Read Files and Exception and Flow Charts

File Class:

· New type of class found inside java.io so import java.io.File

· Documentation:

o standard doc: http://docs.oracle.com/javase/7/docs/api/?java/io/File.html

o Clearer tutorial: http://tutorials.jenkov.com/java-io/file.html

· Represents filename and the path. Use forward slash in paths - '/'

· Constructor parameter – a filename with a path

o File f = new File ("c:/users/pepper/documents/myfile.txt");

o File f = new File("myfile.txt"); // expects the file in your bluej project folder

o File f = new File ("subfolder/myfile.txt"); // expects the file in the subfolder of your bluej project folder

o bluej project folder is current directory or working directory

· Methods:

o exists() Returns a boolean and Tests whether the file or directory denoted by this abstract pathname exists.

o canRead()Returns a boolean and Tests whether the application can read the file denoted by this abstract pathname.

o getAbsolutePath() Returns a string of the absolute pathname string of this abstract pathname.

• length() returns a long integer containing the length the file denoted by this abstract pathname.
• isDirectory
• isFile

Open a file with Scanner

• Scanner can read from a text file instead of from the console
• Instead of System.in, give it a file object

· Scanner throws a checked exception FileNotFoundException if the file is not found

o Surround with a try phrase

o indicate your method plans to ignore this checked exception

• throws FileNotFoundException after method parms and before {

More on Exceptions:

· You already know:

o Create a new Exception of a known type

o Throw an exception from a method : throw new Exception("bad");

o Catch an exception that is thrown

o Look into the exception for information

· New:

o Choose to ignore a certain type of checked exception when using a method

o Syntax: public static void main()

throws FileNotFoundException {

Read a file with Scanner

· Same methods as with screen input

· Loop through until the file ends: while( input.hasNext()) {

o Error: NoSuchElementException (along with line number from file)

· Read the correct type

o Error: InputMismatchException

· Deal with new line characters

o Scanner's nextInt does not read the \n

o Scanner's nextLine does read the \n

· Forward only

o To read the file again, just create a new Scanner

· Exercise: read a file and print it

· Exercise: sum the hours an employee works – id followed by name followed by hours until end

Parse the input

· hasNextInt, nextInt pairs

· Read lines of text : hasNextLine and nextLine

· Keeps spaces

· Read a string with Scanner

o Construct the Scanner with the string:

o Scanner stringScanner = new Scanner("1 a");

o Use all normal Scanner methods

§ int a = stringScanner.nextInt();

§ String b = stringScanner.next();

· Delimiters

o useDelimiter method of scanner: - change the separator

§ input.useDelimiter(",") : uses comma instead of space to separate

§ Ex input.useDelimiter("[,|^]"); lets | , and ^ all act as delimiters because anything inside the brackets will work as a delimiter. (This is a regular expression.)

· Exercise: sum the hours an employee works – id followed by name followed by hours until end for many workers

Write to a File

· Create a printstream object to replace "System.out" and use the same print and println methods

· System.out is a printstream object

· File f = new File ("output.out");

· PrintStream output = new PrintStream (f);

· Exercises: Copy a file all in uppercase

Prompt for filename:

· Continue asking until canread

Scanner console = new Scanner(System.in);

System.out.println("Enter file name");

File f = new File(console.nextLine());

while (!f.canRead()){

System.out.println("not a good filename. Enter another");

f = new File(console.nextLine());

}

Zip code example:

· prompt user for zip

· prompt for proximity

· find coordinates for target

· find coordinates for matching zips

o Flow chart

o gliffy

· Formula for distance given 2 lat/long

o Earth's radius * arcos(sin(lat1) sin(lat2) + cos(lat1)cos(lat2)cos(long1 – long2)

o must all be in radians not degrees (and file has degrees)

o Earth's radius is 3956.6 miles.