Learn Programming With Java

What is the world?

There are different types of software you'll need to write, from small utilities to enterprise software. Despite the size, they will rarely be stand-alone applications functioning within their own environment. More likely they will be working with local storage (a local hard drive), communicating with other applications locally, or reaching out to remote services over the network.

In this chapter, we will cover three kinds of interactions:

1. Getting input from the user

2. Fetching data on the internet

3. Writing to a file

For this, we will ask for a URL, make sure it is correct, fetch the content addressed by the URL, and write it to a file.

Get and validate input from the user

User input can be gathered either when the program is started, using command-line arguments, or by prompting the user at any point during the program execution. 

Checking for a command-line input

The following code is located inside a UserInput class, itself part of a communication package.

When user-defined elements are provided at program startup, they are made available in the  args array of the main function. Here is the code for the class with a main function that:

1. Checks whether an argument has been provided

2. Prompts the user for a URL if not

3. Displays whether the URL is formatted correctly or not

package communication;

import java.net.MalformedURLException;
import java.net.URL;
import java.util.Scanner;

/** Demo of getting and checking User input
 * 
 * @author The OpenClassrooms Education Team
 *
 */
public class UserInput {

/** Check if user has provided a URL from the command line
	 * If not prompt for one
	 * In both cases make sure the URL is well-formed!
	 * @param args either nothing or a URL string
	 */
	public static void main(String[] args) {
		String urlString="";
		
		// assign a value to urlString or exit
		switch(args.length) {
		case 0:
			urlString=askForUrl();
			break;
		case 1:
			urlString=args[0];
			break;
		default:
			System.out.println("Please run the program with the desired URL or no argument at all");
			System.exit(-1);
		}
		
		System.out.println("Checking URL " + urlString);
		if (isValidURL(urlString)) {
			System.out.println(urlString + " is a well-formed URL");
		}else {
			System.out.println(urlString + " is not a well-formed URL");			
		}
	}

The import statements exist for the functions that we will define later in this section. Let's move to the main function. Since we are making a decision based on the different possible values of an expression (the  args.length  property) we make use of a switch statement:

• If the user has not provided any argument on the command line, this length is 0.  Therefore, call the askForUrl() function and assign its result to the urlString variable.

• If the user has provided one argument, assume it is the URL she wants to provide. Therefore, directly assign this value to the urlString variable.

• If the user has provided more than one argument, you don't know what to do! Therefore, exit with the System.exit(-1); statement.

It works! Let's check the implementation of the askforUrl function.

Asking for an input

	/** Prompt user for URL
	 * 
	 * @return a string representation of the URL entered by the user 
	 */

	public static String askForUrl() {
		Scanner readInput=new Scanner(System.in);
		System.out.println("Please enter a valid URL");
		String url=readInput.nextLine(); 
		readInput.close();
		return url;
	}

This program makes use of the  Scanner class located inside the  java.util package. The import  statement import java.util.Scanner; at the beginning of the file, before the class definition, ensures that your program can access it.

First, the  Scanner readInput=new Scanner(System.in); creates an object that contains the  readLine() method and assigns it to the readInput variable.

Then, the String url=readInput.nextLine(); statement pauses the program until the user enters her input and presses the Enter key. Whatever string has been typed on the command line before pressing the key is then assigned to the url variable. 

Finally, the scanner is closed and the result sent to the calling function thanks to the return url;statement.

Let's complete this section on user input by checking the validity of the input.

Checking the validity of the provided URL

Getting user input is a terrific first step. Now there is one rule that is essential both for practical and security reasons: always validate user input.

Some users will make mistakes. Others will try to find ways around the security of your application. In order to check that the string provided by the user really represents a URL, Java offers a URL class that will throw an exception if the provided string representation is not a properly formatted URL.

In order to use this class, you need two import statements before your class declaration:

import java.net.MalformedURLException;
import java.net.URL;

Here is the code for the isValidUrl function that uses the URL class:

	/** Check if a string is a well-formed URL
	 * 
	 * @param urlString
	 * @return true if the URL is well formed, else false
	 */
	private static boolean isValidURL(String urlString) {
		try{
			new URL(urlString); 
			// we will never get here if the URL is not well-formed 
			return true;
		}catch (IOException e) {
			return false;
		}
	}

The gist of this code is the new URL(urlString); statement.

• If the URL is formatted correctly, the instantiation works and it returns true.

• If the URL is not formatted correctly, the MalformedURLException is raised. Catch it directly and return false in the catch statement.

Now that we have all the pieces, let's run the code.

Running the program

We'll use the  javac command to compile the program and run it with the java command followed by the package-prefixed name of the class and a URL as the following argument:

$ javac communication/UserInput.java
$ java communication.UserInput https://www.openclassrooms.com
Checking URL https://www.openclassrooms.com
https://www.openclassrooms.com is a well-formed URL

Alternatively, here is the result of running the program with no argument:

$ java communication.UserInput
Please enter a valid URL
$ https://www.google.com
Checking URL https://www.google.com
https://www.google.com is a well-formed URL

Sweet! Now that we are able to get some input and check that it consists of a well-formed URL, let's grab the content the URL refers to!

Get the content from a URL with networking

Networking consists of managing a request-response combination over a network, typically the internet.

Your code needs to do the following:

• Send a request to the server and wait for a response, or handle the no-network situation.

• Wait for response and process the response/or no response.

A no response situation can occur due to various reasons:

• You sent request to an incorrect URL

• The server is not available (temporarily or permanently)

• Processing of the request took too long

• … etc.

Let's get on with the request-response code. We'll need to:

1. Connect to the resource identified by the URL.

2. Fetch the content and put it in a list.

Connecting to a resource

Let's create a RemoteContent class inside the same communication package as the UserInput  class. This allows you to reuse functions created in the UserInput class. 

In the main function of our RemoteContent class, we'll prompt the user for a URL. Then we'll connect to the resource it links to. Finally, we'll fetch the content of the resource and store it in memory as a list of strings:

package communication;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLConnection;
import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

/** 
 * Demo of a connection to a remote resource
 * 
 * @author the OpenClassrooms Education Team
 *
 */
public class RemoteContent {

	/**
	 * Connect to a resource from a URL and print its content type
	 * @param args not used in this example
	 */
	public static void main(String[] args){
		// define the string representation of the URL to connect to
		 String urlString=UserInput.askForUrl();

		// connect and print the content type of the retrieved resource
		try {	
			URLConnection myURLConnection=connectFromURL(urlString);
			System.out.println("Retrieved a resource of type: " + myURLConnection.getContentType() + " from " + myURLConnection.getURL());		
			System.out.println(getContent(myURLConnection));

		}
		// manage errors if the connection fails
		catch (MalformedURLException e) {
			System.err.println("Malformed URL - " + e.getLocalizedMessage());
		}
		catch(IOException e){
			System.err.println("Cannot establish connection - "+ e.toString());
		}
	}

The first statement of the main function is String urlString=UserInput.askForUrl();. It calls the askForUrl() function from our UserInput class and assigns the user input string into the urlString variable.

We then enter a try block. This means an error encountered during the execution of the block of code will be managed by the corresponding catch statement. Networking errors inside the  try block can happen when we:

1. Connect to the resource by calling the connectFromURL function.

2. Get the content of the resource and put it in a list with the getContent function.

We can define catch blocks for two manageable exceptions:

• catch (MalformedURLException e) will deal with ill-formed URLs

• catch(IOException e) will manage errors with accessing the resource 

In this program, we simply print an informative message in case of an error. More advanced error handling may deal with retrying to connect or prompting the user for another URL.

Let's now see how the connection is handled.

Connecting to a resource identified with an URL

Here is the code for the connectFromURL function:

	/** establish a connection to a provided URL
	 * 
	 * @param urlString the string of the URL to connect to
	 * @return an active connection 
	 * @throws IOException if the connection cannot be established
	 */
	public static URLConnection connectFromURL(String urlString) throws IOException {
		URL mySite = new URL(urlString);
		URLConnection myURLConnection= mySite.openConnection(); 
		myURLConnection.connect();
		return myURLConnection;
	}

Here is a three-step process:

1. Create an instance of an URL from the provided string. As you have already seen, a MalformedURLException exception will be triggered if the URL is not properly formatted. Else, the result is assigned to a mySite variable.

2. Instantiate a URLConnection object from the mySite instance created in step one, using the  openConnection() method the URL class provides. As you can see in the documentation, this method will either:

   • Return an instance of the  URLConnection class 

   • Throw an IOException if the connection cannot be established

3. Call the connect() method from the created instance. Then return the  myURLConnection  object to the calling function.

 In the main function, we assign the returned instance of the URLConnection class to a  myURLConnection variable. Let's see how the getContent function can fetch the content by receiving this instance as an argument.

Fetch the content from the resource

In the getContent function, we create an empty list, read the content line by line, and return the list once we reach the end of the content.

/** read the resource content line by line and add each line to a list
	 * 
	 * @param resource the connected resource
	 * @return a list of lines of content
	 * @throws IOException 
	 */
	public static List<String> getContent(URLConnection resource) throws IOException {
		List<String>lines=new ArrayList<String>();
		BufferedReader in = new BufferedReader(new InputStreamReader(resource.getInputStream()));
		String line;
		while ((line = in.readLine()) != null) {
			lines.add(line);
		}
		return lines;
	}

The getContent function declaration provides us with the following information:

• It needs to be provided with a URLConnection instance as argument that will be assigned to the  resource  parameter.

• It returns a list of strings, as the   List<String> return type indicates.

• It can throw an IOException exception in case a network error happens.

Inside the function definition:

1. The first statement creates a new, empty instance of a  List<String> object that is assigned to the lines variable.

2. Then create an instance of a BufferedReader that is constructed from an InputStreamReader, itself initialized from the getInputStream() method attached to the resource variable that refers to the URLConnection instance.  This provides efficient access to the resource content.

3.  The String line; statement declares a line variable that will receive one line of content at a time before it is added to the list. 

4. Read the content of the resource line by line by calling the readLine() method of the  BufferedReader instance until nothing is returned. Inside the loop, add the content of the line to the list with the lines.add(line); statement.

5. Finally, return the completed list.

In case of an error, an IOException will be triggered, to be dealt with in the calling function. We can now run the program and see it in action.

Running the program

Let's compile and run our RemoteContent program:

$ javac communication/RemoteContent.java
$ java communication.RemoteContent
Please enter a valid URL
$ https://www.google.com
Retrieved a resource of type: text/html; charset=ISO-8859-1 from https://www.google.com
[<!doctype html><html itemscope="" itemtype="http://schema.org/WebPage" lang="fr"><head><meta content="text/html; charset=UTF-8" http-equiv="Content-Type"><meta content="/images/branding/googleg/1x/googleg_standard_color_128dp.png" itemprop="image"><title>Google</title><script 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It works! Let's complete our work by saving the content to a file for easier inspection.

Generate a text file

In this last section of this chapter, we will save our resource content into a file. We will create another class that will make use of our RemoteContent class and add the functionality. Here is the code for the class definition and the main function:

The main function is very similar to the last program. We just add a call to a function that will write to a file:

package communication;

import java.io.FileWriter;
import java.io.IOException;
import java.net.MalformedURLException;
import java.net.URLConnection;
import java.util.List;

/** 
 * Demo of writing to a text file
 * 
 * @author the OpenClassrooms Education Team
 *
 */
public class WriteFile {

	/**
	 * Connect to a resource from a URL and print its content type
	 * @param args not used in this example
	 */
	public static void main(String[] args){
		// define the string representation of the URL to connect to
		 String urlString=UserInput.askForUrl();

		// connect and print the content type of the retrieved resource
		try {	
			URLConnection myURLConnection=RemoteContent.connectFromURL(urlString);
			List<String> content= RemoteContent.getContent(myURLConnection);
			writeToFile(content, "output.txt");

		}
		// manage errors if the connection fails
		catch (MalformedURLException e) {
			System.err.println("Malformed URL - " + e.getLocalizedMessage());
		}
		catch(IOException e){
			System.err.println("Cannot establish connection - "+ e.toString());
		}
	}

The file writing takes place if the connection works, thanks to a call to the writeToFile method. Here is the code:

	/** writes the content of a list of strings to a file
	 * 
	 * @param content the list of strings
	 * @param fileName the name of the output file
	 */
	public static void writeToFile(List<String> content, String fileName) {
		Path pathToFile= Paths.get(fileName);
		try (BufferedWriter writer = Files.newBufferedWriter(pathToFile)){
			for(String line: content) {
				  writer.write(line);
			}
			System.out.println("Content written to " + pathToFile  );
		} catch (IOException e) {
			System.out.println("Could not write file to " + pathToFile);
			e.printStackTrace();
		} 
	}

All the file management is done through the Files.newBufferedWriter method  that creates an instance of a BufferedWriter. Each call to the write method of that object adds a line to the file given as argument.

After running this code, the content of a valid and reachable URL should be available to you in the generated output.txt file.

Summary

In this chapter, you've learned the following:

• Networking is managing a request-response combination over the network which is typically the internet. Networking implies working with URLs.

• In Java, the following components are used to implement the URL connection:

  • URL  - the class used to define a URL object

  • URLConnection  - a class with methods related to URLs

  • The BufferedReader and BufferedWriter classes provide an efficient way to read and write to external resources