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

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 Input � 3 ways of providing input to the program:

6 Input � 3 ways of providing input to the program: � Pass parameters directly to the program

7 Input � 3 ways of providing input to the program: � Pass parameters directly to the program � Command-line input from the user

8 Input � 3 ways of providing input to the program: � Pass parameters directly to the program � Command-line input from the user � Reading in files

9 Passing parameters � When running the program can directly pass parameters to it

10 Passing parameters � When running the program can directly pass parameters to it java ProgramName parameter1 parameter2 ...

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

12 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

13 Command-line input � Receive input from the console during execution

14 Command-line input � Receive input from the console during execution � Use the Scanner class

15 The Scanner class � Used for reading data

16 The Scanner class � Used for reading data � Constructors: � Scanner(InputStream) ■ InputStream = System.in

17 The Scanner class � Used for reading data � Constructors: � Scanner(InputStream) ■ InputStream = System.in � Scanner(String)

18 The Scanner class � Used for reading data � Constructors: � Scanner(InputStream) ■ InputStream = System.in � Scanner(String) � Scanner(File)

19 The Scanner class � Methods: � boolean hasNext() : If scanner has more tokens

20 The Scanner class � Methods: If scanner has more tokens � boolean hasNext() : Returns the next String � String next() : � int nextInt() : Returns the next int � double nextDouble() : Returns the next double

21 The Scanner class � Methods: If scanner has more tokens � boolean hasNext() : Returns the next String � String next() : � int nextInt() : Returns the next int Returns the next double � double nextDouble() : � void useDelimiter(pattern: String) : Set’s the delimiting pattern (“ “ by default)

22 The Scanner class � Methods: If scanner has more tokens � boolean hasNext() : Returns the next String � String next() : � int nextInt() : Returns the next int Returns the next double � double nextDouble() : � void useDelimiter(pattern: String) : Set’s the delimiting pattern (“ “ by default) � void close(): Closes the Scanner

23 Command-line input � Use the “next..” methods to read from the standard input

24 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));

25 File Input � Ability to read files essential to any language.

26 File Input � Ability to read files essential to any language. � Two ways to store data: � Text format:

27 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

28 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

29 The File class � java.io package � Represents a “file” object � Used for input/output through data streams, the file system and serialization.

30 The File class � Constructors: Creates a File object for the specified � File(pathname: String): pathname. pathname = directory or file

31 The File class � Constructors: Creates a File object for the specified � File(pathname: String): 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.

32 The File class � Methods: If the file exists � boolean exists() :

33 The File class � Methods: If the file exists � boolean exists() : If the file exists and we can read it � boolean canRead() : � boolean canWrite() : If the file exists and we can write to it

34 The File class � Methods: If the file exists � boolean exists() : If the file exists and we can read it � boolean canRead() : � boolean canWrite() : If the file exists and we can write to it if the object is a directory � void isDirectory() : if the object is a file � void isFile() :

35 The File class � Methods: If the file exists � boolean exists() : If the file exists and we can read it � boolean canRead() : � boolean canWrite() : If the file exists and we can write to it if the object is a directory � void isDirectory() : if the object is a file � void isFile() : Returns the name of the file � String getName() :

36 The File class � Methods: If the file exists � boolean exists() : If the file exists and we can read it � boolean canRead() : If the file exists and we can write to it � boolean canWrite() : if the object is a directory � void isDirectory() : if the object is a file � void isFile() : Returns the name of the file � String getName() : Deletes the file and returns true � boolean delete() : if succeeded Tries to rename the file and returns true � renameTo (dest: File) : if succeeded

37 Reading Files � Use the Scanner class � new Scanner(File)

38 Reading Files � How does Scanner really work?

39 Reading Files � How does Scanner really work? � Breaks file contents into tokens � Uses a delimiter

40 Reading Files � How does Scanner really work? � Breaks file contents into tokens � Uses a delimiter � Delimiter by default is whitespace

41 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

42 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

43 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()); }

44 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

45 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

46 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(); }

47 Reading files � Have to be careful. Suppose a file contains the line: � 34 567

48 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();

49 Reading files Scanner scanner = new Scanner(“file.txt”); Treats the String “file.txt” as the source, NOT the file “file.txt”

50 Writing Files � Use the PrintWriter class

51 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

52 The PrintWriter class � Methods: Writes a String � void print(String) :

53 The PrintWriter class � Methods: Writes a String � void print(String) : � void print(int) : Writes an int Writes a float � void print(float) :

54 The PrintWriter class � Methods: Writes a String � void print(String) : � void print(int) : Writes an int Writes a float � void print(float) : � void println(String) : Writes a String but also adds a line separator