External Interfaces

Example - Reading a URL

This program, URLdemo, opens a connection to a web site specified by a URL (Uniform Resource Locator), for the purpose of reading text from a file at that site. It constructs an object of the Java API class, java.net.URL, which enables convenient handling of URLs. Then, it calls a method on the URL object, to open a connection.

To read and display the lines of text at the site, URLdemo uses classes from the Java I/O package java.io. It creates an InputStreamReader object, and then uses that object to construct a BufferedReader object. Finally, it calls a method on the BufferedReader object to read the specified number of lines from the site.

Description of URLdemo

The major tasks performed by URLdemo are:

1. Construct a URL Object

The example first calls a constructor on java.net.URL and assigns the resulting object to variable url. The URL constructor takes a single argument, the name of the URL to be accessed, as a string. The constructor checks whether the input URL has a valid form.

• url = java.net.URL(...
    'http://www.mathworks.com/support/tech-notes/1100/1109.shtml')
  

2. Open a Connection to the URL

The second statement of the example calls the method, openStream, on the URL object url, to establish a connection with the web site named by the object. The method returns an InputStream object to variable, is, for reading bytes from the site.

• is = openStream(url)
  

3. Set Up a Buffered Stream Reader

The next two lines create a buffered stream reader for characters. The java.io.InputStreamReader constructor is called with the input stream is, to return to variable isr an object that can read characters. Then, the java.io.BufferedReader constructor is called with isr, to return a BufferedReader object to variable br. A buffered reader provides for efficient reading of characters, arrays, and lines.

• isr = java.io.InputStreamReader(is)
  br = java.io.BufferedReader(isr)
  

4. Read and Display Lines of Text

The final statements read the initial lines of HTML text from the site, displaying only the first 4 lines that contain meaningful text. Within the MATLAB for statements, the BufferedReader method readLine reads each line of text (terminated by a return and/or line feed character) from the site.

• for k = 1:108           % Skip initial HTML formatting lines
     s = readLine(br);
  end
  
  for k = 1:4             % Read the first 4 lines of text
     s = readLine(br)
  end
  

Running the Example

When you run this example, you see output similar to the following.

• url =
  http://www.mathworks.com/support/tech-notes/1100/1109.shtml
  is =
  ice.net.CachedInputStream@7e700c
  isr =
  java.io.InputStreamReader@7e6696
  br =
  java.io.BufferedReader@7e5d1b
  s =
        <p class="standard">This technical note provides an 
  introduction to vectorization techniques.
  s =
        In order to understand some of the tricks available, an 
  introduction to
  s =
        MATLAB indexing is provided. Then several vectorization 
  techniques are
  s =
        discussed, in order of simplest to most complicated.
  

Introduction to Programming Examples

Example - Finding an Internet Protocol Address

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