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

Transcription

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

2 Input/Output (IO) 2

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:! 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

21 21 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)

22 22 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

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:! File(pathname: String): Creates a File object for the specified pathname. pathname = directory or file

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

33 33 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

34 34 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

35 35 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

36 36 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

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:!

48 48 Reading files Have to be careful. Suppose a file contains the line:! 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:! void print(string) : Writes a String

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

54 54 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

55 55 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

56 56 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.

57 57 Writing Files PrintWriter output = null; try { output = new PrintWriter(new File( test )); // creates a file if it does not exist; // discards the current content if the file exists output.print("john T Smith "); output.println(90); output.print("eric K Jones "); output.println(85); output.flush(); } catch(ioexception ioe) { System.out.println(ioe.toString()); } finally { if (output!= null) output.close(); }

58 58 Writing Files Problem: What if you want to append to the file not replace it?

59 59 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.

60 60 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

61 61 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

62 62 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)

63 63 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) )

64 64 Writing Files PrintWriter output = null; try { output = new PrintWriter(new File( test )); // creates a file if it does not exist; // discards the current content if the file exists output.print("john T Smith "); output.println(90); output.print("eric K Jones "); output.println(85); output.flush(); } catch(ioexception ioe) { System.out.println(ioe.toString()); } finally { if (output!= null) output.close(); }

65 65 Writing Files Append PrintWriter output = null; try { output = 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 output.print("john T Smith "); output.println(90); output.print("eric K Jones "); output.println(85); output.flush(); } catch(ioexception ioe) { System.out.println(ioe.toString()); } finally { if (output!= null) output.close(); }

66 66 Summary Use Scanner for reading from command-line and files.! Based on delimiters Use PrintWriter for writing to files