COMP200 INPUT/OUTPUT OOP using Java, based on slides by Shayan - - PowerPoint PPT Presentation

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COMP200 INPUT/OUTPUT OOP using Java, based on slides by Shayan - - PowerPoint PPT Presentation

Jul 23, 2023 427 likes •1.11k views

1 1 COMP200 INPUT/OUTPUT OOP using Java, based on slides by Shayan Javed 2 Input/Output (IO) 3 I/O So far we have looked at modeling classes 4 I/O So far we have looked at modeling classes Not much in the way of Input... 5

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COMP200


INPUT/OUTPUT

OOP using Java, based on slides by Shayan Javed

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Input/Output (IO)

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I/O

So far we have looked at modeling classes

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I/O

So far we have looked at modeling classes Not much in the way of Input...

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Input

3 ways of providing input to the program:

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Input

3 ways of providing input to the program:

Pass parameters directly to the program

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Input

3 ways of providing input to the program:

Pass parameters directly to the program Command-line input from the user

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Input

3 ways of providing input to the program:

Pass parameters directly to the program Command-line input from the user Reading in files

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Passing parameters

When running the program can directly pass

parameters to it

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Passing parameters

When running the program can directly pass

parameters to it

java ProgramName parameter1 parameter2 ...

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Passing parameters

public static void main(String[] args) { // args is the array of all parameters // args[0] would be the first parameter }

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Passing parameters

public static void main(String[] args) { // args is the array of all parameters // args[0] would be the first parameter } Let’s look at an example

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Command-line input

Receive input from the console during

execution

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Command-line input

Receive input from the console during

execution

Use the Scanner class

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The Scanner class

Used for reading data

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The Scanner class

Used for reading data Constructors:

Scanner(InputStream)

■ InputStream = System.in

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The Scanner class

Used for reading data Constructors:

Scanner(InputStream)

■ InputStream = System.in

Scanner(String)

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The Scanner class

Used for reading data Constructors:

Scanner(InputStream)

■ InputStream = System.in

Scanner(String) Scanner(File)

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The Scanner class

Methods:

boolean hasNext() :

If scanner has more tokens

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The Scanner class

Methods:

boolean hasNext() :

If scanner has more tokens

String next() :

Returns the next String

int nextInt() :

Returns the next int

double nextDouble() :

Returns the next double

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The Scanner class

Methods:

boolean hasNext() :

If scanner has more tokens

String next() :

Returns the next String

int nextInt() :

Returns the next int

double nextDouble() :

Returns the next double

void useDelimiter(pattern: String) : Set’s the delimiting pattern

(“ “ by default)

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The Scanner class

Methods:

boolean hasNext() :

If scanner has more tokens

String next() :

Returns the next String

int nextInt() :

Returns the next int

double nextDouble() :

Returns the next double

void useDelimiter(pattern: String) : Set’s the delimiting pattern

(“ “ by default)

void close():

Closes the Scanner

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Command-line input

Use the “next..” methods to read from the standard input

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Command-line input

Use the “next..” methods to read from the standard input

import java.util.Scanner; Scanner scanner = new Scanner(System.in); System.out.print(“Enter number1: “); double number1 = scanner.nextDouble(); System.out.print(“Enter number2: “); double number2 = scanner.nextDouble(); System.out.println(“The addition of the two numbers: “ + (number1 + number2));

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File Input

Ability to read files essential to any language.

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File Input

Ability to read files essential to any language. Two ways to store data:

Text format:

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File Input

Ability to read files essential to any language. Two ways to store data:

Text format:

■ Human-readable form ■ Can be read by text editors

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File Input

Ability to read files essential to any language. Two ways to store data:

Text format:

■ Human-readable form ■ Can be read by text editors

Binary format:

■ Used for executable programs ■ Cannot be read by text editors

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The File class

java.io package Represents a “file” object Used for input/output through data streams,

the file system and serialization.

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The File class

Constructors:

File(pathname: String):

Creates a File object for the specified

  • pathname. pathname = directory or file
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The File class

Constructors:

File(pathname: String):

Creates a File object for the specified

  • pathname. pathname = directory or file

File(parent: String, child: String): Creates a File object for the

child

under the directory parent. child may be a filename or subdirectory.

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The File class

Methods:

boolean exists() :

If the file exists

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The File class

Methods:

boolean exists() :

If the file exists

boolean canRead() :

If the file exists and we can read it

boolean canWrite() : If the file exists and we can write to it

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The File class

Methods:

boolean exists() :

If the file exists

boolean canRead() :

If the file exists and we can read it

boolean canWrite() : If the file exists and we can write to it void isDirectory() :

if the object is a directory

void isFile() :

if the object is a file

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The File class

Methods:

boolean exists() :

If the file exists

boolean canRead() :

If the file exists and we can read it

boolean canWrite() : If the file exists and we can write to it void isDirectory() :

if the object is a directory

void isFile() :

if the object is a file

String getName() :

Returns the name of the file

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The File class

Methods:

boolean exists() :

If the file exists

boolean canRead() :

If the file exists and we can read it

boolean canWrite() :

If the file exists and we can write to it

void isDirectory() :

if the object is a directory

void isFile() :

if the object is a file

String getName() :

Returns the name of the file

boolean delete() :

Deletes the file and returns true if succeeded

renameTo (dest: File) :

Tries to rename the file and returns true if succeeded

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Reading Files

Use the Scanner class new Scanner(File)

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Reading Files

How does Scanner really work?

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Reading Files

How does Scanner really work? Breaks file contents into tokens

Uses a delimiter

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Reading Files

How does Scanner really work? Breaks file contents into tokens

Uses a delimiter Delimiter by default is whitespace

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Reading Files

How does Scanner really work? Breaks file contents into tokens

Uses a delimiter Delimiter by default is whitespace

Reads a token, converts it to the required

type

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Reading Files

How does Scanner really work? Breaks file contents into tokens

Uses a delimiter Delimiter by default is whitespace

Reads a token, converts it to the required type Can change the delimiter – useDelimiter()

method

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Reading Files

// Reads in the file and outputs all the tokens Scanner input = new Scanner(new File(“test.txt”)); while (input.hasNext()) { System.out.println(input.next()); }

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Reading Files

// Reads in the file and outputs all the tokens Scanner input = new Scanner(new File(“test.txt”)); while (input.hasNext()) { System.out.println(input.next()); }

ERROR – WON’T COMPILE

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Reading Files

// Reads in the file and outputs all the tokens Scanner input = new Scanner(new File(“test.txt”)); while (input.hasNext()) { System.out.println(input.next()); }

ERROR – WON’T COMPILE The constructor throws a FileNotFoundException

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Reading Files

// Reads in the file and outputs all the tokens try { Scanner input = new Scanner(new File(“test.txt”)); while (input.hasNext()) { System.out.println(input.next()); } } catch (FileNotFoundException fe) { fe.printStackTrace(); }

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Reading files

Have to be careful. Suppose a file contains

the line:

34 567

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Reading files

Have to be careful. Suppose a file contains

the line:

34 567

What will be the contents of intValue and line after the following code is executed?

Scanner in = new Scanner(new File(“test.txt”)); int intValue = in.nextInt(); String line = in.nextLine();

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Reading files

Scanner scanner = new Scanner(“file.txt”);

Treats the String “file.txt” as the source, NOT the file “file.txt”

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Writing Files

Use the PrintWriter class

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Writing Files

Use the PrintWriter class Constructors:

PrintWriter(File file):

Creates a PrintWriter for the specified File

PrintWriter(String name): Creates a PrintWriter for the specified

File with the name

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The PrintWriter class

Methods:

void print(String) :

Writes a String

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The PrintWriter class

Methods:

void print(String) :

Writes a String

void print(int) :

Writes an int

void print(float) :

Writes a float

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The PrintWriter class

Methods:

void print(String) :

Writes a String

void print(int) :

Writes an int

void print(float) :

Writes a float

void println(String) :

Writes a String but also adds a line separator

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The PrintWriter class

Methods:

void print(String) :

Writes a String

void print(int) :

Writes an int

void print(float) :

Writes a float

void println(String) :

Writes a String but also adds a line separator

void flush() :

Flushes the output stream. Ensures writing to the file

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The PrintWriter class

Methods:

void print(String) :

Writes a String

void print(int) :

Writes an int

void print(float) :

Writes a float

void println(String) :

Writes a String but also adds a line separator

void flush() :

Flushes the output stream. Ensures writing to the file

void close() :

Closes the output stream.

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Writing Files

PrintWriter output = null; try {

  • utput = new PrintWriter(new File(“test”));

// creates a file if it does not exist; // discards the current content if the file exists

  • utput.print("John T Smith "); output.println(90);
  • utput.print("Eric K Jones "); output.println(85);
  • utput.flush();

} catch(IOException ioe) { System.out.println(ioe.toString()); } finally { if (output != null) output.close(); }

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Writing Files

Problem: What if you want to append to the

file not replace it?

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Writing Files

Problem: What if you want to append to the

file not replace it?

Solution 1: Read the whole file, then write it

back.

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Writing Files

Problem: What if you want to append to the

file not replace it?

Solution 1: Read the whole file, then write it

back.

Cumbersome and too much work

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Writing Files

Problem: What if you want to append to the

file not replace it?

Solution 1: Read the whole file, then write it

back.

Cumbersome and too much work

Solution 2: Use the FileWriter class

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Writing Files

Problem: What if you want to append to the

file not replace it?

Solution 1: Read the whole file, then write it

back.

Cumbersome and too much work

Solution 2: Use the FileWriter class

FileWriter(File file, boolean append)

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Writing Files

Problem: What if you want to append to the

file not replace it?

Solution 1: Read the whole file, then write it

back.

Cumbersome and too much work

Solution 2: Use the FileWriter class

FileWriter(File file, boolean append)

PrintWriter pw = new PrintWriter(new FileWriter(file, true) )

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Writing Files

PrintWriter output = null; try {

  • utput = new PrintWriter(new File(“test”));

// creates a file if it does not exist; // discards the current content if the file exists

  • utput.print("John T Smith "); output.println(90);
  • utput.print("Eric K Jones "); output.println(85);
  • utput.flush();

} catch(IOException ioe) { System.out.println(ioe.toString()); } finally { if (output != null) output.close(); }

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Writing Files – Append

PrintWriter output = null; try {

  • utput = new PrintWriter( new FileWriter(new File(“test”),true) );

// creates a file if it does not exist; // appends to the current content if the file exists

  • utput.print("John T Smith "); output.println(90);
  • utput.print("Eric K Jones "); output.println(85);
  • utput.flush();

} catch(IOException ioe) { System.out.println(ioe.toString()); } finally { if (output != null) output.close(); }

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Summary

Use Scanner for reading from command-line

and files.

Based on delimiters

Use PrintWriter for writing to files