Getting Input from a File To open a file for reading: Scanner - - PowerPoint PPT Presentation

Getting Input from a File

To open a file for reading: Scanner inFile = new Scanner (file); Scanner constructor - takes a File parameter and provides methods to get individual pieces of data

Scanner class

Provides methods to get the next data in a file. public String next(); public int nextInt(); public double nextDouble(); Provides methods to check whether the next data has a particular type public boolean hasNext(); public boolean hasNextInt(); public boolean hasNextDouble();

• private void readResultFile() throws FileNotFoundException {
• /

/ Open the file.

• Scanner skierFile = new Scanner(new File (RESULTS_FILE));
• /

/ Find out how many skiers are in the file

• int numSkiers = skierFile.nextInt();
• skiers = new Skier[numSkiers];
• /

/ Read the information for each racer.

• for (int i = 0; i < numSkiers; i++) {
• /

/ Line of file is a skier name, followed by 2 doubles, one for each / / race time

• String skierName = skierFile.next();
• double time1 = skierFile.nextDouble();
• double time2 = skierFile.nextDouble();
• skiers[i] = new Skier(skierName, time1, time2);
• skierList.append(skiers[i].toString() + "\n");
• }
• skierFile.close();
• }

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Closing a file

When you are all done reading from a file, call close: inFile.close();

What if...

There is no file with the name given to the File constructor: new File (“IDontExist.txt”); Throws FileNotFoundException

Handling Exceptions

Call that throws the exception Exception Handler Exception being handled try { File f = new File (“file.txt”); Scanner in = new Scanner (f); ... } catch (FileNotFoundException e) { ... }

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What happens

Call the constructor try { File f = new File (“file.txt”); Scanner in = new Scanner (f); ... } catch (FileNotFoundException e) { ... }

If the file doesn’ t exist

Go to the exception handler try { File f = new File (“file.txt”); Scanner in = new Scanner (f); ... } catch (FileNotFoundException e) { ... }

Not executed

If the file does exist

Go to the next line try { File f = new File (“file.txt”); Scanner in = new Scanner (f); ... } catch (FileNotFoundException e) { ... }

Not executed

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What happens if no handler

public void someMethod() { File f = new File (“file.txt”); Scanner in = new Scanner (f); ... }

Compiler error: Unhandled exception type FileNotFoundException Solution: method must declare that it throws the exception:

public void someMethod() throws FileNotFoundException

Exceptions and Contracts

Exceptions should be documented in a method’ s contract What exception is thrown Under what conditions is the exception thrown

/** Does something really wonderful @throws FileNotFoundException if the file named file.txt does not exist. */

public void someMethod () throws FileNotFoundException

What if...

You call nextInt() and the next thing in the file is not an integer? InputMismatchException You call one of the next methods and there is nothing left in the fille? NoSuchElementException You call one of the next methods but the file is closed? IllegalStateException

Be sure to call has… methods before next… methods to avoid these exceptions.

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