File Input & Output

Sections 10.1-10.3

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

keyboard monitorProgram

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.txt fileOut.txtmyprog.exe

fin(Scanner)

fout(PrintWriter)

Streams and Files

File exists as a whole, on the disk (or key) Stream 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.txt Read3NumbersFromFile.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.txt Read3NumbersFromFile.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

10 15 340 -7 -2 103

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 classesimport 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 streamScanner inStream; input file stream(*)PrintWriter outStream;output file stream(*)

Typical names for file stream variablesin, fin, inStream, inFileout, fout, outStream, outFile also names that suggest the file contentsstuNumFile, 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 objectsint n = fileIn.nextInt();String line = fileIn.nextLine(); fileIn acts just like kbd

Our output streams are PrintWriter objectsfileOut.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 nameString fileName = “3Numbers.txt”;

Create a File object to represent the fileFile dataFile = new File(fileName);

Create a Scanner to read data from the fileScanner fin = new Scanner(dataFile);

Or combine it all into one commandScanner 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” itpublic 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 clausepublic 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)

Javadoc @throws Tag

Add @throws 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/** * … * @throws 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.outimport 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 filefout.close();

You should also close Scanners when you’re done with them frees up the file and system resourcesfin.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 filePrintWriter 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 fileScanner 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 elementswhile (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 classArrayList<Double> myNumbers = new ArrayList<Double>();Scanner fin = new Scanner(new File(fileName));while (fin.hasNextDouble()) { myNumbers.add(fin.nextDouble());}fin.close();

» (still need to catch/declare FileNotFoundException)

Exercise

Revise this code to read integer values instead of doublesArrayList<Double> myNumbers = new ArrayList<Double>();Scanner fin = new Scanner(new File(fileName));while (fin.hasNextDouble()) { 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 ProjectsJ:

Lab03src

myProg.java

classes myProg.class

Lab05Data.txtLab05

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 FileFile 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 readableString 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!