File and Streams continued

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Chapter - 12
File and Streams (continued)
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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

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DataOuputStream

• 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

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• contain the sales price, the number of units
  ordered, and a description of the item,like this
  
• 19.99 12 Java T-shirt
• 9.99 8 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

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String descs "Java T-shirt", "Java
Mug", "Duke Juggling Dolls", "Java
Pin", "Java Key Chain"
for (int i 0 i lt prices.length i )
out.writeDouble(pricesi)
out.writeChar('\t')
out.writeInt(unitsi)
out.writeChar('\t')
out.writeChars(descsi)
out.writeChar('\n')
out.close()
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DataInputStream

• 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.

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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()
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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.

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• 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.

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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.

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Providing Object Serialization for Your Classes

• 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.

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Implementing the Serializable Interface

• 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
• ...