1. Working with files

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
DCS – SWC

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)
DCS – SWC

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…
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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();
DCS – SWC

6. Text files and Java FileReader reader = new FileReader(”input.txt”);
Constructor takes file name as a String (remember ””)
FileReader reader = new FileReader(”input.txt”);
Scanner in = new Scanner(reader);
String line = in.nextLine();
Several next… methods available
next()
nextInt()
nextDouble() …
FileReader itself cannot be used for reading – need a Scanner object
DCS – SWC

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);
DCS – SWC

8. Text files and Java
When providing a file path as a constant string, the string must 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
DCS – SWC

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
DCS – SWC

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”);
DCS – SWC

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
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12. Text files and Exceptions
Code for handling files should include proper handling of exceptions!
IOException class, and subclasses thereof
We can define more specific classes as well
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13. 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
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14. 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
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15. 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
DCS – SWC

16. Binary files and Java
FileInputStream 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();
DCS – SWC

17. Binary files and Java
FileOutputStream outS = new FileOutputStream(”output.dat”);
byte nextByte;
boolean done = false;
while (!done) {
... // We get bytes to write from somewhere
outS.write(nextByte); }
outS.close();
DCS – SWC

18. 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
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19. 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 5 1 2 9
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20. 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();
DCS – SWC

21. 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
DCS – SWC

22. 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…
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23. 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
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24. 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
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25. Object streams
public interface Serializable {
// Empty... }
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26. 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
DCS – SWC

27. Object streams
This is very convenient, but BE AWARE of a common ”trap” in serialization!
To serialize an object, all fields in the object must themselves be serializable!
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28. Object streams Example: We want to make ItemCatalog serializable
An ItemCatalog class contains an…
ArrayList<Item> instance field, which contains several…
Item objects
We want to make ItemCatalog serializable
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29. Object streams
We must then mark the ItemCatalog class as being serializable
ArrayList is itself serializable, so all is good…
…BUT we must also mark the Item class itself as being serializable!
Otherwise, an IOException will be thrown
DCS – SWC

30. Object streams public class Bank implements Serializable {...}
Bank myBank = new Bank();
// Enter data into myBank object
FileOutputStream fos = new FileOutputStream(”bank.dat”);
ObjectOutputStream oos = new ObjectOutputStream(fos);
oos.writeObject(myBank);
oos.close();
fos.close();
DCS – SWC

31. Object streams public class Bank implements Serializable {...}
Bank myBank = new Bank();
// Reading Bank data from a file
FileInputStream fis = new FileInputStream(”bank.dat”);
ObjectInputStream ois = new ObjectInputStream(fis);
myBank = (Bank)ois.readObject();
ois.close();
fis.close();
DCS – SWC