Files and Streams Part1-Java Programming-Lecture Slides, Slides of Java Programming

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Chapter 16 – Files and

Streams

Goals

 To be able to read and write text files

 To become familiar with the concepts of text

and binary formats

 To learn about encryption

 To understand when to use sequential and

random file access

 To be able to read and write objects using

serialization

MEM

CPU

HDD

keyboard

monitor terminal console

standard input stream

standard output stream file input stream LOAD READ file output stream SAVE WRITE

Streams files

What does information

travel across?

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Reading and Writing Text Files

 Text files – files containing simple text

 Created with editors such as notepad, html, etc.

 Simplest way to learn it so extend our use of

Scanner

 Associate with files instead of System.in

 All input classes, except Scanner, are in java.io

 import java.io.;*

Numerical Input

 2 ways (we’ve learned one, seen the other)

 Use int as example, similar for double

 First way:

 Use nextInt()

int number = scanner.nextInt();

 Second way:

 Use nextLine(), Integer.parseInt()

String input = scanner.nextLine(); int number = Integer.parseInt(input);

Numerical Input

 Exceptions

 nextInt() throws InputMismatchException

 parseInt() throws NumberFormatException

 Optimal use

 nextInt() when there is multiple information on

one line

 nextLine() + parseInt() when one number

per line

Reading Files

 To read from a disk file, construct a FileReader

 Then, use the FileReader to construct a Scanner

object

FileReader rdr = newFileReader("input.txt");

Scanner fin = new Scanner(rdr);

Reading Files

 You can use File instead of FileReader

 Has an exists() method we can call to avoid

FileNotFoundException

File file = new File ("input.txt");

Scanner fin;

if(file.exists()){

fin = new Scanner(file);

} else {

//ask for another file

}

File Class

 java.io.File

 associated with an actual file on hard drive  used to check file's status

 Constructors

 File()  File(, )

 Methods

 exists()  canRead() , canWrite()  isFile() , isDirectory()

File Class

 java.io.FileReader

 Associated with File object

 Translates data bytes from File object into a

stream of characters (much like InputStream vs.

InputStreamReader)

 Constructors

 FileReader( );

 Methods

 read(), readLine()

 close() docsity.com

Writing to a File

 The out field of the System class is a PrintWriter

object associated with the console

 We will associate our PrintWriter with a file now

PrintWriter fout = new PrintWriter("output.txt"); fout.println(29.95); fout.println(new Rectangle(5, 10, 15, 25));

fout.println("Hello, World!");

 This will print the exact same information as with

System.out (except to a file “output.txt”)!

Closing a File

 Only main difference is that we have to close

the file stream when we are done writing

 If we do not, not all output will written

 At the end of output, call close()

fout.close();

File Locations

 When determining a file name, the default is to

place in the same directory as your .class files

 If we want to define other place, use an absolute

path (e.g. c:\My Documents)

in = new FileReader(“c:\homework\input.dat”);

 Why \?

Sample Program

 Two things to notice:

 Have to import from java.io

 I/O requires us to catch checked exceptions

 java.io.IOException