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) " );
  }

Really, a Scanner is an object who can group individual characters into meaningful units (words, doubles, etc.).

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...

`nextInt` vs. `nextLine`

Unintutively, consider:

import java.util.Scanner;
Scanner s1 = new Scanner( "line 1  \n   lion 2  \n    lyin' 3\n Last line; bye!" );
s1.next()              // returns "line"
s1.nextInt()           // returns 1

s1.next()              // returns "lion"
s1.nextInt()           // returns 2
s1.nextLine()          // returns "  " -- I guess that makes sense.

s1.next()              // returns "lyin'"
s1.nextInt()           // returns 3
s1.nextLine()          // returns "" -- beware!

In general, when you intermingle calls to nextLine and next/nextInt/etc., be very careful!

Alternate strategy:

Scanner s2 = new Scanner( "line 1  \n   lion 2  \n    lyin' 3\n Last line; bye!" );

String entireLine = s2.nextLine();
Scanner scanAssistant = new Scanner( entireLine );
scanAssistant.next();    // returns "line"
scanAssistant.nextInt(); // returns 1

String entireLine = s2.nextLine();
Scanner scanAssistant = new Scanner( entireLine );
scanAssistant.next();    // returns "lion"
scanAssistant.nextInt(); // returns 2

String entireLine = s2.nextLine();
Scanner scanAssistant = new Scanner( entireLine );
scanAssistant.next();    // returns "lyin'"
scanAssistant.nextInt(); // returns 3

¹ In this case, if you try it out, you realize that web pages are written in html; in html the notion of the next word is different from where the next whitespace occurs. Indeed, people have written specialized classes which extend Scanner to so that the next method returns the next word in a way that is appropriate for html. ↩

This page licensed CC-BY 4.0 Ian Barland
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Reading from a file

reading files and web pages

nextInt vs. nextLine

`nextInt` vs. `nextLine`