Reading Interactive Data

1. Reading Interactive Data • We can use javax.swing package and JOptionPane class • JOptionPane class has a method call showInputDialog that takes a string (as prompt) and allows the user to enter a string • Eg: s1 = JOptionPane.showInputDialog("Enter an integer"); • S1 is a string that can now be used.

2. Reading Interactive Data - an example import javax.swing.JOptionPane; public class Example1 { public static void main(String[] args) { String s1, s2; int n1, n2, max; // prompt for integers s1 = JOptionPane.showInputDialog("Enter an integer"); s2 = JOptionPane.showInputDialog("Enter another integer"); // convert these strings to integers n1 = Integer.parseInt(s1); n2 = Integer.parseInt(s2); // find the maximum max = Math.max(n1,n2); // print the message and the max value JOptionPane.showMessageDialog(null, "The biggest is " + max); System.exit(0); } }

3. Disk Files • Advantages • Persistence: data in a file lasts longer than data on the monitor • Capacity: more information can be stored on a disk than displayed at one time on a monitor • Attributes • Contents (data) • A file name • Operations • Writing to a file (creating and storing data for the first time) • Deleting a file • Renaming a file • Overwriting a file • Obtaining contents of a file

4. Modeling disk files • Java provides a predefined class to model disk files, called File • Constructor for the file class • Accepts the files name (a String reference) as the argument • Example: File f1 = new File(“myData.txt”); • IMPORTANT: • We have created a reference to a file object • This does not create a file on the disk • If this file is not on the disk, we cannot do anything with this object

5. If the file is on the disk • The File object provides two methods that model some of the operations we listed earlier: • delete() File f = new File(“junk.txt”); f.delete(); • renameTo() File f1, f2; f1 = new File(“junk.txt”); f2 = new File(“garbage.txt”); f1.renameTo(f2); Download FileStuff.zip demo

6. Writing output to disk • To create a new file, or overwrite an existing file, we need a pathway, or stream • Java has a predefined class to model this stream, FileOutputStream • The constructor for FileOutputStream uses a reference to a File object as its argument File f = new File(“stuff.txt”); FileOutputStream fstream = new fileOutputStream(f); • Opens the file so that it can receive data • Creates a new file if the file does not exist • However, it does not provide any convenient methods for output

7. Using PrintStream methods for disk files • We want a PrintStream object to be associated with a disk file and not a monitor • The PrintStream constructor takes a reference to a FileOutputStream as its argument • The PrintStream object is associated with the FileOutPutStream • Using println() and print() methods on this PrintStream object means that output will go to the disk file File f = new File(“stuff.txt”); FileOutputStream fstream = new FileOutputStream(f); PrintStream target = new PrintStream(fstream); target.println(“This is a new disk file.”);

8. Catching exceptions (first pass) • In Karel we had runtime errors that would halt program flow • Try to pick a beeper when none were there • To to move through a wall that was in the way • A similar type of problem may be trying to access a file that is unavailable • We will add the line throws Exception to the heading of the main() method of our program class public static void main(String args[]) throws Exception {

9. Throwing exceptions and methods • Any method that can throw an exception must acknowledge it in the header with a throws clause • A throws clause consists of the keyword throws and a list of all Exceptions thrown in the method • Example public static void main(String args[]) throws Exception {

10. Throwing Exceptions • To handle the unexpected, Java uses the throw statement throw reference • Reference is an object of a subclass of the class Exception • See the API and the Java Interlude on page ??? of the text • The throw statement usually takes the following form throw new Exception-class(String-argument) • This is called throwing an exception

11. Handling the unexpected • By default the throws statement launches a chain of method terminations from the method that executes the throw to the main() method, terminating the program itself • Often the program can respond to the unexpected and recover, by catching the Exception • Breaking the cascade of method throwing along the invocation chain

12. try … catch() { } • To catch an Exception, the statements containing invocations of Exception-throwing methods are surrounded by braces and preceded with the keyword try try { someObject.someMethod(); … } • Following is thekeyword catch and a declaration of an reference variable that will refer to the thrown Exception object in parentheses catch (Exception e) { statements } • Finally comes a group of statements surrounded by braces that will be executed when the Exception reaches this method

13. Implementing a try…catch in file output • We could also implement the try…catch mechanism in the following way File f = new File("stuff.txt"); try { FileOutputStream fstream = new FileOutputStream(f); PrintStream target = new PrintStream(fstream); target.println("This is a new disk file."); } catch (FileNotFoundException e) { System.out.println("File not found."); } • We will not worry about the details of this implementation at this time

14. Creating or Overwriting a File: Summary • Create a File object to represent the file • Create a FileOutputStream object to represent the pathway to the file (using the File reference as an argument) • Create a PrintStream object to provide a convenient output pathway to the file (using the FileOutputStream reference as an argument) • Use print() or println() methods of PrintStream as needed • Be sure to watch for exceptions appropriately

15. Input from keyboard, file, or network connection • Sequence of steps similar to output • Construct an object that models some sort of input stream • FileInputStream • BufferedInputStream • Use the input stream object to construct an InputStreamReader object • Models stream of input as a stream of characters • Does not recognize boundaries between Strings in the input • Use the InputStreamReader object to construct a BufferedReader object • Provides method readLine() reads an entire line, creates a String object, and returns a reference to that object

16. Input: the keyboard • Java provides a predefined BufferedInputStream object to represent a stream of input that comes from the keyboard, System.in • Unlike System.out, System.in cannot be used right away (different classes) • System.in must be used to construct an InputStreamReader object • Which is used as an argument to construct a BufferedReader object • Which can then be sent readLine() messages to read lines as Strings

17. Getting a line from the keyboard InputStreamReader isr; BufferedReader keyBoard; String inputLine; isr = new InputStreamReader(System.in); keyBoard = new BufferedReader(isr); inputLine = keyBoard.readLine(); System.out.print(inputLine); System.out.println("s");

18. Interactive Input/Output • A program that expects input from a keyboard, must provide that information to the user as it runs • A general form for interactive input and output: System.out.print(prompt goes here); string reference variable = keyboard.readLine() possibly compute something System.out.println(output string goes here);

19. Input: Disk Files • Only slightly more involved that obtaining input from a keyboard • Instead of BufferedInputStream object (System.in), we need a FileInputStream object • Construct a FileInputStream object the same as constructing a FileOutputStream object File f = new File(“myData.txt”); FileInputStream fs = new FileInputStream(f); • Now we can use the FileInputStream object as we used System.in in getting data from the keyboard

20. Getting a line from a disk file File f = new File(“myData.txt”); FileInputStream fs = new FileInputStream(f); InputStreamReader isr; BufferedReader input; isr = new InputStreamReader(fs); input = new BufferedReader(isr); String inputLine; inputLine = input.readLine(); System.out.println(inputLine);

21. Network Computing • Network concepts • Computer network: group of computers that can directly exchange information with each other • Internet: a computer network of networks. Allows computers on one network to exchange information with another computer on a different network • THE Internet • Each computer has it’s own unique Internet address • Internet addresses on the Net are Strings • Information on the Net is organized into units called network resources (usually a file of some type) • Each resource is uniquely identified by its URL (Universal Resource Locator) • Form of URL: protocol://internet address/file name

22. Network Input • Reading input from the World Wide Web is as simple as reading data from a disk file • Obtain an InputStream that models a stream of input from a Web site • Class URL provided by Java to model URLs • The constructor for the URL class takes a String argument URL u = new URL(“http://www.whitehouse.gov”); • It also provides a method, openStream, that takes no arguments, but returns an InputStream InputStream ins = u.openStream();

23. openStream() • Does a lot of work behind the scenes • Sets up communication software on your machine • Contacts the remote machine • Waits for response • Sets up connection • Constructs an InputStream object to model the connection • Returns a reference to this object • We then construct a BufferedReader as we did for disk files and keyboards Download WHWWW.zip demo