Working with files

RHS – SOC 2

Motivation

• All our programs so far have only worked with data stored in primary storage (RAM)

• Data is lost when program is terminated

• Almost all ”real” programs allow us to save the state – by some definition – of the program to permanent storage (e.g disk)

• Such permanent storage is often a file

RHS – SOC 3

What is a file?

• A collection of bytes, stored on a perma-nent storage media

• Bytes can be interpreted as text, numerics, picture, etc.

• Files have a name, and a position (path)– Myschedule.doc (name)– C:\documents\per\rhs (path)

RHS – SOC 4

Text files

• A text file just contains characters (bytes inter-preted as characters)

• Usually named .txt

• Created with simple text editor like Notepad, or NetBeans…

RHS – SOC 5

Text files and Java

• It is rather easy to read a text file in Java – we use the FileReader class and the Scanner class

FileReader reader = new FileReader(”input.txt”);

Scanner in = new Scanner(reader);

String line = in.nextLine();

RHS – SOC 6

Text files and Java

FileReader reader = new FileReader(”input.txt”);

Scanner in = new Scanner(reader);

String line = in.nextLine();

Constructor takes file name as a String (remember ””)

FileReader itself cannot be used for reading – need a Scanner object

Several next… methods available next() nextInt() nextDouble() …

RHS – SOC 7

Text files and Java

FileReader reader = new FileReader(”input.txt”);

Scanner in = new Scanner(reader);

int lineCount = 0;

while (in.hasNextLine())

{

String line = in.nextLine();

lineCount++;

}

in.close();

System.out.println(”Lines in file: ” + lineCount);

RHS – SOC 8

Text files and Java

• When providing a file path as a constant string, the string mus look like this:

”c:\\documents\\psl\\work.txt”;

• Notice the use of double backslash!

• Recall that \ is used for escape characters!

• We do not need double backslash when e.g the user types the path in a dialog

RHS – SOC 9

Text files and Java

• It is also easy to write to a file in Java – we use the PrintWriter class

PrintWriter out = new PrintWriter(”output.txt”);

• That’s it – we can now use the well-known print(…) and println(…) methods

RHS – SOC 10

Text files and Java

PrintWriter out = new PrintWriter(”output.txt”);

for (int i = 0; i < 10; i++){

out.println(”Writing to a file is easy!”);}

out.close();System.out.println(”Done writing to file”);

RHS – SOC 11

Text files and Exceptions

• Working with files is easy, but…

• …errors beyond the control of the pro-grammer can happen!– File was not found– File was not a text file– File was different format than expected– …and so on

RHS – SOC 12

Text files and Exceptions

• Code for handling files should include proper handling of exceptions!

• IOException class, and subclass thereof

• We can define more specific classes as well

RHS – SOC 13

Exercises

• Self-check: --

• Review: R11.1, R11.2

• Programming: P11.1, P11.7

RHS – SOC 14

Binary files

• A binary file contains ”raw” bytes that can be interpreted as almost anything

• Can have any extension

• Can be created from various sources

RHS – SOC 15

Binary files

• Note that a binary file does usually not contain information about itself (metadata)

• We must know the meaning of the file in advance– Picture– Object data– Encrypted data

RHS – SOC 16

Binary files and Java

• In Java, a binary file is considered a ”stream” of bytes

• We generally use InputStream and OutputStream to handle binary data

• For binary files, we use:– Read: FileInputStream– Write: FileOutputStream

RHS – SOC 17

Binary files and JavaFileInputStream inS = new FileInputStream(”input.dat”);

boolean done = false;

while (!done)

{

int next = inS.read();

byte nextByte;

if (next != -1)

{

nextByte = (byte)next;

... // Do something with nextByte

}

else {done = true; }

}

inS.close();

RHS – SOC 18

Binary files and JavaFileOutputStream outS = new

FileOutputStream(”output.dat”);

byte nextByte;boolean done = false;

while (!done){

... // We get bytes to write from somewhereoutS.write(nextByte);

}

outS.close();

RHS – SOC 19

Binary files – random access

• Sequential processing of binary files is fairly simple – but possibly inefficient

• What if a binary file contained data for thousands of bank accounts, and we just needed data for one…?

• Java also allows ”random access” to data in a binary file

RHS – SOC 20

Binary files – random access

• A file also has a file pointer

• The file pointer indicates where to read/write the next time

1 9 7 7 0 5 1 2 9

RHS – SOC 21

Binary files – random access

• Random access in a file is done using the RandomFileAccess class– new RandomAccessFile(”data.bin”,”rw”)– f.seek(pointerPos)– int pointerPos = f.getFilePointer()– long fileLen = f.length();

RHS – SOC 22

Binary files – random access

• Random access can be efficient when dealing with large binary files

• Again, no help from Java – you must decide and obey a data format yourself

• If you overwrite data – too bad…

• A little bit of help – Java does offer methods for read/write of numeric types from a binary file

RHS – SOC 23

Binary files and Java

• Java only provides simple methods for reading/writing binary files – what else could it do…?

• We are responsible for interpreting and processing the stream of bytes

• Java gets the bytes for us, it does not try to analyse them…

RHS – SOC 24

Object streams

• Wouldn’t it be nice, if we could– Convert an object to binary format– Write the binary data to a file– Close the program– Open the program anew– Read the binary data from the file– Convert the data back to an object

RHS – SOC 25

Object streams

• An object stream can do just that!

• Two classes available– ObjectOutputStream– ObjectInputStream

• Only prerequisite; a class must implement the Serializable interface, in order to be writable and readable by an object stream

RHS – SOC 26

Object streams

public interface Serializable{// Empty...

}

RHS – SOC 27

Object streams

• Typical way to use object streams:– Define a class where one object can contain

all relevant data (for instance a Bank class)– Let the class implement Serializable– Use the class for writing/reading data

• This is extremely useful for saving in-memory data to a file

RHS – SOC 28

Object streamspublic class Bank implements Serializable {...}

// Writing Bank data to a file

Bank myBank = new Bank();

... // Enter data into myBank object

FileOutputStream fos = new FileOutputStream(”bankA.dat”);

ObjectOutputStream oos = new ObjectOutputStream(fos);

oos.writeObject(myBank);

// Reading Bank data from a file

FileInputStream fis = new FileInputStream(”bankB.dat”);

ObjectInputStream ois = new ObjectInputStream(fis);

myBank = (Bank)ois.readObject();

RHS – SOC 29

Exercises

• Self-check: 1, 6, 7, 8

• Review: R19.3, R19.5, R19.7

• Programming:– The Car Rental Project