Java I/O

1
Java I/O

• Input information brought to program from an
  external source
• Output information sent from program to an
  external destination
• External source or destination can be
• Disk file or memory
• Another program (piping)
• Network location (URL)
• Information type can be
• Primitive types or objects
• Images or sounds

2
I/O Streams

• A stream is a data structure facilitating
  handling of an information source for sequential
  access

reads
Source
Program
Stream
writes
Destination
Program
Programs read from and write to streams, and
not the actual source or destination
3
Standard Input and Output Streams

• Java provides references to the standard input
  stream (keyboard) and standard output stream
  (display)
• System.in an instance of the standard Java
  Platform InputStream class
• System.out an instance of the PrintStream class
• InputStream and PrintStream are in the java.io
  package

4
InputStream Class
An input stream provides 8-bit bytes. The
InputStream class is a direct descendant of the
Object class.
The shaded subclasses implement data sink
streams unshaded subclasses implement processing
streams.
5
Input Streams Used for Processing

• InputStream abstract superclass of all input
  stream classes
• All subclasses of InputStream must implement
  read() method that returns one byte as an int
• FilterInputStream designed to be extended to
  provide extra functionality involving the
  transformation (filtering) of data
• SequenceInputStream concatenates several streams
  into one logical stream
• ObjectInputStream reads data and objects that
  have been serialized to an output destination

6
Subclasses of FilterInputStream

• DataInputStream reads bytes that are parts of
  primitive types (int, double, etc.) and returns a
  specific value.
• BufferedInputStream buffers input bytes for
  efficiency
• PushbackInputStream adds the ability to push
  back'' or unread'' one byte
• LineNumberInputStream deprecated. Use
  LineNumberReader class instead

7
Input Streams Used on Data Sinks (all subclasses
of InputStream)

• FileInputStream obtains input bytes from a file
  in a file system (intended for raw data, as for
  image files, only)
• PipedInputStream gets bytes from a piped output
  stream, usually from another program thread
• ByteArrayInputStream gets bytes from an internal
  buffer which is a byte array
• StringBufferInputStream deprecated. Use
  StringReader class instead

8
OutputStream Class
An output stream outputs 8-bit bytes. The
OutputStream class is a direct descendant of the
Object class.
The shaded subclasses implement data sink
streams unshaded subclasses implement processing
streams.
9
Output Streams Used for Processing

• OutputStream abstract superclass of all output
  stream classes
• All subclasses of OutputStream must implement
  write() method that writes one byte stored as an
  int
• FilterOutputStream designed to be extended to
  provide extra functionality involving the
  transformation (filtering) of data
• ObjectOutputStream writes data and objects that
  have been serialized to an output destination

10
Subclasses of FilterOutputStream

• DataOutputStream writes primitive data types
  (int, double, etc.)
• BufferedOutputStream buffers output bytes for
  efficiency
• PrintStream converts primitive data types to
  string representation for printing
• can be created to perform automatic flushing

11
Output Streams Used on Data Sinks (all
subclasses of OutputStream)

• FileOutputStream sends output bytes to a file in
  a file system (intended for raw data, as for
  image files, only)
• PipedOutputStream sends bytes to a piped input
  stream, usually from another program thread
• ByteArrayOutputStream sends bytes to an internal
  buffer which is a byte array

12
Example Echoing Bytes from Standard Input to
Standard Output
import java.io. public class Echo
public static void main(String args) throws
IOException int c while ((c
System.in.read()) ! -1)
System.out.write(c)
13
Notes on the Example

• Must import the I/O package, of which all the
  input and output stream classes are part
• Since the read() method may encounter an I/O
  error, the main() method must either
• catch and deal with an IOException, or
• throw the IOException to the calling procedure
• Even though the streams read and write bytes, the
  internal storage is an int
• The read() method returns -1 if EOF is encountered

14
Example Output
Output is in red
82 javac Echo.java 83 java Echo now is the
timeReturn now is the time for all good
menReturn for all good men to come to the
aidReturn to come to the aid ctl-D 84
15
Unix Redirection
84 java Echo gt myfile now is the time for all
good men to come to the aid ctl-D 85 cat
myfile now is the time for all good men to come
to the aid 86 java Echo lt myfile gt newfile 87
cat newfile now is the time forall good men to
come to the aid 88
16
Streams vs. Readers/Writers

• The InputStream and OutputStream methods work on
  bytes
• The Echo example notwithstanding, the stream
  classes are used primarily for binary data as in
  sound and images
• In Java, characters are represented using 16-bit
  Unicode
• To insure internationalization, programs dealing
  with character I/O should use descendants of the
  Reader and Writer classes

17
Readers
Think of the Reader class like InputStream
that has been specialized to deal with
characters. Reader is a direct, abstract
subclass of Object.
Note similarity to the InputStream hierarchy.
18
Writers
Think of the Writer class like OutputStream
that has been specialized to deal with
characters. Writer is a direct, abstract
subclass of Object.
Note similarity to the OutputStream hierarchy.
19
New Echo Class
import java.io. public class Echo
public static void main(String args) throws
IOException InputStreamReader
r new InputStreamReader(System.in)
OutputStreamWriter w new OutputStreamWriter(Syst
em.out) int c while ((c
r.read()) ! -1) w.write(c)
w.flush()
20
Notes on the New Echo Class

• InputStreamReader and OutputStreamWriter are
  built on top of InputStream and OutputStream
• so their constructors require streams as
  arguments
• Internal character storage is an int
• Processing loop is exactly the same as for
  streams
• The writer requires flushing

21
New Echo Example Output
Output is in red
93 java Echo now is the timeReturn for all
good menReturn to come to the
aidReturn ctl-D now is the time for all good
men to come to the aid 94
22
File Handling Example
import java.io. public class FileCopy
public static void main(String args)
throws IOException FileReader
r new FileReader("infile") FileWriter
w new FileWriter("outfile") int c
while ((c r.read()) ! -1)
w.write(c) w.flush()
23
Notes on FileCopy

• FileReader and FileWriter constructors can take
  file names as strings for arguments
• Processing loop and flush call the same as before
• This class is of limited use since the file names
  are hard coded

24
FileCopy Example Output
105 ls FileCopy.class FileCopy.java infile 106
cat infile Here is a test file to show that the
FileCopy class really works. 107 java
FileCopy 108 ls FileCopy.class FileCopy.java infi
le outfile 109 cat outfile Here is a test
file to show that the FileCopy class really
works. 110
25
Exception Example
111 rm outfile 112 rm infile 113
ls FileCopy.class FileCopy.java 114 java
FileCopy Exception in thread "main"
java.io.FileNotFoundException infile (No such
file or directory) at java.io.FileInputStream.ope
n(Native Method) at java.io.FileInputStream.ltinit
gt(FileInputStream.java103) at
java.io.FileInputStream.ltinitgt(FileInputStream.jav
a66) at java.io.FileReader.ltinitgt(FileReader.jav
a39) at FileCopy.main(FileCopy.java7) 115
Note The FileNotFoundException class is a
subclass of IOException. The
exception generates a stack trace with line
numbers of offending code.
26
Reading Strings

• The BufferedReader class has a readLine() method
  for reading strings

import java.io. public class FileCopy
public static void main(String args) throws
IOException BufferedReader br
new BufferedReader(new
InputStreamReader(System.in))
System.out.print("Source file name ")
String inFileName br.readLine()
System.out.print("Destination file name ")
String outFileName br.readLine()
FileReader r new FileReader(inFileName)
FileWriter w new FileWriter(outFileName)
int c while ((c r.read()) ! -1)
w.write(c) w.flush()

27
Example Output
121 ls FileCopy.class FileCopy.java 122 java
FileCopy Source file name FileCopy.java Destinati
on file name X.java 123 ls FileCopy.class
FileCopy.java X.java 124 cat X.java import
java.io. public class FileCopy public
static void main(String args) throws
IOException BufferedReader br
new BufferedReader(new
InputStreamReader(System.in))
System.out.print("Source file name ")
String inFileName br.readLine()
System.out.print("Destination file name ")
String outFileName br.readLine()
...
28
Reading Strings and Converting to Numbers

• The java.lang package has classes Integer,
  Double, etc., whose instances have int, double,
  etc. as data members
• These classes have static methods for converting
  strings to numbers
• For example, the Integer class has a
  parseInt(ltstringgt) method that returns an int
  given a String
• Similarly, the Double class has a
  parseDouble(ltstringgt) method that returns a double

29
Example
import java.io. public class ConvertTest
public static void main(String args) throws
IOException int sum 0
BufferedReader br new
BufferedReader(new InputStreamReader(System.in))
System.out.print("Enter an integer ")
String s br.readLine() while ( s
! null ) int num
Integer.parseInt(s) sum num
System.out.print("Enter an integer ")
s br.readLine()
System.out.println("The sum is " sum)
30
Notes on the Example

• The readLine() method returns a null string if
  end of stream is reached
• The parseInt() method must be fully qualified
  with its class name since the method is static
• The parseInt() method might throw a
  NumberFormatException if its argument is not
  syntactically correct
• Unlike an IOException, a NumberFormatException is
  not checked by the compiler

31
Example Output
136 java ConvertTest Enter an integer 3 Enter
an integer 417 Enter an integer 29 Enter an
integer 1234 Enter an integer ctl-D The sum
is 1683 137 java ConvertTest Enter an integer
3 Enter an integer 417 Enter an integer
trash Exception in thread "main"
java.lang.NumberFormatException trash at
java.lang.Integer.parseInt(Integer.java426) at
java.lang.Integer.parseInt(Integer.java476) at
ConvertTest.main(ConvertTest.java12) 138
32
Lining Output Up in Columns
import java.io. public class ColumnTest
public static void main(String args) throws
IOException int sum 0 int
nums new int10 int i 0
BufferedReader br new
BufferedReader(new InputStreamReader(System.in))
System.out.print("Enter an integer ")
String s br.readLine() while ( s
! null ) numsi
Integer.parseInt(s) sum
numsi System.out.print("Enter an
integer ") s br.readLine()
System.out.println("Numbers")
System.out.println("-------") for (int
j 0 j lt i j) System.out.println(
numsj) System.out.println("-------")
System.out.println(sum " total")
33
Output
144 java ColumnTest Enter an integer 3 Enter an
integer 417 Enter an integer 29 Enter an
integer 1234 Enter an integer
ctl-D Numbers ------- 3 417 29 1234 ------- 1683
total 145
34
Decimal Formatting

• The java.text package has a DecimalFormat class
  for formatting decimals as strings
• The DecimalFormat class has a format() method
  that takes an integer and formats it according to
  a pattern string given to the DecimalFormat
  object's constructor
• The pattern string can have regular characters as
  well as special formatting characters
• The special character 0'' indicates a decimal
  digit
• E.g. the pattern string 0000'' indicates a
  string with three leading blanks then an integer
  in 4-digit field

35
Decimal Formatting Example
import java.io. import java.text. public
class ColumnTest public static void
main(String args) throws IOException
int sum 0 int nums new int10
int i 0 BufferedReader br
new BufferedReader(new InputStreamReader(Sy
stem.in)) System.out.print("Enter an
integer ") String s br.readLine()
while ( s ! null ) numsi
Integer.parseInt(s) sum
numsi System.out.print("Enter an
integer ") s br.readLine()
DecimalFormat f new DecimalFormat("
0000") System.out.println("Numbers")
System.out.println("-------") for
(int j 0 j lt i j)
System.out.println(f.format(numsj))
System.out.println("-------")
System.out.println(f.format(sum) " total")

36
Decimal Formatting Example Output
144 java ColumnTest Enter an integer 3 Enter an
integer 417 Enter an integer 29 Enter an
integer 1234 Enter an integer
ctl-D Numbers ------- 0003 0417 0029
1234 ------- 1683 total 145
If you can figure out how to get the leading
zeros to print as blanks, please let me know.