File class in Java Programmers refer to input/output as - PowerPoint PPT Presentation

File class in Java Programmers refer to input/output as "I/O". ■ Input is received from the keyboard, mouse, files. output is sent to ■ the console, monitor, files, … File Input and Output The File class represents files as objects, and is defined in the ■ (Savitch, Chapter 10) java.io package. Creating a File object allows you to get information about a file ■ TOPICS (on the hard disk or optical drive). Creating a File object does NOT create a new file on your disk. ■ • File Input File f = new File("example.txt"); • Exception Handling if (f.exists() && f.length() > 1000) { • File Output f.delete(); } CS 160, Summer Semester 2016 2 File methods Scanner reminder ■ Some methods in the File class: The Scanner class reads input and processes strings ■ Method name Description and numbers from the user. The constructor can be called with the following three returns whether file can be ■ canRead() arguments: read ❑ A String literal removes file from disk delete() Scanner scan = new Scanner(“Hello there”); ■ ❑ System.in whether this file exists on disk exists() Scanner scan = new Scanner(System.in); returns name of file ■ getName() ❑ A File object returns number of characters length() Scanner scan = new Scanner(new File(<fileName>)); ■ in file changes name of file renameTo( filename ) CS 160, Summer Semester 2016 3 CS 160, Summer Semester 2016 4

Scanner reminder Scanner for reading a file To read a file, pass a File object as a parameter when ■ ■ Common methods called on Scanner : constructing a Scanner String variable Read a line ❑ Scanner for a file: or string literal ■ String str = scan.nextLine(); Scanner <name> = new Scanner(new File( <filename> )); Read a string (separated by whitespace) ❑ Example: String str = scan.next( ); ■ Read an integer Scanner scan = new Scanner(new File("numbers.txt")); ❑ int ival = scan.nextInt( ); or: ■ Read a double ❑ double dval = scan.nextDouble( ); File file = new File("numbers.txt"); Scanner scan= new Scanner(file); CS 160, Summer Semester 2016 5 6 CS 160, Summer Semester 2016 File names and paths File names and paths ■ relative path : does not specify any top-level folder, so ■ When you construct a File object with a relative path, Java the path is relative to the current directory: assumes it is relative to the current directory . In Directory : "names.dat" ❑ Scanner scan = In Subdirectory : "code/Example.java" ❑ new Scanner(new File("data/input.txt")); ■ absolute path : The complete pathname to a file starting If our program is in ~/workspace/P4 ❑ at the root directory /: Scanner will look for ~/workspace/P4/data/input.txt ❑ In Linux : "/users/cs160/programs/Example.java" ❑ In Windows: "C:/Documents/cs160/programs/data.csv" ❑ CS 160, Summer Semester 2016 7 CS 160, Summer Semester 2016 8

Compiler error with files Compiler error with files Here is the compilation error that is produced: ■ Question: Why will the following program NOT compile? ■ ReadFile.java:6: unreported exception import java.io.*; // for File java.io.FileNotFoundException; import java.util.*; // for Scanner must be caught or declared to be thrown public class ReadFile { Scanner scan = new Scanner(new File("data.txt")); public static void main(String[] args) { The problem has to do with error reporting. File file = new File("input.txt"); ■ Scanner scan = new Scanner(file); What to do when a file cannot be opened? ■ String text = scan.next(); File may not exist, or may be protected. System.out.println(text); ■ } Options: exit program, return error, or throw exception ■ } Exceptions are the normal error mechanism in Java. Answer: Because of Java exception handling! ■ ■ CS 160, Summer Semester 2016 9 CS 160, Summer Semester 2016 10 Exceptions Checked exceptions ■ exception : An object that represents a program ■ checked exception : An error that must be error. handled by our program (otherwise it will not ❑ Programs with invalid logic will cause exceptions. compile). ❑ Examples: ❑ We must specify what our program will do to handle dividing by zero ■ any potential file I/O failures. calling charAt on a String with an out of range index ■ trying to read a file that does not exist ❑ We must either: ■ declare that our program will handle (" catch ") the exception, or ❑ We say that a logical error results in an exception ■ being thrown . state that we choose not to handle the exception (and we ■ accept that the program will crash if an exception occurs) ❑ It is also possible to catch (handle) an exception. CS 160, Summer Semester 2016 11 CS 160, Summer Semester 2016 12

Throwing Exceptions Handling Exceptions ■ throws clause : Keywords placed on a method's header ■ When doing file I/O, we use IOException . to state that it may generate an exception. public static void main(String[] args) { ■ It's like a waiver of liability: try { "I hereby agree that this method might throw an exception, and I ❑ File file = new File(“input.txt”); accept the consequences (crashing) if this happens.” Scanner scan = new Scanner(file); General syntax: ❑ String firstLine = scan.nextLine(); public static <type> <name> ( <params> ) throws <type> ... { … } } catch (IOException e) { When doing file open, we throw IOException . ❑ System.out.println(“Unable to open input.txt”); public static void main(String[] args) System.exit(-1); throws IOException { } } CS 160, Summer Semester 2016 13 CS 160, Summer Semester 2016 14 Fixing the compiler error Using Scanner to read a file ■ Consider a file numbers.txt that contains ■ Throwing an exception or handling the exception both this text: resolve the compiler error. 308.2 ■ Throwing Exceptions: User will see program terminate 14.9 7.4 2.8 with exception, that’s not very friendly. 3.9 4.7 -15.4 ■ Handling Exceptions: User gets a clear indication of 2.8 problem with error message, that’s much better. ■ A Scanner views all input as a stream of ■ We will handle exceptions when reading and writing files in programming assignments. characters: 308.2\n\t14.9 7.4 2.8\n\n3.9 4.7\t-15.4\n\t2.8\n ❑ CS 160, Summer Semester 2016 15 CS 160, Summer Semester 2016 16

Consuming tokens First problem ■ Each call to next / nextLine / nextInt / nextDouble , ■ Write code that reads the first 5 double etc. advances the position of the scanner to the end of values from a file and prints. the current token, skipping over any whitespace: 308.2\n 14.9 7.4 2.8\n\n\n3.9 4.7 -15.4\n2.8\n ^ scan.nextDouble(); 308.2 \n 14.9 7.4 2.8\n\n\n3.9 4.7 -15.4\n2.8\n ^ scan.nextDouble(); 308.2\n 14.9 7.4 2.8\n\n\n3.9 4.7 -15.4\n2.8\n ^ CS 160, Summer Semester 2016 17 CS 160, Summer Semester 2016 18 First solution Second problem public static void main(String[] args) ■ How would we modify the program to read all try { the file? File file = new File(“input.txt”); Scanner scan = new Scanner(file); for (int i = 0; i <= 4; i++) { double next = scan.nextDouble(); System.out.println("number = " + next); } } catch (IOException e) { System.out.println(“Unable to open input.txt”); System.exit(-1); } } CS 160, Summer Semester 2016 19 CS 160, Summer Semester 2016 20

Second solution Refining the problem public static void main(String[] args) ■ Modify the program again to handle files that try { also contain non-numeric tokens. File file = new File(“input.txt”); Scanner scan = new Scanner(file); ❑ The program should skip any such tokens. while (scan.hasNextDouble() { ■ For example, it should produce the same double next = scan.nextDouble(); output as before when given this input file: System.out.println("number = " + next); } } catch (IOException e) { 308.2 hello System.out.println(“Unable to open input.txt”); 14.9 7.4 bad stuff 2.8 System.exit(-1); } 3.9 4.7 oops -15.4 } :-) 2.8 @#*($& CS 160, Summer Semester 2016 21 CS 160, Summer Semester 2016 22 Refining the program Reading input line-by-line Given the following input data: ■ while (scan.hasNext()) { 23 3.14 John Smith "Hello world" 45.2 19 if (scan.hasNextDouble()) { The Scanner can read it line-by-line: ■ double next = scan.nextDouble(); 23\t3.14 John Smith\t"Hello world"\n\t\t45.2 19\n System.out.println("number = " + next); ^ } else { scan.nextLine(); 23\t3.14 John Smith\t"Hello world" \n\t\t45.2 19\n // consume the bad token ^ scan.next(); scan.nextLine(); } 23\t3.14 John Smith\t"Hello world"\n \t\t45.2 19 \n } ^ The \n character is consumed but not returned. ■ CS 160, Summer Semester 2016 23 CS 160, Summer Semester 2016 24