ITEC 120 2012fall dbraffitt ibarland

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lab11b
Reading from a file

reading files and web pages

We've seen we can use java.util.Scanners to read input from the keyboard:

java.util.Scanner scnr = new java.util.Scanner( System.in );
System.out.print("Shout! (or ^C to indicate end-of-file) ");
while (scnr.hasNext()) {
  String msg = scnr.next();
  System.out.println( msg + ", echo, echo..." );
  System.out.print( "Shout more! (or ^C to indicate end-of-file) " );
  }

But a Scanner can read information from any source of characters: they can read information from a file, a web page, or even a String inside your program. When you call the constructor, you just pass in a source of characters.

• Read information from a file on the disk.
  We'll demo this in Code Pad:

  // Any (existing) filename will do: s = new java.util.Scanner( new java.io.File( "H:/itec120/lab02a/PizzaServer.java" ) ); s.next() s.next() s.next() s.next()

  However, to include this in a method, there's one caveat: We need to tell people using our method that it's possible that we could trigger a "file not found" error. We do so by including the words “throws java.io.FileNotFoundException” in between the signature and the method body:

  class Foo { public static void FileScanDemo() throws java.io.FileNotFoundException { scnr = new java.util.Scanner( new java.io.File("/etc/motd") ); while (scnr.hasNext()) { String msg = scnr.next(); System.out.println( msg + ", echo, echo..." ); } } }

• Read from a file across the web.
  Try this in Code Pad:
  s = new java.util.Scanner( new java.net.URL( "http://google.com" ).openStream() ) ¹

  In order to use java.net.URL and its method openStream inside a method of your own, you have to announce that errors might occur -- “you must declare which exceptions your method might throw”. We won't talk about exceptions in this course, but here's a working function which reads a web file:

  1 /** Open up a URL, and read the input. 2 * As a side-effect (for demonstration only), print each line to the console. 3 * @return The number of lines in a given URL. 4 */ 5 public static int countLines() throws java.net.MalformedURLException, java.io.IOException { 6 final String sourcePage = 7 "http://www.radford.edu/~itec120/2010fall-ibarland/" 8 // "http://www.radford.edu" 9 // "file:///Users/ian/120/index.html" 10 ; 11 12 // Open sourcePage for input: (Create a URL object based on sourcePage, open that URL as a stream-of-characters, 13 // and create a Scanner who will group that stream's characters into Strings etc.) 14 java.util.Scanner src = new java.util.Scanner( new java.net.URL( sourcePage ).openStream() ); 15 16 int lineCount = 0; // Keep track of how many lines we've seen. 17 String pageSoFar = ""; // All the lines of text seen so far. 18 19 while ( src.hasNext() ) { 20 pageSoFar = pageSoFar + src.nextLine(); 21 lineCount = lineCount + 1; 22 } 23 24 System.out.println( "We read:\n" + pageSoFar ); 25 return lineCount; 26 }

• Read information from a String.
  Try this in Code Pad:

  java.util.Scanner s3; s3 = new java.util.Scanner( "4 score and 7 years" ); s3.hasNextInt() // If we try reading an int next, will it succeed? s3.nextInt() s3.hasNextInt() // If we try reading an int next, will it succeed? s3.next() s3.next() s3.hasNextInt() s3.nextInt() s3.hasNext() // If we try reading anything next, will it succeed? s3.next() s3.hasNext() s3.next() // Try reading anyway...

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©2012, Ian Barland, Radford University Last modified 2012.Nov.08 (Thu)

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