Goals:
- understand these terms:
- network
- encoding
- protocol
- create and manipulate URLs
- retrieve data via HTTP
Important: Please fork this repo and then clone your fork.
In Java, you can encode a char as an int simply by casting it.
int encoded = (int) 'x';Likewise, to decode a character, simply cast it back.
char decoded = (char) 120;Note: Java uses a text encoding named Unicode, which is a superset of ASCII. It encoded values are the same for English letters, digits, and punctuation, but characters from other languages have encodings too.
Exercises:
- Write a method that determines whether a character is an uppercase letter.
public static boolean isUppercaseLetter(char c) { // Write me. }
Write a method that determines whether a character is a lowercase letter.
Rot-13 is a (very simple) cryptographic cipher: a code for transferring text or other information in such a way that the contents are understandable only to intended recipient. It works by rotating each letter 13 positions forward in the alphabet.
For example, A becomes N, B becomes O, C becomes P, etc. The alphabet wraps around from Z to A, so, for example, X becomes K. Lower letters work similarly. All other characters are unchanged.
Write a method that encodes a string in rot-13.
public static String rot13Encode(String string) { // Write me. }
Write
rot13Decode(), which decodes the cipher.Generalize your methods to rotate any number of positions through the alphabet, not just 13.
Use the HTTP.stringToUrl(string) method to convert a string to a java.net.URL object. We've provided you this "factory method" because it's a bit easier to use than the standard constructors for the URL class.
See: http://docs.oracle.com/javase/7/docs/api/java/net/URL.html
Exercises:
- Write a method that constructs an HTTP URL from the host name, port number, and path. Include the port number only if it is not 80.
public static URL makeHttpUrl(String host, int port, String path) { // Write me. }
Using overloaded methods, write a similar method that doesn't take a port number, and assumes the port number is 80.
Write a method that neatly prints out the parts of a URL, in the correct order.
We've provided a method HTTP.get(url) method to connect to a web server, request the correct path, and return you the data. The argument is a URL object and the return value is a String. You don't have to worry about the protocol at all. Just pass it a URL object, and it will return to you a string (unless somethign went wrong).
Exercises:
Write a program that asks for a URL from the user. Retrieve the document via HTTP and print them out. If the URL is invalid, print an error message stating that.
Write a program that asks for a URL and retrieves the document. Guess whether the document is HTML by checking whether it begins with the string
<!doctype html>(case-insensitive). Print out whether the document is HTML.Test your program on,
- http://www.google.com/
- http://espn.go.com/
- http://canitbesaturdaynow.com/images/fpics/355/04_06_09__-(89).jpg
- http://www.gutenberg.org/cache/epub/1251/pg1251.txt
- Write a program that asks for a URL and a word. The program retrieves the document and counts the number of times the word occurs in that document.
For example, you could use it to retrieve Alice in Wonderland from Project Gutenberg and count how many times the word "Alice" appears.
enter URL: http://www.gutenberg.org/cache/epub/11/pg11.txt enter word: Alice That word occurs 403 times in the document.
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Project Gutenberg is a web site that provides thousands of free public domain ebooks. They are available in a variety of formats, including "plain text", which is easy for computers to process.