File Input & Output Sections

Outcomes  Know the difference between files and streams  Use a Scanner to read from a file  add “throws” annotations to methods  use the hasNext family of methods to read every element in a file  Use a PrintWriter object to write to a file  Know when and why to close file streams  Know how to read an entire text file  including files of numbers (!)

Input & Output Streams  System.in and System.out are streams  carry information from one place to another  System.in: from keyboard to program  System.out: from program to monitor keyboardmonitorProgram System.in (kbd) System.out

File Streams  Can get program to read from/write to files  connect Scanner to a file instead of System.in  use a PrintWriter instead of System.out  carry information to/from files fileIn.txtfileOut.txt myprog.exe fin (Scanner) fout (PrintWriter)

Streams and Files nFile exists as a whole, on the disk (or key) nStream just connects it to the program  data comes in/goes out in pieces Input File 3Numbers.txt Scanner fin Program Read3NumbersFromFile.class fout PrintWriter Output File SumOf3.txt

Streams and Files  File exists in secondary memory  File has a name  Data is in the file  Scanner carries the data 3Numbers.txtRead3NumbersFromFile.class

(Possible) Program Variables  A String variable for the file name  A File variable for the file  A Scanner variable for the Scanner  Variable(s) for the data 3Numbers.txtRead3NumbersFromFile.class “3Numbers.txt”

Text Files  We will only read from text files  have.txt extension on a Windows system »tho’ Windows may hide it from you!  Text files can contain numbers (numerals)  and may contain only numbers 3Numbers.txt poem.txt Twas brillig and the slithy toves

Why Use Files?  Permanence  can re-use input; don’t need to copy output  can use output of one program as input to next  Accuracy  correct before the program gets it  even “invisible” output characters stored  Ease of use  large quantities of input hard to type correctly

Using Files  Need to import some classes import java.util.Scanner; import java.io.PrintWriter; import java.io.File; import java.io.FileNotFoundException;  File I/O like console I/O, except: »file streams need to be declared »file streams need to be opened before use »file streams should be closed when done »there are pesky “exceptions” to deal with

Declaring File Streams  Create a file stream Scanner inStream;input file stream(*) PrintWriter outStream;output file stream(*)  Typical names for file stream variables in, fin, inStream, inFile out, fout, outStream, outFile  also names that suggest the file contents stuNumFile, encodedFile, …

(*) File Streams  There are actually classes called InputFileStream and OutputFileStream  these are the very basic file streams  they’re not very useful (for us)  Also lots of other kinds of file streams  useful for different kinds of tasks  Scanners and PrintWriters are more useful for the kinds of things we want to do

Using File Streams  Our input streams are Scanner objects int n = fileIn.nextInt(); String line = fileIn.nextLine();  fileIn acts just like kbd  Our output streams are PrintWriter objects fileOut.println(“The number you entered was ” + n); fileOut.print(“The line was: ‘” + line + “’\n”);  fileOut acts very much like System.out Actually, System.out is a PrintStream, not a PrintWriter. PrintWriters are “new and improved” PrintStreams.

Opening a File for Input  Create a String object for the file’s name String fileName = “3Numbers.txt”;  Create a File object to represent the file File dataFile = new File(fileName);  Create a Scanner to read data from the file Scanner fin = new Scanner(dataFile);  Or combine it all into one command Scanner fin = new Scanner(new File(“3Numbers.txt”)); NOTE: new File(…) does not mean that the file is new! We’re just creating a new object to represent that file.

Problem  An error message when we try to open a file  “… java.io.FileNotFoundException must be caught or declared to be thrown”  We will declare it  later we’ll talk about “catching” it public static void main(String[] args) throws FileNotFoundException { »NOTE where it is – before the body’s opening brace  remember to import the exception class

When to Use a throws Clause  Use throws FileNotFoundException in  any method that opens a file »input or output  any method that calls any method that has a throws FileNotFoundException clause public static void doFileIO() throws FileNotFoundException {... } public static void main(String[] a) throws FileNotFoundException { doFileIO(); } »later we’ll learn how to avoid this annoyance

Example (Part 1) import java.util.Scanner; import java.io.File; import java.io.FileNotFoundException; public class Read3NumbersFromFile { public static void main(String[] args) throws FileNotFoundException { int n1, n2, n3; Scanner fin = new Scanner(new File(“3Nums.txt”)); Continued next slide…

Example (Part 2) n1 = fin.nextInt(); n2 = fin.nextInt(); n3 = fin.nextInt(); System.out.println(“The numbers in the file are: ” + n1 + “, ” + n2 + “, and ” + n3); }}  no fin.nextLine(). Why not? »file creator didn’t have to press the enter key!

Names of Objects  Name of the file itself: 3Nums.txt  the file has the numbers in it  Name of the Scanner: fin  the Scanner is “attached” to the file »gets input from the file instead of from the user  Don’t get them confused: int n1 = 3Nums.txt.nextInt();  errors: illegal name/unknown name

Exercise  Modify the previous program to read four numbers and print their sum  3Nums.txt actually has four numbers in it.  Last value is a sentinel (see next slide)

Tag  tag to javadoc comment  says that this method throws an exception  says what kind(s) of exception it throws  says why it might be thrown /** * … * … FileNotFoundException if 3Nums.txt cannot be found FileNotFoundException if 3Nums.txt cannot be found */ */

Reading a Whole File  Suppose list of numbers is in a file  How shall we read the whole list?  ask user how many numbers in file?  have file start with many numbers it has?  end file with “sentinel” value?  But even better:  we can ask the Scanner »it can check to see if there are more numbers!

Ask the Scanner  Can ask Scanner if file has more data  hasNextInt(), hasNextDouble(), … fin = new Scanner(new File(“3Nums.txt”)); double sum = 0.0; while(fin.hasNextDouble()) { sum += fin.nextDouble(); } System.out.println(“The sum of the #s in the file is ” + sum);

The hasNext Family  There’s a “hasNext” for each “next”  nextDouble  hasNextDouble »will nextDouble work?  nextInt  hasNextInt »will nextInt work?  next  hasNext  nextLine  hasNextLine  hasNext... just checks if it’s there  you have to use next... to read it

Exercise  Write a program fragment to  open “MyEssay.txt” for input  count the number of words in it  report the number of words to the user

File Output  Just like file input, except:  use PrintWriter instead of Scanner »imported from java.io, not java.util  use print & println instead of next, nextInt, &c. »just like System.out import java.io.PrintWriter; PrintWriter fout = new PrintWriter(new File(“t.txt”)); fout.println(“...”);// print to t.txt

Exercise  Modify the number summing program to send the output to the file Summary.txt

Problem  We open the file for output  We write the sum to the file  But when we look at the file, it’s empty!  Why? Buffering!  takes a long time to find the place to print, but then the printing itself is (relatively) fast  PrintWriter saves up output till it has lots  But if the program ends before it prints it out….

Closing Streams  Need to tell the PrintWriter when you’re done printing stuff to the file  so it knows to send what’s remaining to the file fout.close();  You should also close Scanners when you’re done with them  frees up the file and system resources fin.close();

Output Files Start Empty  Output files get created if they didn’t already exist  If an output file did already exist, its contents get erased  once you open it (don’t need to print to it)  If you want to add to the old file PrintWriter fout = new PrintWriter( new FileOutputStream(“out.txt”, true));

Reading File Names  Can mix file and user i/o  e.g. ask user for the name of the file Scanner kbd = new Scanner(System.in); System.out.print(“Enter name of file: ”); String fileName = kbd.nextLine(); Scanner fin = new Scanner(new File(fileName)); int n1 = fin.nextInt();  Reads file name from user  Reads int value from the named file

Exercise  Revise the summing program to read the name of the file the numbers are in  use 3Nums.txt, 100Nums.txt, 1000Nums.txt  use a file that doesn’t exist. What happens?  Revise it again to ask for the output file  use a file that doesn’t already exist  use a file that does already exist »what happens?

Read and Save a File’s Data  When reading from a file, you often don’t know how many elements there will be  use an ArrayList to store the data  Loop until file ends, adding elements while (fin.hasNext()) { myWords.add(fin.next());}  reads all the words of that file into myWords  don’t need to check for filled array

Reading All Numbers from a File  Loop until no more numbers  remember to use wrapper class ArrayList myNumbers = new ArrayList (); Scanner fin = new Scanner(new File(fileName)); while (fin.hasNextDouble()) { myNumbers.add(fin.nextDouble()); myNumbers.add(fin.nextDouble());}fin.close(); »(still need to catch/declare FileNotFoundException)

Exercise  Revise this code to read integer values instead of doubles ArrayList myNumbers = new ArrayList (); Scanner fin = new Scanner(new File(fileName)); while (fin.hasNextDouble()) { myNumbers.add(fin.nextDouble()); myNumbers.add(fin.nextDouble());}fin.close();

Questions?  Next time:  how do we catch FileNotFoundExceptions?  why would we?  what other kinds of exceptions can we catch?  what the heck is an exception, anyway?

Files & Folders  Files exist inside folders  J:\ NetBeans Projects\Lab03\src\myProg.java  J:\ NetBeans Projects\Lab05\Lab05Data.txt NetBeans Projects J: Lab03 src myProg.java classes myProg.class Lab05Data.txt Lab05

Files & Folders  If you just give the name of the file…  myData.txt  …then looks in the project folder  not the src folder!  But you can specify other folders  absolute/relative paths

File Paths  Somewhat system dependent  but can always use / to separate folder names  remember to “escape” \ if inside quote marks  Absolute path  J:/JCreator LE/Lab05/Lab05Data.txt  Relative path (from L03 folder) ../Lab05/Lab05Data.txt

File Paths  Use / in Strings in your code  will work on any machine, not just Windows  “..” means the folder my folder is in  “../..” means the folder that it’s in »and so on!  Put data files in project folder  Try to use relative paths

The File Class  Why do we write “new File(…)”?  it’s not always a new file, after all!  Creating a File object  a program object to represent the file  (remember, the Scanner/PrintWriter is for communicating with the file)  actually just holds the name of the file, but…  …it knows it’s supposed to be the name of a file

Yet Another Variable  You can create a variable for the File File f = new File(fileName);  You can ask the File object about the file  does this file exist? f.exists()  can we read from it? f.canRead()  can we write to it? f.canWrite()  is it a folder/directory? f.isDirectory()  and more these methods may throw SecurityExceptions, but they don’t need to be checked (they are RunTime Exceptions)

Getting a Readable File  Keep asking for file names until we get one that says it’s readable String fileName; File f; do { System.out.print(“Enter a readable file name: ”); fileName = kbd.nextLine(); f = new File(fileName); } while (!f.canRead());

But Still Need to Catch…  Knowing that a file is readable/writable doesn’t mean you can skip the try/catch  file could get deleted between the time you ask if it’s readable and the time you try to create the Scanner/PrintWriter!