Programming with Exceptions In this part of the lab, you will write a program th

Question

Programming with Exceptions

In this part of the lab, you will write a program that fetches the information stored at a give URL on the web and saves that data to a file. This will also include networking and file operations and partly an exercise in using exceptions.

For doing I/O, Java has a pair of nice abstractions: InputStream and OutputStream. These are abstract classes in the package java.io. An InputStream is a place from which you can read data; an OutputStream is a place to which you can write data. For this lab, you will use an InputStream to represent the data read from the Web URL, and you will use an OutputStream to represent the file where you want to save a copy of the data. Once you have the streams, the data can be copied just by calling the following method, which you can copy into your program:

private static void copyStream(InputStream in, OutputStream out)
throws IOException {
int> while (oneByte >= 0) { // negative value indicates end-of-stream
out.write(oneByte);
> }
}

Aside from this method, you should have a main routine that does the following:

>Declare variables to represent the InputStream and the OutputStream. It would be a good idea to initialize them to null to avoid uninitialized variable errors.

>Read the URL and the file name as strings from the user.

>To connect to the web, you need a variable -- say url -- of type URL (from package java.io). You can create the URL object with the constructor call url = new URL(urlString), where urlString is the string provided by the user. This constructor will throw a MalformedURLException if the string is not a legal URL. (Note: the string must be a complete URL, beginning with "http://".)

>To get the input stream, you can simply call url.openStream(), which returns a value of type InputStream. This can throw an IOException, for example, if the web address that you are asking for does not exist.

>To get the output stream, you can use the constructor new FileOutputStream(fileName), where fileName is the file name that was input by the user. This can throw a FileNotFoundException if it is not possible to open the specified file for reading (for example, if the user is trying to create a new file in a directory where they don't have write permission). Warning: If a file of the same name already exists, the old file will be erased and replaced by the new one, without giving the user any notice!

>Now, copy the data from the web into the file by calling the above method. Note that this can throw an IOException.

>Finally, use a finally to clause to make sure that both streams are closed (if they were successfully opened). Both InputStream and OutputStream have a close() method for closing the stream. Note that you can test whether the stream was opened by testing whether the value of the variable is still null.

>Note that an exception should not crash your program. You should catch the exception and print out a reasonable error message before ending the program. It would be nice if the error message depends on the type of error that occurred (which means using several catch clauses).

>Please only use the given packages: java.io. , java.util.Scanner , TextIO.java

Explanation / Answer

Hello there,

I have written whole code as required in problem statement.

Just to note that. The URL class and its related Exception classes are not in java.io , instead they are in java.net package. So I needed to import that.

Program provided by me will not crash on getting exceptions. I have handled most of the exceptions that were asked in problem statement and also those which I found neccessary.

####################################################################

Following is the code:

Q11.java=>

import java.io.*;
import java.util.Scanner;

/* Need to import following two packages for URL class and
* MalformedURLException class which are not in io
*/
import java.net.URL;
import java.net.MalformedURLException;
import java.net.UnknownHostException;

public class Q11 {

String url=null;
String filename=null;
  
public Q11(String url,String filename) {
this.url=url;
this.filename=filename;
}

private static void copyStream(InputStream in, OutputStream out)
throws IOException {
  
int> while(oneByte>=0) {
out.write(oneByte);
> }
}
  
public static void main(String[] args) throws IOException {
  
Scanner sc= new Scanner(System.in);
System.out.println("Enter url and filename [both on separate lines]");
Q11 obj = new Q11(sc.nextLine(),sc.nextLine());
InputStream urlIn = null;
OutputStream fileOut = null;
  
try {
URL urlObj = new URL(obj.url);
urlIn = urlObj.openStream();
fileOut = new FileOutputStream(obj.filename);
Q11.copyStream(urlIn, fileOut);
}
  
catch(MalformedURLException e) {
System.out.println("Invalid URL: "+e);
main(null);
}
catch(UnknownHostException e){
System.out.println("Unknown Host: "+e);
main(null);
}
catch(IllegalArgumentException e){
System.out.println("Illegal url: "+e);
main(null);
}
catch(FileNotFoundException e) {
System.out.println("File opening failed: "+e);
main(null);
}
catch(IOException e) {
System.out.println("IO failed: "+e);
main(null);
}
finally {
if(urlIn!=null)
urlIn.close();
if(fileOut!=null)
fileOut.close();
}
  
}
}

################################################################

Feel free to ask queries and modifications in solution, If required I will give you updated code also.

Afterall I am here to help you!!!

:)

Thank you!!!

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