input files–java can’twork if the file doesn’texist in the system –checked exception...

Input Files

Using the File Class to Access and Manipulate Files

Using the Scanner Class to Read Files

• File Paths

• Checked Exceptions

• Token by Token Processing

1

2

Data and Files• Electronic data are stored in files

– Java programs– Word documents– Excel Spreadsheets– Images

• Java programs can read files, process files, and write files– Called input/output (I/O)

• Reading a file is very similar to reading input from the command line!

3

File Objects• Use File objects to represent and interact

with files on the computer

– import java.io.*;

• Create aon disk.

– Creating a File object in a Java program

does not create a new file on the system

File object to get info about a file

File f = new File("data.txt");

4

File Methodshttps://docs.oracle.com/javase/8/docs/api/java/io/File.html

if (f.exists()

}

&& f.length() > 1000) {

f.delete();

Method Description

canRead() Returns whether file is able to be read

delete() Deletes the given file

exists() Whether or not this file exists on the system

getAbsolutePath() The full path to where this file is located

getName() The name of this file as a String, without any path attached

isDirectory() Whether this file represents a directory/folder on the system

isFile() Whether this files represents a file (non-folder) on the system

length() The number of characters in this file

renameTo(file) Changes this files name to be the given file’s name


https://docs.oracle.com/javase/8/docs/api/java/io/File.html

5

File paths• absolute path: specifies the file’s location starting at the top level

folder

– / in Linux (/afs/unity.ncsu.edu/users/m/mlglatz/csc116)– C:\ in Windows (C:\users\mlglatz\csc116 or C:\csc116)– Specify file paths using / in Java. Even if you are on a Windows

machine, Java will handle the paths properly

• relative path: does not specify any top-level foldernames.dat

input/kinglear.txt

– Assumed to be relative to the current directory:

File f = new File("data/readme.txt");

If our program is in ~/csc116/exercises/Lab13

Filewill look for ~/csc116/exercises/Lab13/data/readme.txt

6

Using File Methods• Download the following file from Lecture 13 on our course Moodle

page (this was copied from the book website):

• hamlet.txt

• In the same directory, create a file named FileInfo.java with the following code:

import java.io.*;

public class FileInfo {

public static void main(String[] args) {

File f = new File("hamlet.txt");

System.out.println("exists returns " + f.exists());

System.out.println("canRead returns " + f.canRead());

System.out.println("length returns " + f.length());

System.out.println("getAbsolutePath returns " +

f.getAbsolutePath());

}

}

7

Using File Methods

• Compile and run FileInfo.java

• Output:

exists returns true canRead returns true length returns 196197getAbsolutePath returns

/afs/unity.ncsu.edu/users/m/mlglatz/csc116/Lab13/hamlet.txt

C:\CSC116\exercises\Lab13\hamlet.txt

8

Reading files• Previously, we used Scanner objects to read

input from the console window

– Uses System.in as source of input

– Need to prompt the user for information

• We can also use Scanner objects to read from

a file.

– Uses a File object as source of input

– Do NOT need to prompt for information

• We need separate Scanners for reading from

the console and reading from a File

9

Creating Scanners for Files

• Using a specific File object that may be used later


Scanner input = new Scanner(f);

• Using a File object that is just read once

Scanner input = new Scanner(new File("mydata.txt"));

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Scanner Methods

Now we have access to the Scanner methods, such

as:

– next()

– hasNext()

– nextInt()

– hasNextInt()

– nextDouble()

– hasNextDouble()

11

Using File Input• In the directory where you put hamlet.txt, create a file named

FileInputTest.java with the following code:

import java.util.*;

import java.io.*;

public class FileInputTest {

public static void main(String[] args) {


Scanner input = new Scanner(f);

String data = input.next();

System.out.println("data from file: " + data);

}

}

• Try to compile your completed FileInputTest.java program.

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Compiler Error

The program fails to compile:

FileInputTest.java:7: error: unreported exception FileNotFoundException;

must be caught or declared to be thrown

Scanner input = new Scanner(f);^

1 error

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Exceptions

Recall: Exceptions are errors that prevent a program

from continuing normal execution

calling charAt on a String and passing too large an index

trying to read the wrong type of value from a Scanner

trying to read a file that does not exist

– Exceptions are “thrown” by the program

– We can “catch” exceptions to handle them

FileNotFoundException

– Java can’t work if the file doesn’t exist in the system

– Checked Exception (compiler ‘checks’ that we are aware of the possibility of this exception)

– We must specify how our program will handlefile I/O failures.

– Can’t ignore this exception like you could an InputMismatchException (unchecked exception)

– If program does not acknowledge or handle I/O exception:

• Get unreported exception compile error

8

The throws clause• throws clause: declaration on a method's

header that states that the method will notattempt to handle a particular type of exception.– Explicit acknowledgement that a statement in the

method may cause java to throw the exception– Passing the exception on to the client of our

method– Add @throws to method javadoc

• Syntax:public static type name(params) throws type {

throws

– Example:public class ReadFile {

main(String[]public static void args)

FileNotFoundException{

10

16

Correcting the Compiler Error• Add the following throws clause to your FileInputTest program:

import java.util.*;

import java.io.*;

public class FileInputTest {

public static void main(String[] args) throws FileNotFoundException {


Scanner in = new Scanner(f);

String data = in.next();

System.out.println("data from file: " + data);

}

}

• Compile and run your corrected FileInputTest.java

program.

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Token-Based Processing

• Token-Based Processing: Processing input token

by token

– Recall: A token is a single element of input (e.g.

one word, one number). With Scanner objects,

tokens are separated by whitespace (spaces,

tabs, newline characters).

• Use while loops to process unknown contents of

a file

• Use for loops if bounds of the loop are known

Input tokens token: A unit of user input, separated by whitespace.

A Scanner splits a file's contents into tokens.

If an input file contains the following:

23 3.14

"John Smith"

The Scanner can interpret the tokens as the following types:

Token23

3.14

"John

Smith"

Type(s)int, double, String double, String String

String

18Copyright 2008 by Pearson Education

19

The Scanner and Files

Consider a file numbers.txt that contains this text:

308.2

14.9 7.4 2.8

3.9 4.7

2.8

-15.4

A Scanner views all input as a stream of characters:

308.2\n 14.9 7.4 2.8\n\n3.9 4.7 -15.4\n 2.8\n

^

input cursor: The current position of the Scanner.

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Input CursorA Scanner always keeps track of its current location in a file using an input cursor

The input cursor starts at the beginning of the File.

Scanner scan = new Scanner(new File("numbers.txt"));

308.2 14.9 7.4\n2.8\n\n3.9 4.7 -15.4\n2.8

Consuming tokens consuming input: Reading input and advancing the cursor.

Calling nextInt etc. moves the cursor past the current token.

308.2\n 14.9 7.4 2.8\n\n3.9 4.7 -15.4\n 2.8\n

^

^

21Copyright 2008 by Pearso Education

^

double x = input.nextDouble(); // 308.2

308.2\n 14.9 7.4 2.8\n\n3.9 4.7 -15.4\n 2.8\n

String s = input.next(); // "14.9"

308.2\n 14.9 7.4 2.8\n\n3.9 4.7 -15.4\n 2.8\n

Note that the cursor is placed immediately after the token

that has been processed (at the whitespace character).

^

String l = input.nextLine(); // " 7.4 2.8"

308.2\n 14.9 7.4 2.8\n\n3.9 4.7 -15.4\n 2.8\n

String l = input.nextLine(); // ""

308.2\n 14.9 7.4 2.8\n\n3.9 4.7 -15.4\n 2.8\n

^

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Scanner: Forward Processing

• Scanners are designed for file processing in

a forward manner.

– There is no support for reading the input

backwards

• There is NO way to reset the Scanner back to the

beginning of input

– Instead you will have to construct a new

Scanner object, which would position the input

cursor at the beginning of the file

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Scanners should be closed

• Every Scanner to an input file must be closed after

you are done using it

Scanner input = new Scanner(new File("filename"));

...

input.close();

• If you do not close the Scanner and then try to

rename or move or delete the file, it will not work.

• Closing a Scanner to the Console Window is NOT

really necessary, but it's a good habit.

File input question Recall the input file numbers.txt:

308.2

14.9 7.4 2.8

3.9 4.7 -15.4

2.8

Write a program that reads the first 5 values from the file and prints them along with their sum.

Sum = 337.2


number = 308.2

number = 14.9

number = 7.4

number = 2.8

number = 3.9


File input answer// Displays the first 5 numbers in the given

// and displays their sum at the end.

file,

public static void main(String[] args)

throws FileNotFoundException {

Scanner input = new Scanner(new File("numbers.txt"));

double sum

for (int i

double

= 0.0;

= 1; i <= 5; i++) {

next = input.nextDouble();

System.out.println("number = " + next);

sum = sum + next;

}

input.close();

System.out.printf("Sum = %.1f\n", sum);

}

}

import java.io.*; // for File

import

public

java.util.*;

class Echo {

// for Scanner

26

Scanner Exceptions

InputMismatchException

You read the wrong type of token (e.g. read "hi" as int).

NoSuchElementException

You read past the end of the input.

Finding and fixing these exceptions:

Read the exception text for line numbers in your code (the first line that mentions your file; often near the bottom):

Exception in thread "main" java.util.NoSuchElementExceptionat java.util.Scanner.throwFor(Unknown Source)at java.util.Scanner.next(Unknown Source)at FileInputTest.main(FileInputTest.java:11)

27

Avoiding NoSuchElementException

To detect the end of a file:

Scanner input = new Scanner(new File("example.txt"));

while (input.hasNext()) {

String token = input.next();

System.out.println("token: " + token);

}

Team ExerciseGiven the following 3 lines of text (source:

http://www.lipsum.com , Mr. John Slankas) in

SampleText.txt:

Lorem ipsum 10 dolor 25.5 sit amet,

consectetur adipiscing elit.

Donec 23 ut 76.6 0.25 5 orci ipsum.

What values are returned from the Scanner

method calls listed to the right:

• assume they are executed sequentially,

• assume we have created a Scanner s:

Scanner s = new Scanner(new

File("SampleText.txt"));

If the value returned is a String, put double

quotes around your answer (ex. "Lorem"), if it

is not a String (integer, double, boolean), do

not put quotes around the answer.

Put the answers in a text file named

FileScanner.txt

1.s.next();

2.s.hasNextInt();

3.s.hasNextDouble();

4.s.next();


6.s.hasNextInt();

7.s.nextDouble();

8.s.next();


10.s.hasNextInt();

11.s.next();

12.s.nextLine();

13.s.nextLine();

14.s.next();

15.s.nextInt();

16.s.next();

17.s.hasNextInt();

18.s.nextDouble();

19.s.hasNextInt();


21.s.next();

22.s.hasNextInt();

23.s.nextDouble();

24.s.nextLine();

25.s.hasNext(); 26.s.next();

http://www.lipsum.com/

Reading an entire file


Suppose we want our program to process the entire file.(It should work no matter how many values are in the file.)

Sum = 329.3

• Can use while loops to process unknowncontents of a file

number = 308.2

number = 14.9

number = 7.4

number = 2.8

number = 3.9

number = 4.7

number = -15.4

number = 2.8

File input question 2


Modify the Echo program to process the entire file:

(It should work no matter how many values are in the file.)

Sum = 329.3

number = 308.2

number = 14.9

number = 7.4

number = 2.8

number = 3.9

number = 4.7

number = -15.4

number = 2.8

File input answer 2


// Displays each number in the given file,

public static void main(String[] args)


Scanner input = new Scanner(new File("numbers.txt"));

double sum = 0.0;

while (input.hasNextDouble()) {

double next = input.nextDouble();

System.out.println("number = " + next);

sum = sum + next;

}

input.close();

System.out.printf("Sum = %.1f\n", sum);

}

}

// and displays their sum at the end.

import

import

java.io.*;

java.util.*;

// for

// for

File

Scanner

public class Echo {

In-class Exercise• Go to the moodle page and work on the CountTokens.java

assignment.

• Your CountTokens program should count all the tokens in the file hamlet.txt.

• You must use a File Scanner to read the tokens from the file and after processing the entire file print the following line:

There are ______ tokens in the file hamlet.txt.

• Where the underlined space is replaced by the number of tokens computed by your program.

• Don’t forget to import both the util and io java packages.

• Don’t forget the throws clause

• Don’t forget to close your Scanner.

File input question 3


Modify the Echo program to handle files that contain non-

numeric tokens (by skipping them).

For example, it should produce the same output as before when given this input file, numbers2.txt:

308.2 hello

14.9 7.4 bad stuff 2.8

3.9 4.7 oops -15.4

:-) 2.8 @#*($&

File input answer 3


// Displays each number in the given file,// and displays their sum at the end.


Scanner input = new Scanner(new File("numbers2.txt")); double sum = 0.0;while (input.hasNext()) {

if (input.hasNextDouble()) {double next = input.nextDouble(); System.out.println("number = " + next); sum = sum + next;

} else {input.next(); // throw away the bad token

}}input.close();System.out.printf("Sum = %.1f\n", sum);

}}

import java.io.*; // for File

import

public

java.util.*;

class Echo2 {

// for Scanner

Election question


Write a program that reads a file polls.txt of poll data.

Format: State Clinton% Trump% ElectoralVotes Pollster

The program should print how many electoral votes each candidate leads in, and who is leading overall in the polls.

Clinton: 214 votes

Trump: 257 votes

CT 56 31 7 Oct U. of Connecticut

NE 37 56 5 Sep Rasmussen

AZ 41 49 10 Oct Northern Arizona U.

Election answer

} 36Copyright 2008 by Pearson Education

// Computes leader in presidential polls, based on input file such as:// AK 42 53 3 Oct Ivan Moore Research

import java.io.*; import java.util.*;

// for File// for Scanner

public class Election {public static void main(String[] args) throws FileNotFoundException {

Scanner input = new Scanner(new File("polls.txt")); int clintonVotes = 0, trumpVotes = 0;

while (input.hasNext()) {if (input.hasNextInt()) {

int clinton = input.nextInt(); int trump = input.nextInt(); int eVotes = input.nextInt(); if (clinton > trump) {

clintonVotes = clintonVotes + eVotes;} else if (trump > clinton) {

trumpVotes = trumpVotes + eVotes;}

} else {input.next(); // skip non-integer token

}}input.close();System.out.println("Clinton: " + clintonVotes + System.out.println("Trump: " + trumpVotes

" votes");+ " votes");}

Scanners as Parameters

• When a Scanner is passed as a parameter to a method, the input cursor does not reset to the beginning of the File

• Need to create a new Scanner if you want to go back and read the file again

14

Scanner as ParameterExample

14

import

import

java.io.*;

java.util.*;

public class ShowSum3 {


Scanner input = new Scanner(new File("numbers.dat"));

processTokens(input, 2);



input.close();

}

double sum = 0.0;

for (int i = 1; i < n; i++) {

double next = input.nextDouble();

System.out.println ("number " + i + " = " + next);

sum += next;

}

System.out.println("Sum = " + sum);

System.out.println();

}

}

public static void processTokens(Scanner input, int n) {

308.2 14.9 7.4

2.8

3.9 4.7 -15.4

2.8

numbers.dat

number 1 = 308.2

number 2 = 14.9

Sum = 323.09999999999997

number 1 = 7.4

number 2 = 2.8

number 3 = 3.9

Sum = 14.1

Output:

number 1 = 4.7

number 2 = -15.4

Sum = -10.7

Getting Filename from User

• Create a Scanner for console input

– Prompt user for filename

– Read in filename

• Check that file exists. If not, re-prompt

• Create a Scanner for file input

14

Getting Filename from User return Scanner

14

public static Scanner getInputScanner(Scanner console)


System.out.print("Enter a file name to process: ");

File file = new File(console.next());

while (!file.exists()) {

System.out.print("File doesn't exist. "

+ "Enter a file name to process: ");

file = new File(console.next());

}

Scanner fileScanner = new Scanner(file);

return fileScanner;

}

Lab Exercise• Go to the moodle page and work on the ProcessFile.java

assignment.

• Finish the given ProcessFile.java program that prompts the user for a filename and reprompts if the file doesn’t exist.

– You will process the data in the file; skipping any text or real (double) numbers.

– You will print the max, min, sum, count, and average of the integers in the file.

– You will want to create test files that contain integers, doubles, and Strings.

• HINT: Use the Election example to help set up your loop to read in the integer data and skip the non-integers.

• The min/max loops material from the Lecture 8 slides may be helpful.

input files–java can’twork if the file doesn’texist in the system –checked exception...

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