Download Binary File I/O

COMP1681 / SE15
Introduction
to Programming
Lecture 34
File I/O -Part 2
Today’s Learning Objectives
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To understand the File Class
To Learn how to use the classes
ObjectOutputStream and ObjectInoutStream to
write and read binary files
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Lecture Outline
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The File Class
Binary file I/O
Object I/O with object Streams
Using the Serializable interface
Savitch Chapter 9
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Using the File Class
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The File class is a wrapper class for file names. The constructor
takes a String as an argument and produces an object that can be
thought of as a file with that name.

It also allows you to test properties of the file such as if a file exists
or if you have permission to read it.
File fileObject = new File(“myfile.txt”);
if(! fileObject.exists())
System.out.println(“No file by that name”);
if(! fileObject.canRead())
System.out.println(“Not allowed to read that
file”);
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Some Methods of the File Class
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exists()
boolean canRead()
boolean canWrite()
boolean delete()
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long length()
String getName()
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tries to delete the file
Returns the name of the file (not path)
String getPath()
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Returns the path name
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Binary File I/O
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Files are stored in the same form as they are in the
computer's memory.
This means they are efficient (compact)
Binary files created by a Java program can be moved
between computers and still read by a Java program –
but only a Java program.
The preferred classes for processing binary files are:


ObjectInputStream
ObjectOutputStream
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Opening a binary file for writing

Opening a binary file for writing is similar to text files

The ObjectOutputStream class has no constructor that takes
a file name as its argument so we use the class
FileOutputStream to create a stream that can be used as an
argument to the ObjectOutputStream constructor e.g.
new FileOutputStream(“out.dat”)

ObjectOutputStream outputStream = new
ObjectOutputStream(new
FileOutputStream(“out.dat”));
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Writing primitive types to binary files
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Once a stream class is connected to a file, you can write various
primitive types to a file using the methods of
ObjectOutputStream.
 e.g. outputStream.writeInt(n);
 which writes the variable integer variable n a file.

There are similar methods:
 writeDouble(double n)
 writeFloat(float n)
 writeChar(int n)
 writeBoolean(boolean b)
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You can mix up the types you write but you need to know the order
in which they appear in the file to be able to read them back again.
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Writing Strings to binary files
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writeUTF(String aString)
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UTF refers to a particular encoding of the string, you
therefore need to use the readUTF method to read it
back from the file.
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writes the String value of aString to the output stream
outputStream.writeUTF(“Yikes it’s Java”);
UTF stands for Unicode Text Format

This encodes all Unicode characters, but favours the ASCII
character set, giving these short efficient codes.
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Opening a binary file for reading

Again this is similar to text files
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The ObjectInputStream class has no constructor that
takes a file name as its argument so we use the class
FileInputStream to create a stream that can be used as
an argument to the ObjectInputStream constructor e.g.
new FileInputStream(“mydata.dat”)
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ObjectInputStream inputStream = new
ObjectInputStream(new
FileInputStream(“mydata.dat”));
If your program attempts to read beyond the end of the file a
EOFException is thrown.
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Reading and Writing Objects
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You can read and write objects to a binary file using the:
writeObject method of the ObjectOuputStream class
and the
readObject method of the ObjectInputStream class.
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This requires the class to Serializable by adding implements
Serializable to the heading of the class definition
public class Library implements Serializable {
The Serializable interface is unusual in that it does not require
any methods to be implemented (it is just a signal to the compiler)

Reading and writing objects also requires import java.io.*
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The Serializable Interface
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What is the effect of making a class serializable?
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Java assigns a serial number to each object it writes to a
stream.
If the object is written more than once then after the first
time Java only writes the serial number
This makes I/O more efficient and files smaller
If a Serializable class has instance variables of a class type
then the class for the instance variables must also be
Serializable and so on for all levels of instance variables
within classess.
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Summary
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Looked at the File Class
Looked at examples of reading and writing binary files
containing
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Primitive types
Strings
Objects using the Serializable interface
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Follow-up Work
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Savitch chapter 9
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