FILE IO
Input and
Output
FILE I/O OVERVIEW
 A File is a 'persistent' collection of data




Persistent means that the data is stored even after the program ends
Files can store text, audio, images, video or anything else
Files are stored in folders
Folders are containers for files and folders
 I/O stands for Input/Output
 In this context it is input to and output from programs
 Input can be from the keyboard or a file or a mouse
 Output can be to the screen or to a file
2
BINARY VERSUS TEXT FILES
 All data and programs are ultimately just zeros and ones
 each digit can have one of two values, hence binary
 a bit is one binary digit
 a byte is a group of eight bits
 a byte can represent only 256 different values
 All files contain sequences of bytes (which are numbers)!
 Text files: the numbers represent characters
 Each datum is one byte
 Each datum represents a character using ASCII code
 Binar y files: the bits represent other types of encoded information,
such as executable instructions or numeric data
 The datum may have varying byte lengths
 these files are easily read by the computer but not humans
3
BINARY FILE AS TEXT FILE
EXAMPLE
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4
ASCII Table
0
1
2
3
4
5
6
7
8
9
10
11
12
0
NUL
LF
DC4
RS
(
2
<
F
P
Z
d
n
x
1
SOH
VT
NAK
US
)
3
=
G
Q
[
e
o
y
2
STX
FF
SYN
SP
*
4
>
H
R
\
f
p
z
3
ETX
CR
ETB
!
+
5
?
I
S
]
g
q
{
4
EOT
SO
CAN
"
,
6
@
J
T
^
h
r
|
5
ENQ
SI
EM
#
7
A
K
U
_
i
s
}
6
7
ACK BEL
DLE DC1
SUB ESC
$
%
.
/
8
9
B
C
L
M
V
W
`
a
j
k
t
u
~ DEL
8
BS
DC2
FS
&
0
:
D
N
X
b
l
v
9
HT
DC3
GS
'
1
;
E
O
Y
c
m
w
Consider a file containing the following sequence of byte values. What appears if this file is opened in NotePad?
74 97 118 97 32 105 115 10 70 117 110 33
5
DECIMAL REPRESENTATION
decimal digit
8
4
5
position name
hundreds
tens
ones
102
101
100
positional value
6
GENERAL REPRESENTATION
 Numbers are represented in an arbitrary base (or
radix) using the formula
a n R n + a n-1 R n-1 + a n-2 R n-2 +
...
+ a1R1 + a0R0
• a is a digit in the base numbering scheme
• R is the base (or radix)
• n is an exponent that corresponds to the positional value of the digit
7
COMMON CS NUMBERING SYSTEMS
Largest Single
Digit
Base
Only Valid Digits
2
binary
1
01
8
octal
7
01234567
10
decimal
9
0123456789
F (f)
0 1 2 3 4 5 6 7 8 9AB
CDEF
(a b c d e f)
16
hexadecimal
Problem:
convert 111102 to decimal (base 10)
convert 368 to decimal
convert 1E16 to decimal
8
CONVERSION TABLE
Conversion Table
Binary
Octal
Decimal
Hexadecimal
0
0
0
0
1
1
1
1
10
2
2
2
11
3
3
3
100
4
4
4
101
5
5
5
110
6
6
6
111
7
7
7
1000
10
8
8
1001
11
9
9
1010
12
10
A
1011
13
11
B
1100
14
12
C
1101
15
13
D
1110
16
14
E
1111
17
15
F
10000
20
16
10
9
DECIMAL TO BINARY CONVERSION
 Algorithm Conver tToBinar y(X)
Input: X is a decimal number
Output: B is a sequence of binary digits representing X
Let B be an empty sequence of binary digits
while X is greater than or equal to 1 do
set A to the remainder of the non-integer division of (X/2)
set X to the quotient (non-remainder) of (X/2)
place A at the beginning of the sequence B
return B
Problem: convert 81910 to binary
Answer: 11001100112
10
PRIMITIVE DATA T YPES
MEMORY STORAGE AND RANGES
Type
Kind of Value
Memory
Used
Size Range
byte
Integer Number
1 byte
-128 to 127
short
Integer Number
2 bytes
-32768 to 32767
int
Integer Number
4 bytes
-2,147,483,648 to 2,147,483,647
long
Integer Number
8 bytes
-9,223,372,036,854,775,808 to
9,223,374,036,854,775,808
float
Real Number
4 bytes
+/- 3.4028… x 10+38 to
+/- 1.4023… x 0-45
double
Real Number
8 bytes
+/- 1.767… x 10+308 to
+/- 4.940… x 0-324
char
Single character
2 bytes
0 to 65535
11
TEXT VERSUS BINARY FILES
 Text files are more readable by humans
 Binar y files are more ef ficient
 computers read and write binary files more easily than text
 Java binar y files are por table
 they can be used by Java on different machines. Reading and
writing binary files is normally done by a program
 text files are generally used only to communicate with humans
Java Text Files



Source files
Occasionally input files
Occasionally output files
Java Binary Files



Executable files (created by
compiling source files)
Usually input files
Usually output files
12
TEXT FILES
 Two primary classes for manipulating text files:
 PrintWriter is used for creating and writing to files
 BufferedReader is used for reading files
 These classes are found in the “java.io” package. You must
include an “import java.io.*;” line if you use these classes.
 Text files contain only “words” or “Strings”. Everything written
into a text file is first converted to a String (even numbers!).
13
FILE SYSTEM
Users
smith
jones
hw1.java
hw2.java
Root Directory
hw1.class
Java provides a File class for working with
directories and files. The File class is not used
to read/write from/to files, but to provide
information about files.
hw1.class
smith
Stuff.txt
14
FILE CLASS
 To create a File object
 File aFile = new File(“hw1.java”);
 The file is NOT created on disk. The file may not exist.
 Can check properties of the file…
 boolean doesFileExist = aFile.exists();
 boolean isTheFileADirectory = aFile.isDirectory();
 int fileLength = aFile.length();
 Can create and delete files
 boolean operationOK = file.delete(); // deletes the file from disk
 boolean operationOK = file.createNewFile(); //creates empty file
15
STREAMS
 Stream: an object that either delivers data to its destination
(screen, file, etc.) or that takes data from a source (keyboard,
file, etc.)
 Input stream: a stream that provides input to a program (can
be from a keyboard, mouse, or file)
 System.in is an input stream
 Output stream: a stream that takes output from a program
 System.out is an output stream
16
STREAMS
Input Stream
Program
Source
The program reads from the
input stream
Output Stream
Program
Destination
The program write to the
output stream
17
READING AND WRITING
 No matter where the information is coming from
or going to and no matter what type of data is
being read or written, the algorithms for reading
and writing data are usually the same :
Reading
Writing
open a stream
while more data exists
read some data
process the data
close the stream
open a stream
while more data exists
process the data
write the data
close the stream
18
PRINTWRITER OVERVIEW
 To open/create a text file for output, create a PrintWriter
object.
PrintWriter fout =
new PrintWriter(new FileWriter("out.txt"));
 Then you can use print and println to write to the file
just like you would use System.out.print() and
System.out.println() to write to the terminal.
fout.println(“hello”)
\n
o
l
l
e
h
19
PRINTWRITER
 Constructor Summary
 PrintWriter output = new PrintWriter(new FileWriter(“name.txt”));
 Method Summary
 void print(boolean) Prints a boolean.
May throw an IOException
 void print(char) Prints a character.
 void print(double) Prints a double.
 void print(float) Prints a float.
 void print(int) Prints an integer.
 void print(long) Prints a long.
 void print(Object) Prints an object.
 void print(String) Prints a String.
 Also has println methods corresponding to each of the above
20
PRINTWRITER
A PrintWriter always outputs TEXT (STRINGS).
34
false
1993.128
“Hello”
a JFrame
int
boolean
double
String
Object
fout.print(
);
String
21
PRINTWRITER
 A print writer must be closed!
 Use the “close()” method!
 If data is printed to a print writer but the print writer is never closed,
the output file may not contain all of the printed data.
PrintWriter fout = new PrintWriter(new FileWriter(“out.jpg”));
for(int i=0; i<10; i++){
fout.println(“Hello”);
}
Writing
fout.close();
open a stream
while more data exists
process the data
write the data
close the stream
22
WARNING: OVERWRITING A FILE
 Opening a print-writer file creates an empty file
 Opening a print-writer file creates a new file if it does not already
exist
 Opening a print-writer file that already exists eliminates the old
file and creates a new, empty one. Data in the original file is
lost!
Warning Will
Robinson
23
COMMAND-LINE OPTIONS
(A BRIEF DESCRIPTION)
 When a program is executed the computer is
“commanded” to run the program.
 When a program is executed, the meaning is
“invoke the main method”.
 The main method specifies an “array of
Strings” as input
 The value of this array is determined by the
“command-line” that was used to executed
the program
 Each “word” following the name of the class
to execute on the command-line is one
element of the array
24
COMMAND-LINE OPTIONS
(A BRIEF DESCRIPTION)
 Use the class below to experiment with command -line options.
Tr y to predict the output given the following scenarios:
java CommandLineDemo
java CommandLineDemo opt1 opt2 opt3 opt4
java CommandLineDemo file1.txt file2.txt
java CommandLineDemo 1 2 356
java CommandLineDemo 1-x 18 numbers
java CommandLineDemo “once upon a time”
class CommandLineDemo {
public static void main(String[] args) {
for(int i=0; i<args.length; i++){
System.out.println(args[i]);
}
}
}
25
SAVING DATA TO A TEXT FILE
i mpor t j ava.i o.*;
Write 10 randomly generated (0-100)
numbers to a text file where each number is
c lass Ar rayWr iter {
pu bli c sta tic voi d mai n(St rin g[] a rgs) {on the same line and separated by a space
character.
Pr intWr iter fo ut = null ;
The
// crea te a nd initi aliz e a n arr ay o bje
ctcommand-line arguments are:
arg[0] is the name of the file to create
in t[] d ata = n ew in t[10 ];
fo r(int i=0 ; i <data .len gth ; i++ ) {
data [i] = ( int)( Math .ra ndom( )*10 1);
}
tr y {
fou t = n ew P rin tWrit er(n ew
F ileW rit er(ar gs[0 ])) ;
for(i nt i =0; i<da ta.l eng th; i ++) {
fo ut.p rin t(dat a[i] + " ");
}
fou t.clo se() ;
} catch (IOE xce ption ioe ) {
Syste m.ou t.p rintl n("E rro r wri ting to " +
a rgs[ 0]) ;
}
}
May throw an
IOException!
}
26
BUFFEREDREADER
 Used for reading text files
 Constructor Summary
BufferedReader fin =
new BufferedReader(new
FileReader(“data.txt”));
 Method Summary
 int read() – reads a single character but converts it to an int
 String readLine() – reads an entire line of text, returns “null” if
the end of the file is encountered
 Both of the “read” methods throw an IOException
 BufferedReader has no methods to read in numbers so you
must read in a String and convert
27
READING DATA FROM A TEXT FILE
import java.io.*;
class ArrayReader {
public static void main(String[] args) {
BufferedReader fin = null;
String line;
int[] data = new int[10];
Read 10 numbers from a file where each
number is on its own line. Prints the largest,
smallest, and average of the numbers.
The command-line arguments are:
arg[0] is the name of the file to read from
try {
fin = new BufferedReader(new FileReader(args[0]));
for(int i=0; i<10; i++) {
data[i] = fin.readLine(); // THERE IS A PROBLEM HERE!
}
fin.close();
} catch(IOException ioe) {
System.out.println("Error reading from " + args[0]);
System.exit(0);
}
int max = 0; // index of the maximum value
int min = 0; // index of the minimum value
int total = 0; // cumulative total
for(int i=0; i<data.length; i++) {
if(data[i] > data[max]) max = i;
if(data[i] < data[min]) min = i;
total = total + data[i];
}
System.out.println("Minimum value is " + data[min]);
System.out.println("Maximum value is " + data[max]);
System.out.println("Average value is " + total/(double)10);
}
}
28
READING DATA FROM A TEXT FILE
import java.io.*;
import java.util.*;
class ArrayReader {
public static void main(String[] args) {
BufferedReader fin = null;
String line;
int[] data = new int[10];
Read 10 numbers from a file where each
number is on its own line. Prints the largest,
smallest, and average of the numbers.
The command-line arguments are:
arg[0] is the name of the file to create
try {
fin = new BufferedReader(new FileReader(args[0]));
for(int i=0; i<10; i++) {
data[i] = Integer.valueOf(fin.readLine()).intValue(); //
Convert String to int
}
fin.close();
} catch(IOException ioe) {
System.out.println("Error reading from " + args[0]);
System.exit(0);
}
int max = 0; // index of the maximum value
int min = 0; // index of the minimum value
int total = 0; // cumulative total
for(int i=0; i<data.length; i++) {
if(data[i] > data[max]) max = i;
if(data[i] < data[min]) min = i;
total = total + data[i];
}
System.out.println("Minimum value is " + data[min]);
System.out.println("Maximum value is " + data[max]);
System.out.println("Average value is " + total/(double)10);
}
}
29
DETECTING THE END OF A FILE
 A common programming situation is to read data from an
input file but not know how much data the file contains . In
these situations you need to check for the end of the file.
 There are three common ways to test for the end of a file:
1. Test for a special value that signals the end of the file
2. Throw and catch an end-of-file exception
3. Place a sentinal value at the end of a file and test for it
30
TESTING FOR END OF FILE
IN A TEXT FILE
 When readLine tries to read beyond the end of a text
file it returns the special value null
 so you can test for null to stop processing a text file
 read returns -1 when it tries to read beyond the end of a
text file
 the int value of all ordinary characters is nonnegative
31
USING READLINE() FOR LINE-BY-LINE FILE
PROCESSING
readLine throws an
IOException
When using
readLine
test for null
import java.io.*;
class FilePrintDemo {
public static void main(String[] args) {
int count = 0;
try {
BufferedReader in = new BufferedReader(new FileReader(args[0]));
String line = in.readLine();
while (line != null) {
count++;
System.out.println(count + “\t" + line);
line = in.readLine();
}
in.close();
} catch(IOException e) {
System.out.println(e.getMessage());
System.out.println(“Error with file “ + args[0]);
System.exit(0);
}
}
}
32
USING READ() FOR CHARACTER-BYCHARACTER FILE PROCESSING
read throws an
IOException
When using read
test for -1
import java.io.*;
class FilePrintDemo2 {
public static void main(String[] args) {
int count = 1;
try {
BufferedReader in = new BufferedReader(new FileReader(args[0]));
int c = in.read();
if(c != -1) System.out.print(count + “\t”);
while (c != -1) {
System.out.print(String.valueOf((char)c));
if(c == ‘\n’) System.out.print(++count + “\t”);
c = in.read();
}
in.close();
} catch(IOException e) {
System.out.println(e.getMessage());
System.out.println(“Error with file “ + args[0]);
System.exit(0);
}
}
}
33
READING DATA FROM A TEXT FILE
import java.io.*;
class ArrayReader {
private static int MAX=1000;
public static void main(String[] args) {
BufferedReader fin = null;
String line;
int[] data = new int[MAX];
Read zero or more (up to some upper limit)
numbers from a file where each number is
on its own line. Prints the largest, smallest,
and average of the numbers.
The command-line arguments are:
arg[0] is the name of the file to create
try {
fin = new BufferedReader(new FileReader(filename));
while((line = fin.readLine()) != null) {
data[i] = Integer.parseInt(line);
}
fin.close();
} catch(IOException ioe) {
System.out.println("Error reading from " + args[0]);
System.exit(0);
}
int max = 0; // index of the maximum value
int min = 0; // index of the minimum value
int total = 0; // cumulative total
for(int i=0; i<data.length; i++) {
if(data[i] > data[max]) max = i;
if(data[i] < data[min]) min = i;
total = total + data[i];
}
System.out.println("Minimum value is " + data[min]);
System.out.println("Maximum value is " + data[max]);
System.out.println("Average value is " + total/(double) 10);
}
}
34
READING DATA FROM A TEXT FILE
import java.io.*;
class ArrayReader {
private static int MAX=1000;
public static void main(String[] args) {
BufferedReader fin = null;
String line;
int[] data = new int[MAX];
int count=0;
Read zero or more (up to some upper limit)
numbers from a file where each number is
on its own line. Prints the largest, smallest,
and average of the numbers.
The command-line arguments are:
arg[0] is the name of the file to create
try {
fin = new BufferedReader(new FileReader(filename));
while((line = fin.readLine()) != null) {
data[count++] = Integer.valueOf(line).intValue();
}
fin.close();
} catch(IOException ioe) {
System.out.println("Error reading from " + args[0]);
System.exit(0);
}
int max = 0; // index of the
int min = 0; // index of the
int total = 0; // cumulative
for(int i=0; i<count; i++) {
if(data[i] > data[max]) max =
if(data[i] < data[min]) min =
total = total + data[i];
}
maximum value
minimum value
total
i;
i;
System.out.println("Minimum value is " + data[min]);
System.out.println("Maximum value is " + data[max]);
System.out.println("Average value is " +
total/(double)count);
}
}
35
READING DATA FROM A TEXT FILE
import java.io.*;
import java.util.*;
class ArrayReader {
private static int MAX=1000;
public static void main(String[] args) {
BufferedReader fin = null;
String line;
int[] data = new int[MAX];
int count=0;
Read zero or more (up to some upper limit)
numbers from a file where each number is
on the same line and separated by a space.
Prints the largest, smallest, and average of
the numbers.
The command-line arguments are:
arg[0] is the name of the file to create
try {
fin = new BufferedReader(new FileReader(filename));
while((line = fin.readLine()) != null) {
data[count++] = Integer.parseInt(line);
}
fin.close();
} catch(IOException ioe) {
System.out.println("Error reading from " + args[0]);
System.exit(0);
}
int max = 0; // index of the
int min = 0; // index of the
int total = 0; // cumulative
for(int i=0; i<count; i++) {
if(data[i] > data[max]) max =
if(data[i] < data[min]) min =
total = total + data[i];
}
}
maximum value
minimum value
total
i;
i;
System.out.println("Minimum value is " + data[min]);
System.out.println("Maximum value is " + data[max]);
System.out.println("Average value is " +
total/(double)count);
}
36
WORD-BY-WORD PROCESSING
 There are BufferedReader methods to read a line and a
character, but not just a single word
 Scanner is a class that provides more flexible 'parsing'
 Parsing is the act of taking individual letters and grouping them
 The scanner groups letters into primitive types and Strings
 Scanners work by
 Separating groups of letters by 'delimiter' characters
 Converting each group of letters into some primitive type or String type
 A delimiter
 any character that is never part of a 'thing'
 defaults to the "white space" characters:
 space (‘ ‘)
 tab (‘\t’)
 newline (‘\n’)
37
WORD-BY-WORD PROCESSING
 Construction
 Scanner s1 = new Scanner( new File("data.txt") );
 Scanner s2 = new Scanner( "3 12 55\n1 9 31" );
 Control the delimiters
 s1.useDelimiters("[ \n\t]"); // uses space, newline, tab
 s1.useDelimiters("[.1]"); // uses period, 1
 s1.reset(); // uses defaults
 Obtain the next 'thing'







s1.nextInt()
s1.nextByte()
s1.nextBoolean()
s1.nextLong()
s1.nextDouble()
s1.next()
…
38
SCANNER API
 You can construct a scanner by passing in a
 String: new Scanner("The quick fox jumped …");
 File: new Scanner( new File( "input.txt" ) );
 InputStream: new Scanner( System.in );
 You can ask the scanner what 'type' of thing comes next.
Each of the "has<T YPE>" methods returns a boolean. Each
also throws NoSuchElementException, IllegalStateException,
and InputMismatchException. These are RuntimeExceptions.





scanner.hasNextInt()
scanner.hasNextDouble()
scanner.hasNextBoolean()
scanner.hasNext()
… others not shown
39
WORD-BY-WORD PROCESSING
 You can read the next item and return it (advancing the scanner).
The return type is implied by the method name.




int x = scanner.nextInt()
double y = scanner.nextDouble()
boolean z = scanner.nextBoolean()
String token = scanner.next().
 Example:









Scanner s1 = new Scanner("9 false 3.2");
boolean b1 = s1.hasNextInt();
boolean b2 = s1.hasNextBoolean();
boolean b3 = s1.hasNextDouble();
int x = s1.nextInt();
b1 = s1.hasNextInt();
b2 = s1.hasNextBoolean();
b3 = s1.hasNextDouble();
b1 = s1.hasNext();
40
SCANNER
 If a scanner is used to read data from a file; be sure to close
the scanner when you are done with it.
 Scanner scanner = new Scanner( new File( "input.txt") );
 // read lots of data from the scanner
 scanner.close(); // THIS IS IMPORTANT
 Note that once a scanner is closed, any "get -the-next-thing"
method will throw an IllegalStateException.
41
READING DATA FROM A TEXT FILE
import java.io.*;
import java.util.*;
class ArrayReader {
private static int MAX=1000;
public static void main(String[] args) {
int[] data = new int[MAX];
int count = 0;
Read zero or more (up to some upper limit)
numbers from a file where each number is
on the same line and separated by a space.
Prints the largest, smallest, and average of
the numbers.
The command-line arguments are:
try {
S c a n n e r s c a n n e r = n e w S c a n n e r ( n e w F i l e ( a r g s [ 0 ] ) ) ;arg[0] is the name of the file to create
while(scanner.hasNextInt()) {
data[count++] = scanner.nextInt() ;
}
scanner.close();
} catch(Exception e) {
System.out.println("Error reading from " + args[0]);
System.exit(0);
}
int max = 0; // index of the
int min = 0; // index of the
int total = 0; // cumulative
for(int i=0; i<count; i++) {
if(data[i] > data[max]) max =
if(data[i] < data[min]) min =
total = total + data[i];
}
maximum value
minimum value
total
i;
i;
System.out.println("Minimum value is " + data[min]);
System.out.println("Maximum value is " + data[max]);
System.out.println("Average value is " + total/(double)count);
}
}
42
READING DATA FROM A TEXT FILE
import java.io.*;
import java.util.*;
class ArrayReader {
private static int MAX=1000;
public static void main(String[] args) {
int[] data = new int[MAX];
int count = 0;
Read zero or more (up to some upper limit)
numbers from a file where there are zero or
more numbers on each line! Prints the
largest, smallest, and average of the
numbers.
The command-line arguments are:
arg[0] is the name of the file to create
try {
Scanner scanner = new Scanner( new File ( args[0] ) );
while(scanner.hasNextInt()) {
data[count++] = scanner.nextInt() ;
}
scanner.close();
} catch(Exception e) {
System.out.println("Error reading from " + args[0]);
System.exit(0);
}
////////////////////////
// other code goes here
////////////////////////
System.out.println("Minimum value is " + data[min]);
System.out.println("Maximum value is " + data[max]);
System.out.println("Average value is " +
total/(double)count);
}
}
43
BINARY FILES
 The most common stream classes for binar y files:
 DataInputStream: used to read a file
 DataOutputStream: used to write to a file
 The output files are binary and can store any of the primitive
data types (int, char, double, etc.) and the Object String
type
 The files can be read by other Java programs but are not
printable
 Must include “import java.io.*;” to use these classes.
 An IOException might be thrown when writing data
44
DATAOUTPUTSTREAM METHOD
SUMMARY
 After the binary file is opened you can send data to
it by using one of the following methods:
 void writeChar(int c)
 void writeInt(int n)
 void writeDouble(double x)
 void writeBoolean(boolean b)
 void writeUTF(String s)
 Note that each write method throws IOException
45
WHAT IS UTF??
 Use the writeUTF method to output a value of type String.
There is no writeString method!
 UTF stands for Universal Text Format which is a special version of
Unicode
 Unicode: a text code that uses 2 bytes per character. Designed to
accommodate languages with a dif ferent alphabet or no alphabet
(such as Chinese and Japanese)
 ASCII: also a text code, but it uses just 1 byte per character. The
most common code for English and languages with a similar
alphabet
 UTF is a modification of Unicode that uses just one byte for ASCII
characters while allowing 2 bytes for other languages
46
OPENING AN OUTPUT FILE
 To open a file named “numbers.dat” for binary output:
DataOutputStream fout =
new DataOutputStream(new
FileOutputStream(“numbers.dat”));
 Consider the following sequence of statements:





fout.writeInt(10);
fout.writeInt(20);
fout.writeBoolean(true);
fout.writeDouble(12.0);
fout.close();
 How many bytes of data does this file contain?
47
SAVING DATA TO A BINARY FILE
i mpor t j ava.i o.*;
Write 10 int values to a binary file. Each
number is randomly generated in the
c lass Ar rayWr iter {
pu bli c sta tic voi d mai n(St rin g[] a rgs) {interval [0..100].
Da taOut putS tre am fo ut = nu ll;
The command-line arguments are:
arg[0]
// crea te a nd initi aliz e a n arr ay o bje
ct is the name of the file to create
in t[] d ata = n ew in t[10 ];
fo r(int i=0 ; i <data .len gth ; i++ ) {
data [i] = ( int)( Math .ra ndom( )*10 1);
}
tr y {
fou t = n ew D ata Outpu tStr eam (new
F ileO utp utStr eam( arg s[0]) );
for(i nt i =0; i<da ta.l eng th; i ++) {
fo ut.w rit eInt( data [i] );
}
fou t.clo se() ;
} catch (IOE xce ption ioe ) {
Syste m.ou t.p rintl n("E rro r wri ting to " + args [0] );
}
}
}
48
DATAINPUTSTREAM METHODS
 For every DataOutputStream method there is a
corresponding DataInputStream method
 You can read data from a binary input file by using methods
defined in DataInputStream




int readInt()
double readDouble()
boolean readBoolean()
String readUTF()
 Note that each write method throws IOException
49
OPENING AN OUTPUT FILE
 To open a file named “numbers.dat” for binary input:
DataInputStream fin =
new DataInputStream(new
FileInputStream(“numbers.dat”));
 Consider the following sequence of statements:





int x= fin.readInt();
int y = fin.readInt();
boolean b = fin.readBoolean();
double z = fin.readDouble();
fin.close();
 What occurs if the first two lines are replaced with
 long z = fin.readLong();
50
INPUT FILE EXCEPTIONS
 A FileNotFoundException is thrown if the file is not found
when an attempt is made to open a file
 Each read method throws IOException
 If a read goes beyond the end of the file an EOFException is
thrown
51
AVOIDING COMMON DATAINPUTSTREAM
FILE ERRORS
 Input files can contain a mix of data types. It is up to the
programmer to know their order and use the correct read
method
There is no error message (or exception)
if you read the wrong data type!
 DataInputStream works with binary, not text files
 As with an output file, close the input file when you are
done with it
52
THE EOFEXCEPTION CLASS
 Many (but not all) methods that read from a file throw an
end-of-file exception (EOFException) when they try to read
beyond the file
 Most of the DataInputStream methods throw it
 The end-of-file exception can be used in an "infinite"
(while(true)) loop that reads and processes data from
the file
 the loop terminates when an EOFException is thrown
 The program is written to continue normally after the
EOFException has been caught
53
USING
EOFEXCEPTION
Problem: Write code to print the values in a binary file named “numbers.dat”
which contains zero or more int values.
try {
DataInputStream inputStream =
new DataInputStream(new FileInputStream("numbers.dat"));
int n;
Intentional "infinite" loop to
process data from input file
Loop exits when end-of-file
exception is thrown
Processing continues after
EOFException: the input file
is closed
System.out.println("Reading ALL the integers");
System.out.println("in the file numbers.dat.");
try {
while (true) {
n = inputStream.readInt();
System.out.println(n);
}
} catch(EOFException e) {
System.out.println("End of reading from file.");
}
inputStream.close();
} catch(FileNotFoundException e) {
System.out.println("Cannot find file numbers.dat.");
} catch(IOException e2) {
System.out.println("Problem with input from file numbers.dat.");
}
Note order of catch blocks:
the most specific is first
and the most general last
54