COP 3503 FALL 2012 Shayan Javed Lecture 12

1. COP 3503 FALL 2012ShayanJavedLecture 12 Programming Fundamentals using Java

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 ProgramNameparameter1 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: • booleanhasNext() : If scanner has more tokens

20. The Scanner class • Methods: • booleanhasNext() : If scanner has more tokens • String next() : Returns the next String • intnextInt() : Returns the next int • doublenextDouble() : Returns the next double

21. The Scanner class • Methods: • booleanhasNext() : If scanner has more tokens • String next() : Returns the next String • intnextInt() : Returns the next int • doublenextDouble() : Returns the next double • voiduseDelimiter(pattern: String) : Set’s the delimiting pattern (“ “ by default)

22. The Scanner class • Methods: • booleanhasNext() : If scanner has more tokens • String next() : Returns the next String • intnextInt() : Returns the next int • doublenextDouble() : Returns the next double • voiduseDelimiter(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 importjava.util.Scanner; Scannerscanner = newScanner(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: • File(pathname: String): Creates a File object for the specified pathname. pathname = directory or file

31. 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.

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

33. The File class • Methods: • boolean exists() : If the file exists • booleancanRead() : If the file exists and we can read it • booleancanWrite() : If the file exists and we can write to it

34. The File class • Methods: • boolean exists() : If the file exists • booleancanRead() : If the file exists and we can read it • booleancanWrite() : 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

35. The File class • Methods: • boolean exists() : If the file exists • booleancanRead() : If the file exists and we can read it • booleancanWrite() : 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

36. The File class • Methods: • boolean exists() : If the file exists • booleancanRead() : If the file exists and we can read it • booleancanWrite() : 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

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 = newScanner(newFile(“test.txt”)); while (input.hasNext()) { System.out.println(input.next()); }

44. Reading Files // Reads in the file and outputs all the tokens Scanner input = newScanner(newFile(“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 = newScanner(newFile(“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 = newScanner(newFile(“test.txt”)); while (input.hasNext()) { System.out.println(input.next()); } } catch (FileNotFoundExceptionfe) { 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 = newScanner(newFile(“test.txt”)); intintValue = in.nextInt(); String line = in.nextLine();

49. Reading files Scannerscanner = newScanner(“file.txt”); Treats the String “file.txt” as the source, NOT the file “file.txt”

50. Writing Files • Use the PrintWriter class