Understanding Java Input/Output Streams: Mark, Reset, and Object Serialization

1. Mark and Reset The abstract classes InputStream and Reader provide: void mark (int byteLimit) * Marks the current position in this input stream. A subsequent call to the reset method repositions this stream at the last marked position so that subsequent reads re-read the same bytes. * byteLimit is the number of bytes that can be read before this mark becomes invalid void reset() * repositions stream to position when mark was last called boolean markSupported () indicates if a stream supports these methods

2. int[] intList; intList = new int[100]; //this file contains one number 66 FileReader file = new FileReader("number.txt"); BufferedReader ifile = new BufferedReader(file); // all array elements are initialized to 66 ifile.mark(5); for (int i = 0; i<100; i++){ String sVal = ifile.readLine(); ifile.reset(); intList[i] = Integer.parseInt(sVal); }

3. Object Output Another byte (binary) I/O class which extends from OutputStream is ObjectOutputStream ObjectOutputStream class can save entire objects to disk !

4. Serializable • Objects that are written to an object stream must belong to a class that implements the Serializable interface. class Coin implements Serializable { ... } ** Serializable interface has no methods.

5. ObjectOutputStream methods Stream does not just write objects …….. write( ) - writes a byte writeInt(int) - writes a 32 bit int writeDouble(double) - writes a 64 bit double writeChar(char) - writes a 16 bit char writeObject(Object) - writes the specified object to the output stream (if Serializable) close() - closes stream

6. public static void main (String[] args) throws IOException{ int iVal = 15; int[] oVal; oVal = new int[10]; //init some primitive and an Array object double dVal = 55.6; for (int j = 0; j< 10; j++) oVal[j] = j; //set up output file for receiving objects OutputStream ofile = new FileOutputStream("data.dat"); ObjectOutputStream ostrm = new ObjectOutputStream(ofile); //output the data ostrm.writeInt(iVal); ostrm.writeObject(oVal); //Java arrays implement Serializable ostrm.writeDouble(dVal); //close file ostrm.close(); } Writing an Object to a File

7. Object Input • Another byte (binary) I/O class which extends from InputStream is ObjectInputStream ObjectInputStream class can read from a file, written by ObjectOutputStream (Data must by read with respect to datatype and order with which it was written)

8. ObjectInputStream methods read( ) - reads a byte readInt() - reads a 32 bit int readDouble() - reads a 64 bit double readChar() - reads a 16 bit char readObject( ) - reads the specified object to the input stream (if Serializable) close() - closes stream

9. public static void main (String[] args) throws IOException, ClassNotFoundException { int iVal ; int[] oVal; double dVal ; //set up output file for receiving objects InputStream ifile = new FileOutputStream("data.dat"); ObjectInputStream istrm = new ObjectInputStream(ifile); //output the data iVal = istrm.readInt(); oVal = (int []) istrm.readObject(); dVal =istrm.readDouble(); // data has been read into variables //close file ostrm.close(); } Reading from File containing Objects

10. Reading a Coin Object from a File ObjectInputStream in = new ObjectInputStream(newFileInputStream("coins.dat")); Coin c = (Coin)in.readObject();

11. Random .vs. Sequential Access • Sequential access • A file is processed a byte at a time. • Random access • Allows access at arbitrary locations in the file

12. Random and Sequential Access To open a random-access file for reading and writing RandomAccessFile f = new RandomAcessFile("bank.dat","rw");

13. To move the file pointer to a specific byte f.seek(n); • To get the current position of the file pointer. long n = f.getFilePointer(); • To find the number of bytes in a file long fileLength = f.length();