![File Handling
▪ Now that we have the file, we have to input to and
output from the file.
▪ We can do low-level File I/O or high-level File I/O.
▪ In order to do either of these, we need a stream object.
▪ Streams are simply sequences of data items.
– Need streams for input and streams for output.
▪ NOTE: the method that is doing file handing needs to
handle an IOException or needs to throw it back to the
calling method.
public static void main(String[] args) throws IOException{](https://image.slidesharecdn.com/lecture11-230823214225-23244f67/85/Lecture-11-pdf-7-320.jpg)
![File Handling
Low level file output
▪ At this stage all we can send to the stream is a
sequence of bytes:
▪ After we are done writing, we need to close the stream:
byte[] byteStream = {2, 92, 17, 64, 9};
outStream.write(byteStream);
outStream.close();](https://image.slidesharecdn.com/lecture11-230823214225-23244f67/85/Lecture-11-pdf-10-320.jpg)
![File Handling
Low level file input
▪ Again, create a File object. Also create a
FileInputStream object and associate with the File
object.
▪ We are going to read into a byte array, so we need to
set it up. The length of the array will be the length of
File object
File myFile = new File("someFile.dat");
FileInputStream inStream = new FileInputStream(myFile);
int fileLength = (int) myFile.length();
byte[] byteStream = new byte[fileLength];](https://image.slidesharecdn.com/lecture11-230823214225-23244f67/85/Lecture-11-pdf-11-320.jpg)
![File Handling
Low level file input
int fileLength = (int) myFile.length();
byte[] byteStream = new byte[fileLength];
inStream.read(byteStream);](https://image.slidesharecdn.com/lecture11-230823214225-23244f67/85/Lecture-11-pdf-12-320.jpg)
Lecture 11.pdf
- 3. File Handling ▪ File class – creating objects for files ▪ Specify the file name (assume the current directory) or the directory and file name to the constructor. ▪ exists() method returns true if the file exists. File myFile = new File("someFile.dat"); File myFile = new File("D:Documents","someFile.dat");
- 4. File Handling ▪ Can also associate the File object with a directory instead: ▪ If it is a directory, then the list() method will return an array of Strings of all the files in the directory. ▪ isFile() method returns true if it is associated with a File, false if associated with a directory. File myDir = new File("D:Documents");
- 5. File Handling ▪ There is also a GUI object that can be used to select files or directories (from the javax.swing.* package) JFileChooser choose = new JFileChooser(); int status = choose.showOpenDialog(null);
- 6. File Handling ▪ The returned value from showOpenDialog() tells us whether or not the “open” button was clicked. It returns an int value, JFileChooser.APPROVE_OPTION if the “open” button was clicked. ▪ If it was, we can use the getSelectedFile() method or the getCurrentDirectory() method to get the selected file or directory. ▪ Lots of other methods with File class and JFileChooser class that have been omitted.
- 7. File Handling ▪ Now that we have the file, we have to input to and output from the file. ▪ We can do low-level File I/O or high-level File I/O. ▪ In order to do either of these, we need a stream object. ▪ Streams are simply sequences of data items. – Need streams for input and streams for output. ▪ NOTE: the method that is doing file handing needs to handle an IOException or needs to throw it back to the calling method. public static void main(String[] args) throws IOException{
- 8. Low level file handling
- 9. File Handling Low level file output ▪ Start by creating a File object: ▪ Next, create a FileOutputStream object and associate it with the File object. File myFile = new File("someFile.dat"); FileOutputStream outStream = new FileOutputStream(myFile);
- 10. File Handling Low level file output ▪ At this stage all we can send to the stream is a sequence of bytes: ▪ After we are done writing, we need to close the stream: byte[] byteStream = {2, 92, 17, 64, 9}; outStream.write(byteStream); outStream.close();
- 11. File Handling Low level file input ▪ Again, create a File object. Also create a FileInputStream object and associate with the File object. ▪ We are going to read into a byte array, so we need to set it up. The length of the array will be the length of File object File myFile = new File("someFile.dat"); FileInputStream inStream = new FileInputStream(myFile); int fileLength = (int) myFile.length(); byte[] byteStream = new byte[fileLength];
- 12. File Handling Low level file input int fileLength = (int) myFile.length(); byte[] byteStream = new byte[fileLength]; inStream.read(byteStream);
- 13. File Handling Low level file input/output File FileInputStream File FileOutputStream
- 14. High level file handling
- 15. File Handling ▪ Writing and reading individual bytes is tedious if we are not working with byte-sized data. ▪ Java gives us another layer of stream objects that handle the conversion from other data types to the byte array.
- 16. File Handling High level file output ▪ Still need the FileOutputStream object, but now we don’t interact with it directly ▪ Instead use a DataOutputStream object, associate it with the FileOutputStream object ▪ DataOutputStream object allows us to write primitive data types. File myFile = new File("someFile.dat"); FileOutputStream outStream = new FileOutputStream(myFile); DataOutputStream outDataStream = new DataOutputStream(outStream);
- 17. File Handling ▪ Now, we can write any primitive data type using the DataOutputStream objects: outDataStream.writeDouble(...); outDataStream.writeFloat(...); outDataStream.writeLong(...); outDataStream.close();
- 18. File Handling High level file input: ▪ Set up File object, FileInputStream object and DataInputStream object. ▪ Use the read methods of the DataInputStream objects to read from the file. File myFile = new File("someFile.dat"); FileInputStream inStream = new FileInputStream(myFile); DataInputStream inDataStream = new DataInputStream(inStream); double myDouble = inDataStream.readDouble(); float myFloat = inDataStream.readFloat(); long myLong = inDataStream.readLong(); inDataStream.close();
- 19. File Handling High level file input/output: File DataOutputStream FileOutputStream writeFloat() writeDouble() writeInt() File DataInputStream FileInputStream readFloat() readDouble() readInt()
- 20. File Handling ▪ FileOutputStream and FileInputStream objects have been useful for reading and writing primitive variables. ▪ But for text files: ▪ Use PrintWriter instead of DataOutputStream ▪ Use FileReader instead of FileInputStream ▪ Use BufferedReader instead of DataInputStream
- 21. File Handling High level text file output: ▪ Set up File object, FileOutputStream object and PrintWriter object. ▪ Use the println() methods to write to the file – can write any primitive data type which is then converted to a String. File myFile = new File("someTextFile.txt"); FileOutputStream outStream = new FileOutputStream(myFile); PrintWriter outPrintStream = new PrintWriter(outStream); outPrintStream.println(99.0); outPrintStream.println("123"); outPrintStream.close();
- 22. File Handling High level text file input: ▪ Set up File object, FileReader object and BufferedReader object. ▪ Use the readLine() methods to read from the file – everything is read as a String, convert to other type if necessary File myFile = new File("someTextFile.txt"); FileReader inFileReader = new FileReader(myFile); BufferedReader inBufferedReader = new BufferedReader(inFileReader); String line = inBufferedReader.readLine(); inBufferedReader.close();
- 23. File Handling High level text file input: ▪ We can also use the Scanner class, and associate it directly to a file, instead of associating it with the console ▪ Use Scanner methods to read and automatically convert File myFile = new File("someTextFile.txt"); Scanner scanFile = new Scanner(myFile); String s = scanFile.next(); Double d = scanFile.nextDouble(); Boolean b = scanFile.nextBoolean();
- 24. File Handling High level text file input/output: File PrintWriter FileOutputStream println() File BufferedRead FileReader readLine() File Scanner next() nextInt() nextFloat()
- 25. File Handling ▪ What about reading and writing objects? ▪ We can read and write the individual data members using a Data Stream ▪ Better to use ObjectOutputStream and ObjectInputStream objects. ▪ Note: to write an object, we need to add to its class definition: ▪ But we don’t need to do anything else – no compulsory methods to write… class Student implements Serializable
- 26. File Handling High level object file output: ▪ Set up File object, FileOutputStream object and ObjectOutputStream object. ▪ Write using the writeObject() method: File myFile = new File("someFile.dat"); FileOutputStream outStream = new FileOutputStream(myFile); ObjectOutputStream outObjStream = new ObjectOutputStream(outStream); Student s = new Student(); outObjStream.writeObject(s);
- 27. File Handling High level object file input: ▪ Set up File object, FileInputStream object and ObjectInputStream object. ▪ Use the readObject methods to read from the file. Note: we need to cast the object to the correct object class. ▪ We can read and write objects from different classes in one file. File myFile = new File("someFile.dat"); FileInputStream inStream = new FileInputStream(myFile); ObjectInputStream inObjectStream = new ObjectInputStream(inStream); Student s; s = (Student) inObjectStream.readObject();
- 28. File Handling High level - object File ObjectOutputStream FileOutputStream writeObject() File ObjectInputStream FileInputStream readObject()