meljun cortes jedi slides intro2 chapter05 text based applications

7/31/2019 MELJUN CORTES JEDI Slides Intro2 Chapter05 Text Based Applications

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Introduction to Programming 2 1

5 Text-Based Applications


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Topics

Command-Line Arguments and System Properties

Reading from Standard Input

File Handling

Reading from a File

Writing to a File


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Command-Line Arguments

Java allows user to input data from the command line

Purpose of declaring String args[]as a parameter in the mainmethod

When using thejava command, specifying data after the class nameindicates you are passing data via the args parameter

Example:

java Calculate 1 2

args[0]has the value 1

args[1]has the value 2


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Command-Line Arguments:Using NetBeans

Right click on project icon/name on Projects panel Select Properties from the drop-down menu


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Click on Run from Categories panel


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Enter arguments in the Arguments field & click on the OKbutton

Compile and run program as usual


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System Properties

Java also also allows you to manipulate system propertiesfrom the command line

System property Quite similar to environment variables

But is not platform-dependent

Property Mapping between the property name to its corresponding value

Represented in Java with the Properties class.


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System Properties

System class

Provides a methods for determining the current system properties,the getProperties method that returns a Properties object

Also provides the overloaded getPropertymethod


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System Properties

Including a new property

Use the -D option with thejava command

java -D=value

Example:java -Duser.home=philippines

Display the list of system properties

Use the getProperties methodSystem.getProperties().list(System.out);


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System Properties

Truncated sample list of system properties

-- listing properties --

java.runtime.name=Java(TM) 2 Runtime Environment,Stand...

sun.boot.library.path=C:\ProgramFiles\Java\jdk1.5.0_06\jre...

java.vm.version=1.5.0_06-b05

java.vm.vendor=Sun Microsystems Inc.

java.vendor.url=http://java.sun.com/path.separator=;

java.vm.name=Java HotSpot(TM) Client VM

file.encoding.pkg=sun.io

user.country=US

...


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Reading from Standard Input:

Streams Can use streams to read from standard input

Stream

Abstraction of a file or a device that allows a series of items to beread or written

Connected to physical devices

Two general kinds of streams:

Character streams

Byte streams


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Streams Character Streams For Unicode characters

Byte Streams For binary data

Predefined examples

System.in (keyboard by default)

System.out(console by default)


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Reading from Standard Input:BufferedReader

Reading characters from the keyboard

Use the System.in byte stream warped in a BufferedReaderobject

BufferedReader br = new BufferedReader(newInputStreamReader(System.in));

Use readmethod of the BufferedReaderobject

ch = (int) br.read();

//read method returns an integer


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Reading from Standard Input:BufferedReaderExample

1 import java.io.*;2 class FavoriteCharacter {

3 public static void main(String args[])

4 throws IOException {

5

System.out.println("Hi, what's your favorite6 character?");

7 char favChar;

8 BufferedReader br = new BufferedReader(new

9 InputStreamReader(System.in));

10 favChar = (char) br.read();

11 System.out.println(favChar +

12 " is a good choice!");

13 }

14 }


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Output:

Hi, what's your favorite character?

a

a is a good choice!


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Reading from Standard Input:BufferedReader

Reading an entire line

Use the System.in byte stream warped in a BufferedReaderobject

BufferedReader br = new BufferedReader(newInputStreamReader(System.in));

Use the readLine method

str = br.readLine();


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1 import java.io.*;

2 class GreetUser {



5 System.out.println("Hi, what's your name?");

6 String name;



9 name = br.readLine();

1 System.out.println("Nice to meet you, " +

2 name + "! :)");

3 }

4 }


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Output:

Hi, what's your name?rebecca

Nice to meet you, rebecca! :)


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Reminders Don't forget to import thejava.io package as shown below:import java.io.*;

Reading from streams may cause checked exceptions tooccur

Handle these exceptions using try-catch statements

Or handle by indicating the exception in the throws clause of themethod


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File Handling:

Reading from a File Can use the FileInputStream class One of the constructors of this class

FileInputStream(String filename)

Creates a connection to an actual file whose filename is specified asan argument

A FileNotFoundException is thrown when the file does not exist or itcannot be opened for reading

Using the read method Returns an integer representation of data read

Returns -1 when the end of the file is reached


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File Handling:Reading from a File

1 import java.io.*;

2 class ReadFile {



5 System.out.println("What is the name of the6 file to read from?");

7 String filename;


9

InputStreamReader(System.in));10 filename = br.readLine();

11 System.out.println("Now reading from " +

12 filename + "...");

13 //continued...


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14 FileInputStream fis = null;

15 try {

16 fis = new FileInputStream(filename);

17 } catch (FileNotFoundException ex) {

18 System.out.println("File not found.");19 }

20 try {

21 char data;

22

int temp;23 //continued...


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Output:

What is the name of the file to read from?

temp.txt

Now reading from temp.txt...

temporary file


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File Handling:

Writing to a File Can use the FileOutputStream class One of the constructors of this class

FileOutputStream(String filename)

Links an output stream to an actual file to write to A FileNotFoundException is thrown when the file cannot be opened

for writing

Using the write method

void write(int b)

where,

b refers to the data to be written to the actual file

File Handling:


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File Handling:Writing to a File

1 import java.io.*;2 class WriteFile {



5 System.out.println("What is the name of the6 file to be written to?");

7 String filename;



10 filename = br.readLine();

11 System.out.println("Enter data to write to " +

12 filename + "...");

13 //continued...


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14 System.out.println("Type q$ to end.");

15 FileOutputStream fos = null;

16 try {

17 fos = new FileOutputStream(filename);

18 } catch (FileNotFoundException ex) {

19 System.out.println("File cannot be opened

20 for writing.");

21 }

22 try {

23 boolean done = false;

24 int data;

25 //continued...

File Handling:


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26 do {

27 data = br.read();

28 if ((char)data == 'q') {

29 data = br.read();

30 if ((char)data == '$') {31 done = true;

32 } else {

33 fos.write('q');

34 fos.write(data);

35 }

36 } else {

37 fos.write(data);

38 //continued...


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39 }

40 } while (!done);

41 } catch (IOException ex) {

42 System.out.println("Problem in reading from

43 the file.");

44 }

45 }

46 }

Fil H dli


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Output:

What is the name of the file to be writtento?

temp.txt

Enter data to write to temp.txt...

Type q$ to end.

what a wonderful world1, 2, step

q$

Fil H dli


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Content oftemp.txtafter sample run

what a wonderful world

1, 2, step


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Summary

Command-Line Arguments and System Properties

Getting input from the command line

Manipulating system properties

java -D=

Reading from Standard Input

Use System.in

Use BufferedReader Use readmethod


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Summary

File Handling

Reading from a File

Use FileInputStream

Use readmethod

Writing to a File

Use FileOutputStream

Use write method

meljun cortes jedi slides intro2 chapter05 text based applications

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