceptions (it will make more sense after we've covered inheritance).
  • IOExceptions are checked exceptions, i.e., it is necessary to handle them or pass them up to the calling code.

• Reading character files
  • These files store data in character format
  • The FileReader class reads characters from a file.
    • The constructor takes a file name or File object argument, and may throw a FileNotFoundException (which is a type of IOException)
    • The read method reads the next character from the file, returning it as an int. It may throw a IOException. Returns -1 when the end of the file is reached
    • The close method closes the file (good habit there). It may throw a IOException.
  • Example: FileCat.java
  • Catching the IOExceptions is nice, but sometimes gets to be a pain. One solution is just to hand them up.
  • Example: FileCatUp.java
  • Example: FileCatUpMain.java

• Writing character files
  • The FileWriter class writes characters to a file.
    • The constructor takes a file name or a File object argument.
    • The write method writes a character to the file. It may throw an IOException.
    • The close method closes the file (good habit there). It may throw an IOException.
  • Example: FileCopyChar.java

• Files of objects
  • These files store objects in serialized format
  • Serialization converts an object to a stream of bytes
    • An object is serializable only if its class implements the Serializable interface.
    • Making instances of your classes serializable is easy. You just add the implements Serializable clause to your class declaration like this:

      public class MySerializableClass implements Serializable {
          ...
      } 

    • More about serialization after inheritance
  • The ObjectOutputStream class writes objects to a file, in a serialized form. The constructor takes a FileOutputStream object argument.
  • The ObjectInputStream class reads serialized objects from a file. The constructor takes a FileInputStream object argument.
  • These classes have methods for manipulating files of objects
    • Constructors initialize ObjectOutputStream and ObjectInputStream objects. They may throw an IOException.
    • writeObject writes an object to an ObjectOutputStream. It may throw an IOException.
    • readObject reads an object from an ObjectInputStream. It is necessary to type cast the objects that are read in. It may throw an IOException or a ClassNotFoundException. The end-of-file is recognized by catching the EOFException.
    • close closes a stream (and you should always do that when you've finished using a stream, for many reasons). It may throw an IOException.
  • Example: FileCar.java, FileCarDealer.java, FileShonkyCars.java (Class picture)
  • When an object is written, all objects referenced from within are also written. This allows large structures to be written to file in a single step.
  • Example: FileCar.java, FileAllCarDealer.java, FileAllShonkyCars.java (Class picture)

• Files of text
  • These files store data in text format (lotsa characters)
    • The BufferedReader class reads lines from a file. The constructor takes a FileReader object argument.
    • The PrintWriter class writes Strings to a file. The constructor takes a file name or FileOutputStream object argument (yes, that's weird).
    • Example: FileTextEdit.java
  • The Scanner class reads common data types from a text file. The constructor takes a FileInputStream object argument.
    • Example: FileScanner.java (run on FileScannerInput.txt)
  • PrintWriter provides the print, println, and printf methods.
    • Example: FilePrintf.java

• Files of Java data types
  • These files store data in binary format
  • FileInputStreams and FileOutputStreams are used to construct the more general DataInputStreams and DataOutputStream.
  • The DataInputStream class reads Java types from a file. The constructor takes a FileInputStream object argument.
  • The DataOutputStream class writes Java types to a file. The constructor takes a FileOutputStream object argument.
  • Example: FileJavaTypes.java

• The Hierarchy (of the above)

  

Exercises

Exam Style Questions

• What are the two basic types of data that Java can store in files?
• What type of exception is thrown by many file operations?
• A FileReader object reads characters from a file using its read method. Write a program that reads the text file named as its command line argument and prints it out to the screen. You must catch exceptions.
• A BufferedReader object, which is created from a FileReader object, reads characters from a file using its readLine method. Write a program that reads the text file named as its command line argument and prints it out to the screen. You must catch exceptions.
• What does "serialization" do? How do you make objects serializable? When reading in serialized objects, why is it necessary to typecast them to the type of object that you are reading?