COMP 121
Week 6: Files and Streams
Objectives
To be able to read and write text files
 To become familiar with the concepts of
text and binary formats
 To understand when to use sequential and
random file access
 To be able to read and write objects using
serialization

Reading Text Files

Scanner class
 Simplest

way to read text
To read a disk file:
a FileReader object
 Use the FileReader object when constructing
a Scanner object
 Construct
FileReader reader = new FileReader("input.txt");
Scanner in = new Scanner(reader);
 Use

Scanner methods to read data from file
next, nextLine, nextInt, and nextDouble
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Writing Text Files

To write to a text file, construct a
PrintWriter object
PrintWriter out = new PrintWriter("output.txt");
If file already exists, it is reset before the
new data are written to it
 If file doesn't exist, a new, empty file is
created

Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Writing Text Files (continued)

Use print and println to write to a
PrintWriter
out.println(29.95);
out.println(new Rectangle(5, 10, 15, 25));
out.println("Hello, World!");

You must close a file when you are done
processing it
out.close();

Otherwise, some output may not be
written to the disk file
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Example Program

Reads all lines of a file and writes them to
the output file, preceded by line numbers
 Can
be used to number lines in a Java source
file

Sample input file
Mary had a little lamb
Whose fleece was white as snow.
And everywhere that Mary went,
The lamb was sure to go!
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Example Program (continued)

Program produces the output file:
/*
/*
/*
/*
1
2
3
4
*/
*/
*/
*/
Mary had a little lamb
Whose fleece was white as snow.
And everywhere that Mary went,
The lamb was sure to go!
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
LineNumberer.java
import
import
import
import
java.io.FileReader;
java.io.IOException;
java.io.PrintWriter;
java.util.Scanner;
public class LineNumberer
{
public static void main(String[] args)
{
Scanner console = new Scanner(System.in);
System.out.print("Input file: ");
String inputFileName = console.next();
System.out.print("Output file: ");
String outputFileName = console.next();
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
LineNumberer.java (continued)
try
{
FileReader reader = new FileReader(inputFileName);
Scanner in = new Scanner(reader);
PrintWriter out = new PrintWriter(outputFileName);
int lineNumber = 1;
try
{
while (in.hasNextLine())
{
String line = in.nextLine();
out.println("/* " + lineNumber + " */ " + line);
lineNumber++;
}
}
finally
{
out.close();
}
}
catch (IOException exception)
{
System.out.println("Error processing file:" + exception);
}
}
}
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Question:

What package includes the Scanner class
(needed for the import statement)?
Answer: java.util
Text and Binary Formats

Two ways to store data
 Text
format
 Binary format
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Text Format
Human-readable form
 Sequence of characters

 1234567890
stored as 10 characters, one byte each
Use Reader and Writer and their
subclasses to process input and output
 For input:

FileReader reader = new FileReader("input.txt");

For output:
FileWriter writer = new FileWriter("output.txt");
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Reading a Single Character
from a File (not using Scanner)

Use read method of Reader class to
read a single character
the next character as an int or the
integer -1 at end of file
 returns
Reader reader = . . .;
int next = reader.read();
char c;
if (next != -1)
c = (char) next;
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Binary Format
Data items are represented in bytes
 1234567890 in binary format is stored in 4
bytes (int)
 Use InputStream and OutputStream
and their subclasses to read and write
 More compact and more efficient

Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Binary Format

For input:
FileInputStream inputStream
= new FileInputStream("input.bin");

For output:
FileOutputStream outputStream
= new FileOutputStream("output.bin");
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Reading a Single Byte from a
File

Use read method of InputStream class
to read a single byte
the next byte as an int or the integer
-1 at end of file
 returns
InputStream in = . . .;
int next = in.read();
byte b;
if
(next != -1)
b = (byte) next;
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Using Streams, Readers, and
Writers





Use read to read a single character or byte
Use write to write a single character or byte
read and write are the only input and output
methods provided by the file input and output
classes
Each class has a very focused responsibility
To read numbers, strings, or other objects, you
combine stream, reader, and writer classes with
other classes (like Scanner and Serializable)
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Random Access vs. Sequential Access

Sequential access
 A file
is processed a byte at a time, starting at
the beginning of the file
 It can be inefficient

Random access
 Allows
access at arbitrary locations in the file
 Only disk files support random access

System.in and System.out do not
 Each
disk file has a special file pointer
position

You can read or write at the position where the
pointer is pointing
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Random Access vs. Sequential Access
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
RandomAccessFile

You can open a file either for
 Reading
only ("r")
 Reading and writing ("rw")
RandomAccessFile f = new RandomAcessFile("bank.dat","rw");

To move the file pointer to a specific byte
f.seek(n);
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
RandomAccessFile (continued)

To get the current position of the file
pointer:
// Return type is "long" because files can be very large
long n = f.getFilePointer();

To find the number of bytes in a file:
fileLength = f.length();
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Using Random Access Files
Give each “field” a fixed size that is
sufficiently large to hold the largest value
 Every record has the same “fields” and is
the same size
 Easy to skip quickly to a given record

Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Using Random Access Files
(continued)


RandomAccessFile class stores binary data
readInt and writeInt read/write integers
(stored in 4 bytes)
double x = f.readDouble();
f.writeDouble(x);

readDouble and writeDouble read/write
doubles (stored in 8 bytes)
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Example

To find out how many bank accounts are in
the file
public int size() throws IOException
{
// RECORD_SIZE is 12 bytes:
// 4 bytes for the account number and
// 8 bytes for the balance
return (int) (file.length() / RECORD_SIZE);
}
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Example (continued)

To read the nth account in the file
public BankAccount read(int n)
throws IOException
{
file.seek(n * RECORD_SIZE);
int accountNumber = file.readInt();
double balance = file.readDouble();
return new BankAccount(accountNumber, balance);
}
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Example (continued)

To write the nth account in the file
public void write(int n, BankAccount account)
throws IOException
{
file.seek(n * RECORD_SIZE);
file.writeInt(account.getAccountNumber());
file.writeDouble(account.getBalance());
}
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Object Streams
ObjectOutputStream class can save
entire objects to disk
 ObjectInputStream class can read
objects back in from disk
 Objects are saved in binary format;
therefore, you use streams

Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Writing a BankAccount Object
to a File

The ObjectOutputStream saves all
instance variables when writeObject is
called
BankAccount b = . . .;
ObjectOutputStream out = new ObjectOutputStream(
new FileOutputStream("bank.dat"));
out.writeObject(b);
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Reading a BankAccount
Object From a File
readObject returns an Object
reference
 Need to remember the type of the object
that you saved and use a cast

ObjectInputStream in =
new ObjectInputStream(new FileInputStream("bank.dat"));
BankAccount b = (BankAccount) in.readObject();
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Reading an Object From a File

readObject method can throw a
ClassNotFoundException

It is a checked exception
You must catch or declare it

Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Serializable Interface

Objects that are written to an object
stream must belong to a class that
implements the Serializable interface
class BankAccount implements Serializable
{
. . .
}

Serializable interface has no
methods
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Serialization

The process of saving objects to a stream
 Each
object is assigned a serial number on
the stream
 If the same object is saved twice, only serial
number is written out the second time
 When reading, duplicate serial numbers are
restored as references to the same object
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
SerialDemo.java
import
import
import
import
import
import
java.io.File;
java.io.IOException;
java.io.FileInputStream;
java.io.FileOutputStream;
java.io.ObjectInputStream;
java.io.ObjectOutputStream;
/**
This program tests serialization of a Bank object.
If a file with serialized data exists, then it is
loaded. Otherwise the program starts with a new bank.
Bank accounts are added to the bank. Then the bank
object is saved.
*/
public class SerialDemo
{
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
SerialDemo.java (continued)
public static void main(String[] args)
throws IOException, ClassNotFoundException
{
Bank firstBankOfJava;
File f = new File("bank.dat");
if (f.exists())
{
ObjectInputStream in =
new ObjectInputStream(new FileInputStream(f));
firstBankOfJava = (Bank) in.readObject();
in.close();
}
else
{
firstBankOfJava = new Bank();
firstBankOfJava.addAccount(new BankAccount(1001, 20000));
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
SerialDemo.java (continued)
firstBankOfJava.addAccount(new BankAccount(1015, 10000));
}
// Deposit some money
BankAccount a = firstBankOfJava.find(1001);
a.deposit(100);
System.out.println(a.getAccountNumber()
+ ":" + a.getBalance());
a = firstBankOfJava.find(1015);
System.out.println(a.getAccountNumber()
+ ":" + a.getBalance());
ObjectOutputStream out = new ObjectOutputStream
(new FileOutputStream(f));
out.writeObject(firstBankOfJava);
out.close();
}
}
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Summary






When reading text files, use the Scanner class
When writing text files, use the PrintWriter
class and the print and println methods
Close all files when you are done processing
them
A File object is used for a file or directory
A File object can be passed into the
constructor of a file reader, writer, or stream
Streams access sequences of bytes. Readers
and writers access sequences of characters
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Summary (continued)





Use FileReader, FileWriter,
FileInputStream, and FileOutputStream
to read and write disk files
The read method returns an integer (-1 at the
end of the file) which you must cast to a char or
byte
Sequential access processes a file one byte at a
time
Random access allows access at arbitrary
locations in a file without first reading the
preceding bytes
A file pointer is the position in a random access
file
Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Summary (continued)
Object streams can be used to save and
restore all instance fields of an object
 Objects saved to an object stream must
belong to classes that implement the
Serializable interface

Horstmann, C. (2008). Big Java (3rd ed.). New York: John Wiley & Sons.
Any Questions?