File class in Java n Programmers refer to input/output as - PDF document

10/31/15 ¡ File class in Java n Programmers refer to input/output as "I/O". n Input is received from the keyboard, mouse, files. File Input and Output output is sent to the console, monitor, files, … (Savitch, Chapter 10) n The File class represents files as objects, and is defined in the java.io package. TOPICS n Creating a File object allows you to get information about a file (on the hard disk or optical drive). • File Input n Creating a File object does NOT create a new file on • Exception Handling your disk. • File Output File f = new File("example.txt"); if (f.exists() && f.length() > 1000) { f.delete(); } CS 160, Fall Semester 2015 2 File methods Scanner reminder n Some methods in the File class: n The Scanner class reads input and processes strings Method name Description and numbers from the user. n When constructor is called with System.in , the character returns whether file can be canRead() stream is input typed to the console. read n Instantiate Scanner by passing the input character removes file from disk delete() stream to the constructor: whether this file exists on disk exists() Scanner scan = new Scanner(System.in); returns name of file getName() returns number of characters length() in file renameTo( filename ) changes name of file CS 160, Fall Semester 2015 3 CS 160, Fall Semester 2015 4 1 ¡

10/31/15 ¡ Scanner reminder Scanner for reading a file n To read a file, pass a File object as a parameter when n Common methods called on Scanner : constructing a Scanner String variable q Read a line n Scanner for a file: or string literal String str = scan.nextLine(); Scanner <name> = new Scanner(new File( <filename> )); q Read a string (separated by whitespace) n Example: String str = scan.next( ); Scanner scan = new Scanner(new File("numbers.txt")); q Read an integer n or: int ival = scan.nextInt( ); q Read a double File file = new File("numbers.txt"); double dval = scan.nextDouble( ); 
 Scanner scan= new Scanner(file); CS 160, Fall Semester 2015 5 CS 160, Fall Semester 2015 6 File names and paths File names and paths n relative path : does not specify any top-level folder, so n 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 . q In Directory : "names.dat" Scanner scan = q In Subdirectory : "code/Example.java" new Scanner(new File("data/input.txt")); n absolute path : The complete pathname to a file starting q If our program is in ~/workspace/P4 at the root directory /: q Scanner will look for ~/workspace/P4/data/input.txt q In Linux : "/users/cs160/programs/Example.java" q In Windows: "C:/Documents/cs160/programs/data.csv" CS 160, Fall Semester 2015 7 CS 160, Fall Semester 2015 8 2 ¡

10/31/15 ¡ Compiler error with files Compiler error with files n Here is the compilation error that is produced: n 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 Scanner scan = new Scanner(new File("data.txt")); public class ReadFile { n The problem has to do with error reporting. public static void main(String[] args) { File file = new File("input.txt"); n What to do when a file cannot be opened? Scanner scan = new Scanner(file); n File may not exist, or may be protected. String text = scan.next(); n Options: exit program, return error, or throw exception System.out.println(text); n Exceptions are the normal error mechanism in Java. } } n Answer: Because of Java exception handling! CS 160, Fall Semester 2015 9 CS 160, Fall Semester 2015 10 Exceptions Checked exceptions n exception : An object that represents a program n checked exception : An error that must be error. handled by our program (otherwise it will not q Programs with invalid logic will cause exceptions. compile). q Examples: q We must specify what our program will do to handle dividing by zero n any potential file I/O failures. calling charAt on a String with an out of range index n trying to read a file that does not exist q We must either: n declare that our program will handle (" catch ") the exception, or q We say that a logical error results in an exception n being thrown . state that we choose not to handle the exception (and we n accept that the program will crash if an exception occurs) q It is also possible to catch (handle) an exception. CS 160, Fall Semester 2015 11 CS 160, Fall Semester 2015 12 3 ¡

10/31/15 ¡ Throwing Exceptions Handling Exceptions n throws clause : Keywords placed on a method's header n When doing file I/O, we use IOException . to state that it may generate an exception. public static void main(String[] args) { n It's like a waiver of liability: try { q "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); q General syntax: String firstLine = scan.nextLine(); public static <type> <name> ( <params> ) throws <type> ... { … } } catch (IOException e) { q 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, Fall Semester 2015 13 CS 160, Fall Semester 2015 14 Fixing the compiler error Using Scanner to read a file n Consider a file numbers.txt that contains n Throwing an exception or handling the exception both this text: resolve the compiler error. 308.2 n 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 n Handling Exceptions: User gets a clear indication of 2.8 problem with error message, that’s much better. n A Scanner views all input as a stream of n We will handle exceptions when reading and writing files in programming assignments. characters: q 308.2\n\t14.9 7.4 2.8\n\n3.9 4.7\t-15.4\n\t2.8\n CS 160, Fall Semester 2015 15 CS 160, Fall Semester 2015 16 4 ¡

10/31/15 ¡ Consuming tokens First problem n Each call to next / nextLine / nextInt / nextDouble , n 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, Fall Semester 2015 17 CS 160, Fall Semester 2015 18 First solution Second problem public static void main(String[] args) n 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, Fall Semester 2015 19 CS 160, Fall Semester 2015 20 5 ¡

10/31/15 ¡ Second solution Refining the problem public static void main(String[] args) n 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); q The program should skip any such tokens. while (scan.hasNextDouble() { n 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, Fall Semester 2015 21 CS 160, Fall Semester 2015 22 Refining the program Reading input line-by-line n Given the following input data: while (scan.hasNext()) { 23 3.14 John Smith "Hello world" 45.2 19 if (scan.hasNextDouble()) { n 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 } ^ } n The \n character is consumed but not returned. CS 160, Fall Semester 2015 23 CS 160, Fall Semester 2015 24 6 ¡