JAVA Session08

8/8/2019 JAVA Session08

1/20

Slide 1 of 20Session 8Ver. 1.0

Java Programming Language

In this session, you will learn to:

Write a program that uses command-line arguments and

system properties

Write a program that reads from standard input

Describe the C-type formatted input and outputWrite a program that can create, read, and write files

Describe the basic hierarchy of collections

Write a program that uses sets and lists

Write a program to iterate over a collection

Write a program that uses generic collections

Objectives


2/20


3/20



System Properties

System properties are a feature that replaces the concept of

environment variables (which are platform-specific).

System properties include information about the current

user, the current version of the Java runtime, and the

character used to separate components of a file path name.The System.getProperties() method returns a

Properties object.

The System.getProperty(String) method returns a

String representing the value of the named property.

The System.getProperty(String, String)method enables you to supply a default string value (second

parameter), which is returned if the named property does not

exist.


4/20



Console I/O

Applications interact with the user using console I/O.

Java 2 SDK supports console I/O with three publicvariables in the java.lang.System class:

The variable System.out enables you to write to standard

output. It is an object of type PrintStream.The variable System.in enables you to read from standard

input. It is an object of type InputStream.

The variable System.err enables you to write to standard

error. It is an object of type PrintStream.


5/20



Writing to Standard Output

The println() method print the argument and a newline

character (\n).

The print()method print the argument without a newline

character.

The print() and println()methods are overloaded formost primitive types (boolean, char, int, long,

float, and double) and forchar[], Object, and

String.

The print(Object) and println(Object)methods

call the toString() method on the argument.


6/20



The application program can use the following methods ofthe java.io package to read from the standard input:

Read characters from the keyboard and convert the raw bytes

into Unicode characters:

InputStreamReader ir=newInputStreamReader(system.in);

Create a buffered reader to read each line from the keyboard:

BufferedReader in = new BufferedReader(ir);

The BufferedReader(in) provides a readLine() method

to read from standard input one line at a time:s=in.readLine();

The Scanner class ofjava.util package provides

formatted input functionality.

Reading from Standard Input


7/20



Files and File I/O

The java.io package enables you to do the following:

Create File objects

Manipulate File objects

Read and write to file streams


8/20



Files and File I/O (Contd.)

Creating a new File Object:

File myFile;

The File class provides several utilities:

myFile = new File("myfile.txt");

myFile = new File("MyDocs", "myfile.txt");

Directories are treated just like files in Java; the File class

supports methods for retrieving an array of files in the

directory, as follows:

File myDir = new File("MyDocs");

myFile = new File(myDir, "myfile.txt");


9/20




For file input:

Use the FileReader class to read characters.

Use the BufferedReader class to use the readLine()

method.

For file output:Use the FileWriter class to write characters.

Use the PrintWriter class to use the print() and

println() methods.


10/20




The application program can use the following methods ofthe java.io package to read input lines from the keyboard

and write each line to a file:

Create file

File file = new File(args[0]);

Create a buffered reader to read each line from the keyboard

InputStreamReader isr=new

InputStreamReader(System.in);

BufferedReader in = new BufferedReader(isr);

Create a print writer on this filePrintWriter out = new PrintWriter(new

FileWriter(file));


11/20




Read each line from the input stream and print to a file one line

at a time:

s = in.readLine();

out.println(s);

The application program can use the following methods ofthe java.io package to read from a text file and display

each line on the standard output.

Create file:

File file = new File(args[0]);

Create a buffered reader to read each line from the keyboard:BufferedReader in = new BufferedReader(new

FileReader(file));


12/20




Read each line from the file and displays it on the standard

output:

s = in.readLine();

System.out.println(s);


13/20



Demonstration

Lets see how to read data from a file and display the output on

the standard output device. This demo also shows how to run a

program with user provided command line arguments.


14/20



The Collections API

Acollection is a single object representing a group of

objects known as its elements.

The Collections API contains interfaces that group objects

as one of the following:

Collection: A group of objects called elements; any specificordering (or lack of) and allowance of duplicates is specified by

each implementation.

Set: An unordered collection; no duplicates are permitted.

List: An ordered collection; duplicates are permitted.


15/20



Generics

Generics are described as follows:

Provides compile-time type safety

Eliminates the need for casts

Example of before Generics code:

ArrayList list = new ArrayList();

list.add(0, new Integer(42));

int total =

((Integer)list.get(0)).intValue();

Example of afterGenerics code:

ArrayList list = newArrayList();

list.add(0, new Integer(42));

int total = list.get(0).intValue();


16/20



Lets see how to use the Collection API and generics in a Java

program.

Demonstration


17/20



Iterators

Iteration is the process of retrieving every element in a

collection.

An iterator of a set is unordered.

AListIterator of a List can be scanned forwards

(using the next method) or backwards (using the previousmethod).

List list = new ArrayList();

// add some elements

Iterator elements = list.iterator();

while ( elements.hasNext() ) {

System.out.println(elements.next());

}


18/20



Enhanced for Loop

The enhanced for loop has the following characteristics:

Simplified iteration over collections

Much shorter, clearer, and safer

Effective for arrays

Simpler when using nested loopsIterator disadvantages removed

Iterators are error prone:

Iterator variables occur three times per loop.

This provides the opportunity for code to go wrong.


19/20



Summary

In this session, you learned that:

A program can be parameterized by command-line arguments

and system properties.

Applications interaction with the user is accomplished using

text input and output to the console.The Scanner class provides formatted input functionality. It is

a part of the java.util package.

Java.io package enables you to create file objects, manipulate

them, and read and write to the file streams.

The J2SE platform supports file input in two forms:

The FileReader class to read characters.

The BufferedReader class to use the readLine method.

The J2SE platform supports file output in two forms:

The FileWriter class to write characters.

The PrintWriter class to use the print and println methods.


20/20



Summary (Contd.)

A collection is a single object representing a group of objects.

These objects are known as elements. There are two types of

collections:

Set: It is an unordered collection, where no duplicates are

permitted.

List: It is an ordered collection, where duplicates are permitted.

The Iterator interface enables you to scan forward through

any collection. The enhanced for loop can be used to iterate

through a collection.

Generics provide compile-time type safety and eliminate the

need for casts.

JAVA Session08

Documents

Transcript of JAVA Session08