Download File input/output: Almost identical console input/output

Overview
Examples
Tokenizing
File input/output:
Almost identical console input/output
Prof. Dionne Aleman
MIE250: Fundamentals of object-oriented programming
University of Toronto
MIE250: Fundamentals of object-oriented programming (Aleman)
Overview
File I/O
Examples
1 / 11
Tokenizing
File handling
I
File handling refers to reading in and writing to files (file I/O).
I
Java is notable for not having simple ways to do I/O, whether
from/to console or file.
I
Lots of options available for I/O ... we will use BufferedReader.
MIE250: Fundamentals of object-oriented programming (Aleman)
Overview
File I/O
Examples
2 / 11
Tokenizing
Similarity to reading from the console
1
2
// object to read from screen
BufferedReader cin = new BufferedReader ( new InputStreamReader ( System . in ));
3
4
5
// object to read from file
BufferedReader fin = new BufferedReader ( new FileReader ( filename ));
6
7
8
// read from screen
String line1 = cin . readLine () ;
9
10
11
// read from file
String line2 = fin . readLine () ;
MIE250: Fundamentals of object-oriented programming (Aleman)
File I/O
3 / 11
Overview
Examples
Tokenizing
Similarity to writing to the console
1
2
// write to screen
System . out . println ( strSomeText ) ;
3
4
5
// object to write file
BufferedWriter fout = new BufferedWriter ( new FileWriter ( filename ));
6
7
8
9
10
// write to file
fout . write (" This is the first line in my file . " ) ;
fout . newLine () ;
fout . write (" This is the second line in my file . " ) ;
11
12
13
14
// flush the buffer and close the file ( IMPORTANT !)
fout . flush () ;
fout . close () ;
MIE250: Fundamentals of object-oriented programming (Aleman)
Overview
File I/O
Examples
4 / 11
Tokenizing
File I/O notes
I
Exceptions that must be handled:
I
I
IOException
FileNotFoundException
I
Opening a file to write erases the file.
I
Opening a file to write creates the file if the file doesn’t exist.
MIE250: Fundamentals of object-oriented programming (Aleman)
Overview
File I/O
Examples
5 / 11
Tokenizing
Creating a file and then printing it out
1
import java . io .*;
2
3
public class FileIO {
4
public static void main ( String [] args ) throws IOException {
int i = 1;
String line ;
BufferedWriter fout = new BufferedWriter ( new FileWriter (" test . dat "));
do {
fout . write ( i + " " + i * i ) ;
fout . newLine () ;
} while ( i ++ < 5) ;
fout . flush () ;
fout . close () ;
5
6
7
8
9
10
11
12
13
14
15
BufferedReader fin = new BufferedReader ( new FileReader (" test . dat "));
do {
line = fin . readLine () ;
if ( line != null )
System . out . println ( line ) ;
} while ( line != null ) ;
fin . close () ;
16
17
18
19
20
21
22
}
23
24
}
src/FileIO.java
MIE250: Fundamentals of object-oriented programming (Aleman)
File I/O
6 / 11
Overview
Examples
Tokenizing
Checking if a file or directory exists
1
import java . io .*;
2
3
4
public class FileExists {
public static void main ( String [] args ) throws IOException {
5
BufferedReader cin = new BufferedReader ( new InputStreamReader ( System . in )) ;
System . out . print (" Enter a file name or directory to check : ");
String filename = cin . readLine () ;
6
7
8
9
File file = new File ( filename ) ;
10
11
boolean exists = file . exists () ;
if (! exists ) {
System . out . println ( filename + " does not exist ! " ) ;
}
else {
System . out . println ( filename + " exists ! " ) ;
}
12
13
14
15
16
17
18
}
19
20
}
src/FileExists.java
MIE250: Fundamentals of object-oriented programming (Aleman)
Overview
File I/O
Examples
7 / 11
Tokenizing
Some useful File methods
Method
Description
canRead()
Check if the file is readable
canWrite()
Check if the file is writable
delete()
Delete the file/dir
deleteOnExit()
Delete the file/dir when the program quits
exists()
Check if the file/dir exists
isDirectory()
Check if the file/dir is a directory
isFile()
Check if the file/dir is a file
listFiles()
List all the files in the directory
MIE250: Fundamentals of object-oriented programming (Aleman)
Overview
File I/O
Examples
8 / 11
Tokenizing
Tokenizing
I
fin.readLine() returns a String.
I
What if we want to store each value in a line separately?
I
Tokenizing a string is done using str.split(token);
MIE250: Fundamentals of object-oriented programming (Aleman)
File I/O
9 / 11
Overview
Examples
Tokenizing
A tokenizing example
Say we want to read this file and put the values into unique variables:
1
2
3
4
5
1
2
3
4
5
1 1
4 8
9 27
16 64
25 125
// need blank line at the end
6
MIE250: Fundamentals of object-oriented programming (Aleman)
Overview
1
File I/O
Examples
10 / 11
Tokenizing
import java . io .*;
2
3
public class FileIOToken {
4
public static void main ( String [] args ) throws IOException {
String line ;
BufferedWriter fout = new BufferedWriter ( new FileWriter (" test . dat "));
for ( int i = 1; i <= 5; i ++) {
fout . write ( i + " " + i * i + " " + i * i * i ) ;
fout . newLine () ;
}
fout . flush () ;
fout . close () ;
5
6
7
8
9
10
11
12
13
14
BufferedReader fin = new BufferedReader ( new FileReader (" test . dat "));
do {
line = fin . readLine () ;
if ( line != null ) {
String [] splitString = line . split ( " " ) ;
int i1 = Integer . parseInt ( splitString [0]) ;
int i2 = Integer . parseInt ( splitString [1]) ;
int i3 = Integer . parseInt ( splitString [2]) ;
System . out . println ( i1 + " , " + i2 + " , " + i3 ) ;
}
} while ( line != null ) ;
fin . close () ;
15
16
17
18
19
20
21
22
23
24
25
26
}
27
28
}
src/FileIOToken.java
MIE250: Fundamentals of object-oriented programming (Aleman)
File I/O
11 / 11