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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

  1. 1 1 COMP200 
 INPUT/OUTPUT OOP using Java, based on slides by Shayan Javed

  2. 2 Input/Output (IO)

  3. 3 I/O � So far we have looked at modeling classes

  4. 4 I/O � So far we have looked at modeling classes � Not much in the way of Input...

  5. 5 Input � 3 ways of providing input to the program:

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

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

  8. 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. 9 Passing parameters � When running the program can directly pass parameters to it

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

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

  12. 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. 13 Command-line input � Receive input from the console during execution

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

  15. 15 The Scanner class � Used for reading data

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

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

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

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

  20. 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. 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. 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. 23 Command-line input � Use the “next..” methods to read from the standard input

  24. 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. 25 File Input � Ability to read files essential to any language.

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

  27. 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. 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. 29 The File class � java.io package � Represents a “file” object � Used for input/output through data streams, the file system and serialization.

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

  31. 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. 32 The File class � Methods: If the file exists � boolean exists() :

  33. 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. 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. 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. 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. 37 Reading Files � Use the Scanner class � new Scanner(File)

  38. 38 Reading Files � How does Scanner really work?

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

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

  41. 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. 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. 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. 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. 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. 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. 47 Reading files � Have to be careful. Suppose a file contains the line: � 34 567

  48. 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. 49 Reading files Scanner scanner = new Scanner(“file.txt”); Treats the String “file.txt” as the source, NOT the file “file.txt”

  50. 50 Writing Files � Use the PrintWriter class

  51. 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. 52 The PrintWriter class � Methods: Writes a String � void print(String) :

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

  54. 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

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