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Chapter - 12 File and Streams (continued) This chapter includes -  DataOutputStream  DataInputStream  Object Serialization  Serializing Objects 

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Presentation on theme: "Chapter - 12 File and Streams (continued) This chapter includes -  DataOutputStream  DataInputStream  Object Serialization  Serializing Objects "— Presentation transcript:

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2 Chapter - 12 File and Streams (continued)

3 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

4  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

5 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 };

6 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(); }

7  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

8 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();

9 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.

10  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.

11 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.

12  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

13  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


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