10/23/18. File class in Java. Scanner reminder. Files. Opening a file for reading. Scanner reminder. File Input and Output

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1 File class i Java File Iput ad Output TOPICS File Iput Exceptio Hadlig File Output Programmers refer to iput/output as "I/O". The File class represets files as objects. The class is defied i the java.io package. Creatig a File object allows you to get iformatio about a file o the disk. Creatig a File object does NOT create a ew file o your disk. File f = ew File("example.txt"); if (f.exists() && f.legth() > 1000) { f.delete(); 2 Files Scaer remider Some methods i the File class: Method ame caread() delete() exists() getname() legth() reameto(fileame) Descriptio returs whether file ca be read removes file from disk whether this file exists o disk returs ame of file returs umber of characters i file chages ame of file 3 The Scaer class reads iput ad processes strigs ad umbers from the user. Whe costructor is called with System.i, the character stream is iput typed to the cosole. Istatiate Scaer by passig the iput character stream to the costructor: Scaer sca = ew Scaer(System.i); 4 Scaer remider Opeig a file for readig Commo methods called o Scaer: Read a lie Strig str = sca.extlie(); Read a strig (separated by whitespace) Strig str = sca.ext( ); Read a iteger it ival = sca.extit( ); Read a double double dval = sca.extdouble( ); To read a file, pass a File object as a parameter whe costructig a Scaer Scaer for a file: Scaer <ame> = ew Scaer(ew File(<fileame>)); Example: Scaer sca= ew Scaer(ew File("umbers.txt")); or: File file = ew File("umbers.txt"); Scaer sca= ew Scaer(file); Strig variable or strig literal 5 6 1

2 File ames ad paths File ames ad paths relative path: does ot specify ay top-level folder, so the path is relative to the curret directory: "ames.dat" "code/example.java" absolute path: The complete pathame to a file startig at the root directory /: I Liux: /users/cs160/programs/example.java I Widows: C:/Documets/cs160/programs/data.csv Whe you costruct a File object with a relative path, Java assumes it is relative to the curret directory. Scaer sca = ew Scaer(ew File( data/iput.txt )); If our program is i ~/workspace/p4 Scaer will look for ~/workspace/p4/data/iput.txt 7 8 Compiler error with files Compiler error with files Questio: Why will the followig program NOT compile? import java.io.*; import java.util.*; // for File // for Scaer public class ReadFile { public static void mai(strig[] args) { File file = ew File( iput.txt ); Scaer sca = ew Scaer(file); Strig text = sca.ext(); System.out.pritl(text); Aswer: Because of Java exceptio hadlig! 9 Here is the compilatio error that is produced: ReadFile.java:6: ureported exceptio java.io.filenotfoudexceptio; must be caught or declared to be throw Scaer sca = ew Scaer(ew File("data.txt")); The problem has to do with error reportig. What to do whe a file caot be opeed? File may ot exist, or may be protected. Optios: exit program, retur error, or throw exceptio Exceptios are the ormal error mechaism i Java. 10 Exceptios Checked exceptios exceptio: A object that represets a program error. Programs with ivalid logic will cause exceptios. Examples: dividig by zero callig charat o a Strig with a out of rage idex tryig to read a file that does ot exist We say that a logical error results i a exceptio beig throw. It is also possible to catch (hadle) a exceptio. checked exceptio: A error that must be hadled by our program (otherwise it will ot compile). We must specify what our program will do to hadle ay potetial file I/O failures. We must either: declare that our program will hadle ("catch") the exceptio, or state that we choose ot to hadle the exceptio (ad we accept that the program will crash if a exceptio occurs)

3 Throwig Exceptios Hadlig Exceptios throws clause: Keywords placed o a method's header to state that it may geerate a exceptio. It's like a waiver of liability: "I hereby agree that this method might throw a exceptio, ad I accept the coseueces (crashig) if this happes. Geeral sytax: public static <type> <ame>(<params>) throws <type> { Whe doig file ope, we throw IOExceptio. public static void mai(strig[] args) throws IOExceptio { 13 Whe doig file I/O, we use IOExceptio. public static void mai(strig[] args) { File file = ew File( iput.txt); Scaer sca = ew Scaer(file); Strig firstlie = sca.extlie();... catch (IOExceptio e) { System.out.pritl( Uable to ope iput.txt ); 14 Fixig the compiler error Usig Scaer to read file data Throwig a exceptio or hadlig the exceptio both resolve the compiler error. Throwig Exceptios: User will see program termiate with exceptio, that s ot very friedly. Hadlig Exceptios: User gets a clear idicatio of problem with error message, that s much better. We will hadle exceptios whe readig ad writig files i programmig assigmets. Cosider a file umbers.txt that cotais this text: A Scaer views all iput as a stream of characters: 308.2\ \\\ \2.8\ Cosumig tokes First problem Each call to ext, extit, extdouble, etc. advaces the positio of the scaer to the ed of the curret toke, skippig over ay whitespace: 308.2\ \\\ \2.8\ sca.extdouble(); 308.2\ sca.extdouble(); 308.2\ \\\ \2.8\ \\\ \2.8\ Write code that reads the first 5 double values from a file ad prits

4 First solutio Secod problem public static void mai(strig[] args) File file = ew File( iput.txt ); Scaer sca = ew Scaer(file); for (it i = 0; i <= 4; i++) { double ext = sca.extdouble(); System.out.pritl("umber = " + ext); catch (IOExceptio e) { System.out.pritl( Uable to ope iput.txt ); How would we modify the program to read all the file? Secod solutio Refiig the problem public static void mai(strig[] args) File file = ew File( iput.txt ); Scaer sca = ew Scaer(file); while (sca.hasnextdouble() { double ext = sca.extdouble(); System.out.pritl("umber = " + ext); catch (IOExceptio e) { System.out.pritl( Uable to ope iput.txt ); Modify the program agai to hadle files that also cotai o-umeric tokes. The program should skip ay such tokes. For example, it should produce the same output as before whe give this iput file: hello bad stuff oops :-) Refiig the program Readig iput lie-by-lie while (sca.hasnext()) { if (sca.hasnextdouble()) { double ext = sca.extdouble(); System.out.pritl("umber = " + ext); else { // cosume the bad toke sca.ext(); 23 Give the followig iput data: Joh Smith "Hello world" The Scaer ca read it lie-by-lie: 23\t3.14 Joh Smith\t"Hello world"\\t\t \ sca.extlie() 23\t3.14 Joh Smith\t"Hello world"\\t\t \ sca.extlie() 23\t3.14 Joh Smith\t"Hello world"\\t\t \ The \ character is cosumed but ot retured. 24 4

5 File processig uestio Solutio Write a program that reads a text file ad adds lie umbers at the begiig of each lie it cout = 0; while (sca.hasnextlie()) { Strig lie = sca.extlie(); System.out.pritl(cout + + lie); cout++; Problem Scaer o strigs Give a file with the followig cotets: 123 Susa Brad Jeifer Cosider the task of computig hours worked by each perso Approach: Break the iput ito lies. Break each lie ito tokes. A Scaer ca tokeize a Strig, such as a lie of a file. Scaer <ame> = ew Scaer(<Strig>); Example: Strig text = " hello "; Scaer sca = ew Scaer(text); System.out.pritl(sca.ext()); // 1.4 System.out.pritl(sca.ext()); // 3.2 System.out.pritl(sca.ext()); // hello Tokeize a etire file Example We ca use strig Scaer(s) to tokeize each lie of a file: Scaer sca = ew Scaer(ew File(<file ame)); while (sca.hasnextlie()) { Strig lie = sca.extlie(); Scaer liesca = ew Scaer(lie); <process this lie...>; 29 Example: Cout the tokes o each lie of a file. Scaer sca = ew Scaer(ew File("iput.txt")); while (sca.hasnextlie()) { Strig lie = sca.extlie(); Scaer liesca = ew Scaer(lie); it cout = 0; while (liesca.hasnext()) { Strig toke = liesca.ext(); cout++; System.out.pritl("Lie has +cout+" tokes"); Iput file iput.txt: Joh Smith "Hello world" Output to cosole: Lie has 6 tokes Lie has 2 tokes 30 5

6 Opeig a file for writig File output Same story as readig, we must hadle exceptios: public static void mai(strig[] args) { File file = ew File( output.txt ); PritWriter output = ew PritWriter(file); output.pritl( Iteger umber: );... catch (IOExceptio e) { System.out.pritl( Uable to write output.txt ); You ca output all the same thigs as you would with System.out.pritl: Discussio so far has bee limited to text files. output.pritl( Double: " + fmt.format( )); output.pritl("iteger: " ); output.pritl("strig: " + "Hello There"); Biary files store data as umbers, ot characters. Biary files are ot huma readable, but more efficiet