CSC 121 - Chapter 9 Lab - Search Strings and URLs

Lab 09 Objectives

1.      To practice with String: indexOf (str, start), indexOf(str), substring(start), substring(start, end), and
         BufferedReader: readLine() JDK API
2.      To utilize the java.net.URL and java.io classes URL and InputStreamReader.
3.      To build debugging skills by utilizing System.out in loops and catch blocks.
4.      To practice with adding images to JLabel.in a JFrame and working with menus.

In this lab you will view the source code for one or more web pages, discern points of interest, open an input stream to the target URLs, have your program search the source code for the item of interest and return a string with which to build a more specific URL, then display the desired target in your JFrame by adding the target image to a JLabel.

Part I
Define a Comics class that extends JFrame and implements ActionListener. Add a menu with menu items "Dilbert" and "Foxtrot" and a method todaysDilbert. In the actionPerformed method, if the event is an instance of JMenuItem and the action command is "Dilbert", then call the method todaysDilbert which returns a string with which to build the target URL. Create an ImageIcon made from the target URL. Use this ImageIcon to make a JLabel. The size of the JLabel should be determined by the image icon's height and width. Remove all the components from the content pane, then add the newly created JLabel and call repaint(). The following discussion describes a procedure for identifying the target URL in the method todaysDilbert.

Open the Dilbert web page, http://www.dilbert.com, and view the source code. Look for a target of interest.
Then determine the required searchStr and offset with which to build a string to generate a more specific URL. Right click on the image of interest, select properties, and copy that portion of the file name that follows www.dilbert.com. Look at the source code (View -> source) and use control-f to search for the file name in the html source code. In particular look for the html tag <IMG SRC=...> that contains the relative address of the image. Now that you have found the image in the html source code, write a program to automate building a search string based on the directory the comic is stored in. For example, the properties of the image reveals a typical address on a weekend is
"http://www.dilbert.com/comics/dilbert/archive/images/dilbert2004090116225.jpg" and on a weekday is
"http://www.dilbert.com/comics/dilbert/archive/images/dilbert2813550040906.gif".
Our goal¹ is to break this down into three parts -concatenate these parts to recreate the complete address (the target URL):

the url (http://www.dilbert.com),
the search string ("<IMG SRC=\"/comics/dilbert/archive/images/dilbert"), and
the id (2004090116225.jpg or 2813550040906.gif).

todaysDilbert:

    //instantiate a URL named url with the dilbert web address
    //instantiate a BufferedReader wrapped around a new InputStreamReader using url.openStream() as an argument
    //repeatedly store lines of text in a string using bufferedReader.readLine()
    //while this string is not null: //if the indexOf searchStr is greater than 0 in that line
    //assign to offset this index value
    //assign to id, the substring from offset + the number of characters in the searchStr, to the indexOf the closing " following the file type designation
              //be careful here as the to indexOf should begin the search in the line following the end of the search string (otherwise the wrong " is identified)
    //return the concatenation of the url, searchStr and id   Troubleshooting aide: temporarily print this value

Part II
http://foxtrot.com
Visit the foxtrot homepage, right click on the image and select properties. Note that the image is stored on a different computer. We will use a more dirrect approach to build the target URL. The SimpleDateFormat object's format method can be set to format a Date object in any manner you desire.
Date today = new Date ();
SimpleDateFormat dfmt = new SimpleDateFormat ("yyMMdd");
Notice the construction of the id portion of Foxtrot. In the actionPerformed method, if the instance of JMenuItem has action command "Foxtrot", build the URL based on the concatenation of "http://images.ucomics.com/comics/ft/2004/ft" and today's formatted date, and .gif. Use this string to build the target URL. Create an ImageIcon made from the target URL. Use this ImageIcon to make a JLabel. The size of the JLabel should be determined by the image icon's height and width. Remove all the components from the content pane, then add the newly created JLabel and call repaint().

The following method (optional) will print the image to a file:

public static void download(URL url, File file) throws IOException {
    InputStream in = url.openStream();
    FileOutputStream out = new FileOutputStream(file);
    byte[] b = new byte[1024];
    int len;
    while((len = in.read(b)) != -1) {
        out.write(b, 0, len);
    }
    out.close();
}

Email your source code as an attachment to either CSC221.001@uncw.edu or CSC221.002@uncw.edu

1. The goal of the search string algorithm is to read the dilbert.com html code one line at a time to find the line that loads the image into the webpage using the IMG tag. This line contains <IMG SRC="... .gif" or .jpg". Once this line is found, the pertinent pieces of the address begin in the search string at the tenth character in this string and the id begins following the search string and ends at the character before the closing double quotes in the IMG tag. The trick is to limit the indexOf method's search to only the pertinent portions of the line (certainly there are multiple occurrences of double quotes in this line.) This is accomplished, for the id, by using the sum of the offset and the search string's length as a starting point for the search.