lecture 6 streams and serialization

7/27/2019 Lecture 6 Streams and Serialization

1/27

Lecture 6Streams and Serialization


2/27

Stream Is essentially a sequence of bytes, representing a flow of data from a source

to a destination.

Source or destination could be for example disk file, device or networksocket.


3/27

Dealing with Streams

Any read or write is performed in three simple steps:

Step 1. Open the streamyou need to define some objects here

Step 2. Until there is more data, keep reading in a read, or writing in a write.

You need to use the methods of the objects defined in step1 to read the stream.

Step 3. Close the stream.


4/27

Java IO Package

The java.io package contains nearly every class you might ever need to performinput and output (I/O) in Java.

All these streams represent an input source and an output destination.

The stream in the java.io package supports many data such as primitives,

Object, localized characters, etc.


5/27

Java IO Purposes and Features

The Java IO classes, which mostly consists of readers / writers, are addressingvarious purposes. That is why there are so many different classes. The

purposes addressed are summarized below:

File Access

Network Access

Buffering

Parsing

Reading and Writing Text (Readers / Writers)

Reading and Writing Primitive Data (long, int etc.)

Reading and Writing Objects


6/27


7/27

Input Streams


8/27

Reading text from the keyboard usingScanner class

Step1: define a Scannerobject, sc or any name. To

take input from the

keyboard, use (System.in)

Step2: use the Scannerobject to get data

Step3: close the Scannerwhen finished using it!

Import

- java.util.* library thatincludes the Scanner class


9/27

Reading text from a file using Scanner class

Import :

- java.util.* to use Scanner class

- java.io.* to use File class

Step1a: create a File objectthat points to your text file.

Step1b: define a Scannerobject, sc or any name. To

take input from the text file,

use the file object you

created in Step2a

Step2: use the Scannerobject to get data

Step3: close the Scannerwhen finished using it!


10/27

Note

for this code to work properly, we need to write

some error handling code to handle the situationwhen the file is not found.


11/27

Common Scanner methods

input methods:

s = sc.next() Returns next token (i.e. "word).

s = sc.nextLine() Returns an entire input line as a String.

i = sc.nextInt() Returns next integer value.

d = sc.nextDouble() Returns next double value.

x = sc.nextXYZ() Returns value of type XYZ (primitive value if possible), whereXYZ is one of BigDecimal,BigInteger,Boolean,Byte,Float, or Short.

test methods (used for error checking and loops):

b = sc.hasNext() True if another token is available to be read.

b = sc.hasNextLine() True if another line is available to be read.

b = sc.hasNextInt() True if another int is available to be read.

b = sc.hasNextDouble() True if another double is available to be read.

b = sc.hasNextXYZ() XYZ stands for one of the input types available above.


12/27

Getting input using BufferedReader and another input stream reader

The BufferedReaderwraps another Readerand improves performance.

Readers:

(1) InputStreamReaderto get input from keyb. as bytes and translates itto characters

(2) FileReaderto get input from files.

The BufferedReaderprovides a readLine method to read a line of text.

Note that InputStreamReader

doesnt provide areadLine method.

Java also has a class called BufferedWriterthat has a similar function forwriting data.


13/27

Reading from keyboard Using Buffered

Reader

Step1: open aninput stream

Step2: getdata

Step3: closethe inputstream

reading input from a File using FileReader


14/27

Reading from File Using Buffered Reader

Step1: open aninput stream

Step2: get data

Step3: close theinput stream

reading input from keyboard using InputStreamReader


15/27

Output Streams


16/27

Writing text to a file using PrintWriter class

Step2: use the PrintWriter

object to write text

Step3: close the PrintWriterwhen finished using it!

Import

- java.io.* to use the

PrintWriter class

Step1b: create PrintWriterobject that points to your text

file.

Step1a: create a File objectthat points to your text file.


17/27

Note

Any data in the aaa.txt file will be erased before writing thenew data using println method.

For this code to work, we need to write some errorhandling code to handle the situation when the file is notfound.

Solution

Append Mode in File Writer

FileWriter fw = new FileWriter(file, true);

or

PrintWriter out = new PrintWriter(new BufferedWriter(newFileWriter("outfilename", true)));


18/27

Summary

SourceKeyboard, File,

etc

DestinationScreen, File, etc

Classes to use

Scanner

BufferedReaderInputStreamReader

FileReaderetc

Classes to use

PrintWriter

BufferedWriterOutputStreamWrite

FileWriteretc


19/27

SummaryREADING From Keyboard From File

Scanner Scanner in = new Scanner(X);myString = in.next(); myInt = in.nextInt(); //etc

in.close();

Replace Xwith: System.in new File("C:/filename")

BufferedReader BufferedReader in = new BufferedReader(X);myString = in.readLine();

in.close();

Replace Xwith: new InputStreamReader(System.in) new FileReader("C:/filename")

WRITING To Screen To File

PrintWriter PrintWriter out = new PrintWriter(X);out.println("some text here");out.close();

Replace Xwith: System.out new File("C:/filename")

BufferedWriter BufferedWriter out = new BufferedWriter(X);out.write(some text here");

out.close();

Replace Xwith: new outputStreamWriter(System.out) new FileWriter("C:/filename")

OR instead of both classes, simply write:

System.out.println(some text here);


20/27

For More About Other Input / Output stream Classes

http://tutorials.jenkov.com/java-io/index.html
http://tutorials.jenkov.com/java-io/index.htmlhttp://tutorials.jenkov.com/java-io/index.htmlhttp://tutorials.jenkov.com/java-io/index.htmlhttp://tutorials.jenkov.com/java-io/index.html


21/27

Exercise

Develop a simple Employee Class with 3 methods (Save, Load and Print)

Save method saves employee data to a file. The filename is passed to the method as a parameter

Load method loads employee data from a file. The filename is passed to the method as a parameter

Print Method prints employee data to the screen.


22/27

Serialization


23/27

Serializations and Deserialization

Object serialization

is the process of saving an object's state to a sequence ofbytes.

Deserialization

is the process of rebuilding those bytes into a live object.


24/27

How to Serialize an object

1) To Serialize an object you must implement the serializable interface.

implements Serializable

{

}

2) Create output stream

FileOutputStreamf= new FileOutputStream(filename);

3) Create object output stream

ObjectOutputStreamout = new ObjectOutputStream(f);


25/27

3-Serialize it

out.writeObject(ObjectHere);

4-close the stream

out.close();


26/27

How to Deserialize Object

1-create input stream

FileinputStreamf= new FileinputStream(filename);

2-create object input stream

ObjectinputStreamin= new ObjectinputStream(f);

3-DeSerialize it

Object obj= in.readObject();

4-Cast the object

YourClassName x = (YourClassName ) obj;

5-Close the stream.

in.close();


27/27

END

lecture 6 streams and serialization

Documents