Working with files By the end of this lecture you should be able to: explain the principles of input and output and identify a number of different input and output devices; explain the concept of an I/O stream; describe the basic file-handling techniques used in the Java language; distinguish between text, binary and object encoding of data; distinguish between serial access files and random access files; create and access files in Java using all the above encoding and access methods.

Input and output Output: sending information back to the outside world. Input: receiving information from the outside world standard input stream standard output stream file input stream file output stream

Encoding Java supports three different ways of encoding data: “Java” “Charatan Kans” “34.99” TEXT Can be read by text editors as well as programs. BINARY Can only programs. OBJECT Can only read by another Java program.

Types of File Access Serial access Each item of data is read (or written) in turn. Random access We go directly to the record we want.

The Car Class public class Car { private String registration; private String make; private double price; public Car (String registrationIn, String makeIn, double priceIn) { registration = registrationIn; make = makeIn; price = priceIn; } public String getRegistration() { return registration; } public String getMake() { return make; } public double getPrice() { return price; }

Writing to text files “XV1 2AS” “Ford” “2499” Cars.txt FileWriter PrintWriter

private static void writeList (List<Car> carListIn) { try { FileWriter carFile = new FileWriter ("Cars.txt"); PrintWriter carWriter = new PrintWriter (carFile); for (Car item : carListIn) { carWriter.println ( item.getRegistration() ); carWriter.println ( item.getMake() ); carWriter.println ( item.getPrice() ); } carWriter.close(); } catch (IOException e) { System.out.println("There was a problem writing the file"); } }

Effects of writing to a text file

Reading from text files “XV1 2AS” “Ford” “2499” Cars.txt FileReader BufferedReader

private static void readList (List<Car> carListIn) { String tempReg; String tempMake; String tempStringPrice; double tempDoublePrice; try { FileReader carFile = new FileReader ("Cars.txt"); BufferedReader carStream = new BufferedReader (carFile); tempReg = carStream.readLine(); while (tempReg != null) { // code to read data goes here } carStream.close(); } // code to catch exceptions goes here }

tempMake = carStream.readLine(); tempStringPrice = carStream.readLine(); tempDoublePrice = Double.parseDouble(tempStringPrice); carListIn.add (new Car(tempReg,tempMake,tempDoublePrice)); tempReg = carStream.readLine(); Code to read data

catch(FileNotFoundException e) { System.out.println("\nNo file was read"); } catch(IOException e) { System.out.println ("\nThere was a problem reading the file"); } Code to catch exceptions

Writing to binary files Cars.bin FileOutputStream DataOutputStream

private static void writeList (List<Car> carListIn) { Car tempCar; try { FileOutputStream carFile = new FileOutputStream("Cars.bin"); DataOutputStream carWriter = new DataOutputStream(carFile); for (Car item : carListIn) { carWriter.writeUTF(item.getRegistration()); carWriter.writeUTF(item.getMake()); carWriter.writeDouble(item.getPrice()); } carWriter.close(); } catch(IOException e) { System.out.println("There was a problem writing the file"); } }

The effects of writing to a binary file

Reading from binary files Cars.bin FileInputStream DataInputStream

Reading from a binary file private static void readList (List<Car> carListIn) { String tempReg; String tempMake; double tempPrice; boolean endOfFile = false; try { FileInputStream carFile = new FileInputStream ("Cars.bin"); DataInputStream carStream = new DataInputStream (carFile); while(endOfFile == false) { // code to read data here } carStream.close(); } // code to catch FileNotFoundException and IOException here }

Code to read data try { tempReg = carStream.readUTF(); tempMake = carStream.readUTF(); tempPrice = carStream.readDouble(); carListIn.add(new Car(tempReg, tempMake, tempPrice)); } catch(EOFException e) { endOfFile = true; }

Object serialization Enables us to read and write whole objects from and to files; The process of converting an object into a stream of data suitable for storage on a disk is called serialization; Any class whose objects are to be read and written using the above methods must implement the interface Serializable ; import java.io.*; public class Car implements Serializable { // rest of code here }

Writing to object files Cars.obf FileOutputStream ObjectOutputStream

Writing objects to a file private static void writeList(List<Car> carListIn) { try { FileOutputStream carFile = new FileOutputStream("Cars.obf"); ObjectOutputStream carStream = new ObjectOutputStream(carFile); for (Car item : carListIn) { carStream.writeObject(item); } carStream.close(); } catch(IOException e) { System.out.println("There was a problem writing the file"); } }

Reading from object files Cars.obf FileInputStream ObjectInputStream

Reading objects from a file private static void readList(List<Car> carListIn) { boolean endOfFile = false; Car tempCar; try { FileInputStream carFile = new FileInputStream("Cars.obf"); ObjectInputStream carStream = new ObjectInputStream(carFile); tempCar = (Car) carStream.readObject(); while(endOfFile != true) { // code to read data here } carStream.close(); } // code to catch exceptions here }

try { carListIn.add(tempCar); tempCar = (Car) carStream.readObject(); } catch(EOFException e) { endOfFile = true; } Code to read data

catch(FileNotFoundException e) { System.out.println("\nNo file was read"); } catch(ClassNotFoundException e) // thrown by readObject { System.out.println ("\nTrying to read an object of an unknown class"); } catch(StreamCorruptedException e) // thrown by the constructor { System.out.println("\nUnreadable file format"); } catch(IOException e) { System.out.println("There was a problem reading the file"); }