CS111: PROGRAMMING LANGUAGE II … · Files and Streams Java views each file as a sequential stream of bytes. Every operating system provides a mechanism to determine the end of a

CS111: PROGRAMMING

LANGUAGE II

Lecture 13(a): Introduction to Streams Computer Science

Department

Lecture Contents

dr. Amal Khalifa, 2014

Data Hierarchy

Files and streams

Sequential text files

File open

Writing data

File close

2

Introduction to File Processing

Data stored in variables and arrays is temporary.

For long-term retention of data, even after the programs

that create the data terminate, computers use files.

Computers store files on secondary storage devices such as

hard disks, optical disks, flash drives and magnetic tapes.

Data maintained in files is persistent data because it exists

beyond the duration of program execution.

File processing is a subset of Java’s stream-processing

capabilities, which include reading from and writing to

memory, files and network connections.


3

Data Hierarchy

Bit: binary digit

Byte : 8 bits

Character : Unicode

Field : group of characters

Record: group of fields

File : group of related records


4

dr. Amal Khalifa, 2014 5

Data Hierarchy (cont.)

To facilitate the retrieval of specific records from a

file, at least one field in each record is often chosen

as a record key.

A record key identifies a record as belonging to a

particular person or entity and is unique to each

record.

This field typically is used to search and sort records.

In a sequential file, records are stored in order by

the record-key field.


6

Files and Streams

Java views each file as a sequential stream of bytes.

Every operating system provides a mechanism to determine the end of a file (end-of-file marker )

A Java program receives an indication from the operating system when it reaches the end of the stream


7

Files and Streams

Streams that input and output characters

representing data as a sequence of characters.

Files created text files read by text editors.

Streams that input and output bytes

representing data in its binary format.

Files created binary files read by programs that understand the content of the file.

8


character-based streams byte-based streams

Files and Streams in Java

A Java program opens a file by creating an object and associating a stream of bytes or characters with it.

creates three stream objects in Java :

System.in (input stream object)

normally enables a program to input bytes from the keyboard

System.out (output stream object)

normally enables a program to output character data to the screen.

System.err (error stream object) normally enables a program to output character-based error messages to the screen.


9

IO Exception Handling


10

When you programmatically interact with files on

disk, there are many potential problems that could

occur.

Security restrictions on the computer could prevent the

program from opening a file.

If there is something wrong with the disk drive, the

computer could have trouble writing information into

the file.

A program that reads from a file could attempt to open

a file that does not exist, and so on.

IO Exception Handling

IO exceptions are checked exceptions:

To satisfy the catch part of the catch-or-declare requirement, the

code that generates the exception must be wrapped in a try

block and must provide a catch handler for the checked-

exception type (or one of its superclasses).

To satisfy the declare part of the catch-or-declare requirement, the method must provide a throws clause containing the

checked-exception type after its parameter list and before its

method body.

If the catch-or-declare requirement is not satisfied, the compiler

will issue an error message indicating that the exception must be

caught or declared.


11

How to store this data in a file?

Case study 12


The Student class


13

Data

Constructor

set methods

The Student class (cont.)


14

get methods

And…

toString

Case Study: data


15

Store the following Students data on a file :

Name SSN DoB UnvId Faculty Level

Malak Hamad 12345 12-3-1997 22222 Arts 2

Maryam Nabil 34567 10-2-1998 33333 Medicine 1

Salma Hamdy 47689 3-5-1995 44444 Computer

Science

3

Creating Text Files

Formatter

outputs

formatted

Strings to the

screen or a

file


16

Creating Text Files

If the file does

not exist, it

will be

created.

If an existing

file is opened,

its contents

are truncated


17


18

Java

imposes no

structure on

a file

Formatter

object

method

format

a Formatter

object must

be

eventually

closed

File processing


19

Open Write Close

Main!!


20

Three

records

added

Each on a

separate

line

dr. Amal Khalifa, 2014 21

Working on chapter 17…

That’s all for today….. 22

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CS111: PROGRAMMING LANGUAGE II … · Files and Streams Java views each file as a sequential stream of bytes. Every operating system provides a mechanism to determine the end of a