1. CS61B L02 Using Objects (1)Garcia / Yelick Fall 2003 © UCB Kathy Yelick Handout for today: These lecture notes Computer Science 61B Lecture 2 – Using Objects 2003-08-27

2. CS61B L02 Using Objects (2)Garcia / Yelick Fall 2003 © UCB Design Problem Revisited Want to build a web search engine like Google How can we break this into smaller problems? –Design a data structure to store keyword/urls –Design interface to respond to keyword queries –Build a web crawler to collect words on each page »Open a connection »Process strings on that page Find title Eliminate html junk Eliminate useless words (the, and, is, …) »Save results in some files –And much more

3. CS61B L02 Using Objects (3)Garcia / Yelick Fall 2003 © UCB How Long Does It Take? Factoid: 23% of web pages change daily How long does it take to crawl the full web? a)< 2 minutes b)< 2 hours c)< 2 days d)< 2 weeks How long did it take for the 61B web page changes to show up in Google’s own copy? a)< 2 minutes b)< 2 hours c)< 2 days d)< 2 weeks

4. CS61B L02 Using Objects (4)Garcia / Yelick Fall 2003 © UCB Design by Wishful Thinking Approach a problem using top-down design –Break it into pieces –Assume you can build most of them –Concentrate on one at a time –When you’re sure you can build one, go onto the next Rule: don’t write any code until you know how to build the pieces –We won’t follow this rule in lecture Three parts of Java we’ll need in this design: –Strings, Web connections, Files (later)

5. CS61B L02 Using Objects (5)Garcia / Yelick Fall 2003 © UCB Strings are Objects Strings are objects: What does that mean? –We can give them a name that will refer to the object –We can send them messages to have them do things »Tell me your length »What is your first character? »In Java these are called methods: ~40 on strings –It belongs to a class (the “factory” that created it) »We also call this the type of the object »All string objects are of type String s “Hello World”

6. CS61B L02 Using Objects (6)Garcia / Yelick Fall 2003 © UCB Box and Pointer Diagrams To understand complicated data structures draw pictures! A variable is shown as a box with a name –A variable may contain a primitive value (3, 2.5, etc.) –Or it may contain a reference to an object »Drawn as a pointer »The variable “refers to” or “points to” the object An object is also a box –So far we have seen only string objects –We will see that there may be variables inside objects s

7. CS61B L02 Using Objects (7)Garcia / Yelick Fall 2003 © UCB Variables and Objects Declare a variable String s1; s1 Create an object by calling a constructor s1 = new String(); “” Do both together s2 String s2 = new String(“white”); “white” Assign one to another (swing pointer) s1 = s2; X

8. CS61B L02 Using Objects (8)Garcia / Yelick Fall 2003 © UCB Invoking Methods on Strings Can invoke a string method using “.” Strings are immutable: cannot modify string objects Setting one does not affect the other s1 “white” s2 s2 = s2.concat(“house"); x “whitehouse” s2 = s2.substring(0,5); “white” x Wait: “house” is a String object, but it was not created with “new String…” Why? Because Strings are special

9. CS61B L02 Using Objects (9)Garcia / Yelick Fall 2003 © UCB Equality on Strings Two notions of equality: == and.equals Same objectDifferent object, same value s1 “white” s2 s1 “white” s2 “white” (s1 == s2) is true s1.equals(s2) is true (s1 == s2) is false s1.equals(s2) is true s2 = s1 s2 = new String(s1)

10. CS61B L02 Using Objects (10)Garcia / Yelick Fall 2003 © UCB Printing Strings To output strings, use “println” –Can concatenate them using “+” –Can also use + on string with another type System.out.println("s1 is: " + s1); System.out.println("the length of s1 is " + s1.length()); System.out.println(s1 + s2 + s1); Do you feel you know enough about strings in Java to write a program that finds keywords in a string representing a line on a web page?

11. CS61B L02 Using Objects (11)Garcia / Yelick Fall 2003 © UCB Scheme vs. Java: Differences SchemeJava Punctuation Comments ; // or /* */ Separators ( ),;.{ } ( ) Arithmeticprefixinfix Assignment set!= Types Implicit: list, atom, function Explicit int, char, boolean, …

12. CS61B L02 Using Objects (12)Garcia / Yelick Fall 2003 © UCB Scheme to Java SchemeJava Kinds of Values Primitive atom int, char, boolean … Non-primitive built-in ListsString,URL,… Non-primitive user defined Trees, etc. NewTree,… Invoking Functions With objects (ask o ‘m arg …)o.m( arg …) Without objects (f arg …)f( arg …)

13. CS61B L02 Using Objects (13)Garcia / Yelick Fall 2003 © UCB Announcements Lab 1 and homework 1 are online –Homework 1 is due at 11:59 Tuesday night –Homeworks will be auto-graded Textbooks were at Ned’s yesterday –Reading assignments online (do not need to know html) The CS61B Course Reader –Available at Copy Central on Euclid/Hearst –Contains “Scheme to Java”, tool documentation, etc. Labs and discussions –You must attend your assigned lab (change through Telebear if you must) –You may attend a different discussion, although the one that matches your lab has the same TA

14. CS61B L02 Using Objects (14)Garcia / Yelick Fall 2003 © UCB Input/Output (I/O) in Java To build a web crawler we need: Read from files across the web. We’ve seen one simple form of output –System.out.println(“some string”); –System.out is a PrintStream that prints to the screen What about input? Does System have anything? –http://java.sun.com/j2se/1.4.1/docs/api/http://java.sun.com/j2se/1.4.1/docs/api/ –System.in is an InputStream from the keyboard. –And there’s a URL class with an openStream method, which creates an InputStream to the web –But you can’t read a line of text from an InputStream

15. CS61B L02 Using Objects (15)Garcia / Yelick Fall 2003 © UCB Input/Output (I/O) in Java We know how to construct an InputStream An InputStreamReader is one kind of Reader We want to construct a BufferedReader Need to have a Reader to call the constructor To construct an InputStreamReader we can use an InputStream

16. CS61B L02 Using Objects (16)Garcia / Yelick Fall 2003 © UCB Why So Complicated? InputStream objects (like System.in) –read raw data, but don't format the data. InputStreamReader objects –compose the raw data into characters. BufferedReader objects –compose the characters into lines of text. Why are these tasks divided among three different classes? –So one can reimplement part (for efficiency) –Can reuse some parts of code for different kinds of I/O (file, network, user, …)

17. CS61B L02 Using Objects (17)Garcia / Yelick Fall 2003 © UCB Code for Accessing the Web class WHWWW { public static void main(String[] arg) throws Exception { URL u = new URL("http://www.whitehouse.gov/"); InputStream ins = u.openStream(); InputStreamReader isr = new InputStreamReader(ins); BufferedReader whiteHouse = new BufferedReader(isr); System.out.println(whiteHouse.readLine()); }

18. CS61B L02 Using Objects (18)Garcia / Yelick Fall 2003 © UCB Summary Break large designs into pieces –Top-down design –Concentrate on one piece at a time Java and Scheme –Many syntactic differences –Many semantic similarities Strings and I/O Streams –Java Strings and Streams are objects –Use the online reference to the Java library Understanding objects –Draw pictures!