I/O in Java Dennis Burford dburford@cs.uct.ac.za

I/O in Java

Dennis Burford

dburford@cs.uct.ac.za

Input and Output

• Obtaining input:– Using BasicIo methods.

– Through a GUI

• Producing output:– Using System.out methods (System.out.println())

– Through a GUI

• Other input/output: files, sockets etc.

Streams• Java uses the concept of Streams

for I/O

• Data flows from a Writer to a Reader

ReaderWriter Stream

Why Streams?• Stream generalizes input & output

– Keyboard electronics different from disk– Input stream makes keyboard and files seem the

same to a Reader

Input Stream Reader

Simplified Keyboard Input…

• “Java: First Contact” uses BasicIo class– String name = BasicIo.readString();– int age = BasicIo.readInteger();

• Slack textbook uses Keyboard class– String name = Keyboard.readString();– int age = Keyboard.readInt();

Simplified Keyboard Input…

• Both BasicIo and Keyboard are wrapper classes used to hide detail.

• In reality, both classes use System.in and other low-level Java classes to operate.

System class

• System class is inside java.lang package• Contains 2 variables:

public static final InputStream in;public static final PrintStream out;

• System.in is usually connected to the user’s keyboard.

• System.out is usually connected to the user’s screen.

Using System.in

• System.in is an InputStream

• Can use System.in many ways

– Directly (low-level access)– Through layers of abstraction (high-level

access)

Using System.in

• read() method reads one character at a time.• Returns an int• Returns –1 for EOF (End of File = Ctrl-Z)

System.in

'f'

InputStream.read()

'f','i','r','s','t','\n','1','2','3','\n','4','2',' ','5','8','\n', ...

Using System.in

public static void main(String[] args) throws java.io.IOException { char character;

// Prompt for a character and read it System.out.print("Enter a character: "); System.out.flush(); character = (char) System.in.read(); // Display the character typed System.out.println(); System.out.println("You typed " + character); }

Using System.in

int intChar = System.in.read();

while (intChar != -1){ // Convert to character char character = (char) intChar;

System.out.println("Next character is " + character);

// Get next one intChar = System.in.read();}

Reading Strings

• No String-reading methods in System.in

• To read strings from keyboard, first wrap System.in inside InputStreamReader object:

InputStreamReader ISReader = new InputStreamReader(System.in);

InputStreamReader.read()

an InputStreamReader object

InputStream.read()

'f','i','r','s','t','1','2','3','\n','4','2',' ','5','8','\n', ...

System.in stream

'f'

Reading Strings

Reading Strings

• Next, wrap InputStreamReader object in BufferedReader object:

InputStreamReader ISReader

= new InputStreamReader(System.in);

BufferedReader BufReader

= new BufferedReader(ISReader);

Reading Strings

• Can combine these into a single statement:

BufferedReader BufReader

= new BufferedReader(new InputStreamReader(System.in));

Reading Strings

• Can combine these into a single statement:

BufferedReader BufReader

= new BufferedReader(new InputStreamReader(System.in));

InputStream

InputStreamReader

BufferedReader

Reading Strings

• Methods in BufferedReader– read()– readLine()

• Read a string by using readLine() method:

String str = BufReader.readLine();

Reading Strings

InputStreamReader.read()

an InputStreamReader object

InputStream.read()

'f','i','r','s','t','\n','1','2','3','\n','4','2',' ','5','8','\n', ...

System.in stream

"first"

BufferedReader.readLine()

a BufferedReader object

Reading Strings• InputStreamReader, BufferedReader in java.io.*

• Skeleton for reading:

import java.io.*;

class ClassName{ public static void main(String[] args)

throws java.io.IOException { // Create a buffered input stream and attach it to standard // input BufferedReader BufReader = new BufferedReader(new InputStreamReader(System.in)); ... }}

Example: Reading User’s Name// Reads a user's first name and last name. // Demonstrates use of InputStreamReader, // BufferedReader and the readLine() method.

import java.io.*;

class ReadInputAsString{ public static void main(String[] args) throws java.io.IOException { String firstName, lastName; // Create an input stream and attach it to the standard // input stream BufferedReader BufReader = new BufferedReader(new InputStreamReader(System.in));

Example: Reading User’s Name // Read a line from the user as a String

System.out.print("Enter your first name: "); System.out.flush(); firstName = BufReader.readLine();

// Read a line from the user as a String System.out.print("Enter your last name: ");

System.out.flush(); lastName = BufReader.readLine();

// Display the strings System.out.println(); System.out.println("Your name is" + firstName + lastName); }}

Example: Reading User’s Name // Read a line from the user as a String

System.out.print("Enter your first name: "); System.out.flush(); firstName = BufReader.readLine();

// Read a line from the user as a String System.out.print("Enter your last name: ");

System.out.flush(); lastName = BufReader.readLine();

// Display the strings System.out.println(); System.out.println("Your name is" + firstName + lastName); }}

Enter your first name: LindaEnter your last name: Jones

Your name is Linda Jones

Formatting Numbers

• Alternative to Integer.parseInt() etc.

• NumberFormat– Object factory: use getInstance()– parse() takes a string and returns a Number– parse() throws java.text.ParseException

• General Number class– Return value using intValue(), doubleValue() etc.

InputStreamReader.read()

an InputStreamReader object

InputStream.read()

'1','2','3','\n','4','2',' ','5','8','\n', ...

System.in stream

"123"

BufferedReader.readLine()

a BufferedReader object

NumberFormatter.parse()

Number(123)

Number.intValue()

123

Formatting Numbers from Kbd

NumberFormat formatter = NumberFormat.getInstance();

BufferedReader BufReader

= new BufferedReader(new InputStreamReader(System.in));

System.out.print("Enter an integer: ");

System.out.flush();

String response = BufReader.readLine();

Number numberObj = formatter.parse(response);

int numberInt = numberObj.intValue();

Formatting Numbers from Kbd

NumberFormat formatter = NumberFormat.getInstance();

BufferedReader BufReader

= new BufferedReader(new InputStreamReader(System.in));

System.out.print("Enter an integer: ");

System.out.flush();

String response = BufReader.readLine();

Number numberObj = formatter.parse(response);

int numberInt = numberObj.intValue();

Combine

Formatting Numbers from Kbd

NumberFormat formatter = NumberFormat.getInstance();

BufferedReader BufReader

= new BufferedReader(new InputStreamReader(System.in));

System.out.print("Enter an integer: ");

System.out.flush();

int numberInt

= formatter.parse( BufReader.readLine() ).intValue();

Formatting Numbers from Kbd

NumberFormat formatter = NumberFormat.getInstance();

BufferedReader BufReader

= new BufferedReader(new InputStreamReader(System.in));

System.out.print("Enter an integer: ");

System.out.flush();

int numberInt

= formatter.parse( BufReader.readLine() ).intValue();

Don’t forget to throw or catch java.text.ParseException

Don’t forget to throw or catch java.text.ParseException

Input Streams: Multiple Values

• To read several values on one line– Use StringTokenizer object– Breaks one string into component parts– Must still convert numbers (if necessary)

• StringTokenizer– Constructor takes string– nextToken() method– hasMoreTokens() method

"42","58"

StringTokenizer()

StringTokenizer.nextToken()

a StringTokenizer object

InputStreamReader.read()

an InputStreamReader object

InputStream.read()

'4','2',' ','5','8','\n', ...

System.in stream

"42 58"

BufferedReader.readLine()

a BufferedReader object

Input Streams: Multiple Values

StringTokenizer tokenizer = new StringTokenizer("42 58");

String token;token = tokenizer.nextToken();System.out.println(token); // Displays 42token = tokenizer.nextToken();System.out.println(token); // Displays 58

• BETTER…

StringTokenizer tokenizer = new StringTokenizer("42 58");while (tokenizer.hasMoreTokens()){ System.out.println(tokenizer.nextToken());}

FILES

Overview of Files

• How they are accessed:– sequential: data items must be accessed in the order in

which they are stored (ie. start at the beginning and pass through all the items)

– direct (or random): items are accessed by specifying their location

• How information is represented:– text files: data is stored in character form.– binary files: data is stored in internal binary form (faster

and more compact than text files).

Types of Files

Summer.txt

Rough winds do shake the darling buds of May\nAnd Summer’s lease hath all too short a date\nSometime too hot the eye of heaven shines,\nAnd oft is his gold complexion dimmed,\nAnd every fair from fair sometime declines...

Numbers.dat

Œpôw10Žé¿l%®ú€9câÜ(3xLenfˆx¹ª(Ͻ»¼øß:°µœŒÝçMÙ¾à:ˆqfõ�Ñ>|èœ=L¶...

Text Files

• Text file– human-readable with simple tools (Notepad, Ready, ...)– Each line terminated by end-of-line marker (‘\n’)– Example: .java files

• Easy to read and write text files– Advantage of "streams" approach to I/O– Use same classes and methods as System.in and System.out

Text Files: The File Class

• Need File object for each file program uses– File inFile = new File(”Summer.txt");

• Purpose– Contains information about the file

– A “go-between” for the file

– Not the same as the file itself

Text Files: The File Class

• Methods in the File class– exists(): Tells if the file is there– canRead(): Tells if program can read the file– canWrite(): Tells if program can write to the file– delete(): Deletes the file– isDirectory(): Tells if file is really a directory name– isFile(): Tells if the file is a file (not a directory)– length(): Tells the length of the file, in bytes

Text Files: The File Class

• Methods in the File class– exists(): Tells if the file is there– canRead(): Tells if program can read the file– canWrite(): Tells if program can write to the file– delete(): Deletes the file– isDirectory(): Tells if file is really a directory name– isFile(): Tells if the file is a file (not a directory)– length(): Tells the length of the file, in bytes

import java.io.*;

class TellIfExists{

public static void main(String[] args) throws java.io.IOException {

File myFile = new File(“Summer.txt”);

if (myFile.exists()) { System.out.println("File exists"); } else { System.out.println("File does not exist"); }

} // end main

}

import java.io.*;

class TellIfExists{

public static void main(String[] args) throws java.io.IOException {

File myFile = new File(“Summer.txt”);

if (myFile.exists()) { System.out.println("File exists"); } else { System.out.println("File does not exist"); }

} // end main

}

import java.io.*;

class TellIfExists{

public static void main(String[] args) throws java.io.IOException {

File myFile = new File(“Summer.txt”);

if (myFile.exists()) { System.out.println("File exists"); } else { System.out.println("File does not exist"); }

} // end main

}

import java.io.*;

class TellIfExists{

public static void main(String[] args) throws java.io.IOException {

File myFile = new File(“Summer.txt”);

if (myFile.exists()) { System.out.println("File exists"); } else { System.out.println("File does not exist"); }

} // end main

}

import java.io.*;

class TellIfExists{

public static void main(String[] args) throws java.io.IOException {

File myFile = new File(“Summer.txt”);

if (myFile.exists()) { System.out.println("File exists"); } else { System.out.println("File does not exist"); }

} // end main

}

import java.io.*;

class TellIfExists{

public static void main(String[] args) throws java.io.IOException {

File myFile = new File(“Summer.txt”);

if (myFile.exists()) { System.out.println("File exists"); } else { System.out.println("File does not exist"); }

} // end main

}

Text Files: Reading from a File

• Before reading, make sure– File exists and is readable

inFile.exists() && inFile.canRead()

• Attach file to a stream– FileReader object: knows how to read stream from a file– Wrap FileReader object in BufferedReader object

• BufferedReader works on files just like System.in– read(): read a single character, or -1 if EOF– readLine(): read a line, or null if EOF

File

FileReader

BufferedReader

// Initialize the file variableFile inFile = new File(”Summer.txt");

// Make sure the file can be read fromif (inFile.exists() && inFile.canRead()){ // Create input stream and attach to the file BufferedReader BufReader = new BufferedReader(new FileReader(inFile));

// Read from file stream same way as reading // from the keyboard

String line = BufReader.readLine();}else{ // Error: Can't read from file}

// Initialize the file variableFile inFile = new File(”Summer.txt");

// Make sure the file can be read fromif (inFile.exists() && inFile.canRead()){ // Create input stream and attach to the file BufferedReader BufReader = new BufferedReader(new FileReader(inFile));

// Read from file stream same way as reading // from the keyboard

String line = BufReader.readLine();}else{ // Error: Can't read from file}

// Initialize the file variableFile inFile = new File(”Summer.txt");

// Make sure the file can be read fromif (inFile.exists() && inFile.canRead()){ // Create input stream and attach to the file BufferedReader BufReader = new BufferedReader(new FileReader(inFile));

// Read from file stream same way as reading // from the keyboard

String line = BufReader.readLine();}else{ // Error: Can't read from file}

// Initialize the file variableFile inFile = new File(”Summer.txt");

// Make sure the file can be read fromif (inFile.exists() && inFile.canRead()){ // Create input stream and attach to the file BufferedReader BufReader = new BufferedReader(new FileReader(inFile));

// Read from file stream same way as reading // from the keyboard

String line = BufReader.readLine();}else{ // Error: Can't read from file}

// Initialize the file variableFile inFile = new File(”Summer.txt");

// Make sure the file can be read fromif (inFile.exists() && inFile.canRead()){ // Create input stream and attach to the file BufferedReader BufReader = new BufferedReader(new FileReader(inFile));

// Read from file stream same way as reading // from the keyboard

String line = BufReader.readLine();}else{ // Error: Can't read from file}

// Initialize the file variableFile inFile = new File(”Summer.txt");

// Make sure the file can be read fromif (inFile.exists() && inFile.canRead()){ // Create input stream and attach to the file BufferedReader BufReader = new BufferedReader(new FileReader(inFile));

// Read from file stream same way as reading // from the keyboard

String line = BufReader.readLine();}else{ // Error: Can't read from file}

line = “Rough winds do shake the darling buds of May”

Text Files: Reading from a File

• Can do same things we did with System.in

– Read numbers (NumberFormat)

– Read multiple “tokens” (StringTokenizer)

Text Files: Writing to a File

• Before writing, make sure either...– File doesn't exist

!outFile.exists()

– Or file exists, and is writeableoutFile.exists() && outFile.canWrite()

• Combine conditions!outFile.exists() || outFile.canWrite()

Text Files: Writing to a File

• Attach file to a stream

– FileWriter object: knows how to write stream to a file

– Wrap FileWriter object in BufferedWriter object

– Wrap BufferedWriter object in PrintWriter object

File

FileWriter

BufferedWriter

PrintWriter

// Initialize the file variableFile outFile = new File(”New.txt");

// Make sure that the file either doesn't exist or// we can write to itif (!outFile.exists() || outFile.canWrite()){ // Create an output stream and attach it to the file PrintWriter pWriter = new PrintWriter(new BufferedWriter( new FileWriter(outFile)));

// Write to the file pWriter.println( “This is the first line!” );

}else{ // Error: Can't write to the file}

// Initialize the file variableFile outFile = new File(”New.txt");

// Make sure that the file either doesn't exist or// we can write to itif (!outFile.exists() || outFile.canWrite()){ // Create an output stream and attach it to the file PrintWriter pWriter = new PrintWriter(new BufferedWriter( new FileWriter(outFile)));

// Write to the file pWriter.println( “This is the first line!” );

}else{ // Error: Can't write to the file}

// Initialize the file variableFile outFile = new File(”New.txt");

// Make sure that the file either doesn't exist or// we can write to itif (!outFile.exists() || outFile.canWrite()){ // Create an output stream and attach it to the file PrintWriter pWriter = new PrintWriter(new BufferedWriter( new FileWriter(outFile)));

// Write to the file pWriter.println( “This is the first line!” );

}else{ // Error: Can't write to the file}

// Initialize the file variableFile outFile = new File(”New.txt");

// Make sure that the file either doesn't exist or// we can write to itif (!outFile.exists() || outFile.canWrite()){ // Create an output stream and attach it to the file PrintWriter pWriter = new PrintWriter(new BufferedWriter( new FileWriter(outFile)));

// Write to the file pWriter.println( “This is the first line!” );

}else{ // Error: Can't write to the file}

// Initialize the file variableFile outFile = new File(”New.txt");

// Make sure that the file either doesn't exist or// we can write to itif (!outFile.exists() || outFile.canWrite()){ // Create an output stream and attach it to the file PrintWriter pWriter = new PrintWriter(new BufferedWriter( new FileWriter(outFile)));

// Write to the file pWriter.println( “This is the first line!” );

}else{ // Error: Can't write to the file}

Text Files: Writing to a File

• System.out and pWriter both output streams

– System.out is PrintStream object– pWriter is PrintWriter object

– PrintStream and PrintWriter are almost identical (PrintWriter is newer)

– print(), println(), flush() work the same

Text Files: Writing to a File

• System.out and pWriter both output streams

– System.out is PrintStream object– pWriter is PrintWriter object

– PrintStream and PrintWriter are almost identical (PrintWriter is newer)

– print(), println(), flush() work the same

pWriter.print("This goes ");pWriter.println("to the file");

Text Files: Writing to a File

• Once finished writing to the file, close it

pWriter.close();

Java I/O Summary

Java I/O Summary

• Reading from Keyboard– BufferedReader( InputStreamReader( System.in ))

• Writing to Screen– System.out

• Reading from File– BufferedReader( FileReader( File ))

• Writing to File– PrintWriter( BufferedWriter( FileWriter( File )))