Chapter - 12 File and Streams (continued)

This chapter includes -  DataOutputStream  DataInputStream  Object Serialization  Serializing Objects  How to Write To an ObjectOutputStream  How to Read From an ObjectInputStream  Providing Object Serialization For Your Classes  Implementing The Serializable Interface

 What is a DataOutputStream ?  It is a ByteStream  It can Read Different Datatypes.  How to Open a DataOutputStream  First Open a OutputStream such as FileOutputStream to the Source  Then open a DataOutputStream with the OutputStream.  How to Read from DataOutputStream  DataOutputStream has writeXXX methods to read different data types.  Solve the problem  Let us store some tabular data in invoice1.txt. The tabular data is formatted in columns, where each column is separated from the next by tabs. The columns DataOuputStream

contain the sales price, the number of units ordered, and a description of the item,like this: Java T-shirt Java Mug  import java.io.*; public class DataIOTest { public static void main(String[] args) throws IOException { // write the data out DataOutputStream out = new DataOutputStream(new FileOutputStream("invoice1.txt")); double[] prices = { 19.99, 9.99, 15.99, 3.99, 4.99 }; int[] units = { 12, 8, 13, 29, 50 };

String[ ] descs = { "Java T-shirt", "Java Mug", "Duke Juggling Dolls", "Java Pin", "Java Key Chain" }; for (int i = 0; i < prices.length; i ++) { out.writeDouble(prices[i]); out.writeChar('\t'); out.writeInt(units[i]); out.writeChar('\t'); out.writeChars(descs[i]); out.writeChar('\n'); } out.close(); }

 What is a DataInputStream ?  It is a ByteStream  It can Write Different Datatypes.  How to Open a DataInputStream  First Open a InputStream such as FileInputStream to the Source  Then open a DataOutputStream with the OutputStream.  How to Read from DataOutputStream  DataOutputStream has readXXX methods to read different data types.  Solve the problem  Let us now read the invoice1.txt from previous example to read the dataitems written by previous to display a statement summarizing the order and the total amount owed. DataInputStream

import java.io.*; public class DataIOTest2 { public static void main(String[] args) throws IOException { DataInputStream in = new DataInputStream(new FileInputStream("invoice1.txt")); double price; int unit; String desc; double total = 0.0; try { while (true) { price = in.readDouble(); in.readChar(); // throws out the tab unit = in.readInt(); in.readChar(); // throws out the tab desc = in.readLine();

System.out.println("You've ordered " + unit + " units of " + desc + " at $" + price); total = total + unit * price; } } catch (EOFException e) { } System.out.println("For a TOTAL of: $" + total); in.close(); } Object Serialization  Can we read or write an object at a time ?  Yes, Two streams in java.io-- ObjectInputStream and ObjectOutputStream provide this facility.

 The key to writing an object is to represent its state in a serialized form sufficient to reconstruct the object as it is read.  Thus reading and writing objects is a process called object serialization. Serializing Objects  Reconstructing an object from a stream requires that the object first be written to a stream. So let's start there How to write to an ObjectOutputStream  Step 1  Open a OutputStream such as FileOutputStream to the Source  Step 2  Then open a ObjectOutputStream on that OutputStream.  How to write object to ObjectOutputStream  ObjectOutputStream has writeObject (Object o) method which can write supplied object.  Finally close the ObjectOutputStream with close() method.

How to read From a ObjectInputStream  Step 1  Open an InputStream such as FileInputStream to the Source  Step 2  Then open a ObjectInputStream on that InputStream.  How to read object from ObjectInputStream  ObjectInputStream has readObject method which reads an object and returns that.  Finally close the ObjectOutputStream with close() method.

 How to make an object Serializable ?  An object is serializable only if its class implements the Serializable interface.  Thus, if you want to serialize the instances of one of your classes, the class must implement the Serializable interface.  Good News about Serializable Interface!  The good news is that Serializable is an empty interface. That is, it doesn't contain any method declarations.  What is the purpose of an empty Interface  it's purpose is simply to identify classes whose objects are serializable. Providing Object Serialization for Your Classes

 Making instances of your classes serializable is easy.  You just add the implements Serializable clause to your class declaration like this: public class MySerializableClass implements Serializable {... } Implementing the Serializable Interface