cs61b l02 using objects (1)garcia / yelick fall 2003 © ucb kathy yelick handout for today: these...

CS61B L02 Using Objects (1) Garcia / Yelick Fall 2003 © UCB

Kathy Yelick

Handout for today: These lecture notes

Computer Science 61BLecture 2 – Using Objects

2003-08-27


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


How Long Does It Take?•Factoid: 23% of web pages change daily•How long does it take to crawl the full web?a) < 2 minutesb) < 2 hoursc) < 2 daysd) < 2 weeks

•How long did it take for the 61B web page changes to show up in Google’s own copy?a) < 2 minutesb) < 2 hoursc) < 2 daysd) < 2 weeks


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)


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”


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


Variables and Objects• Declare a variable

String s1;

s1

• Create an object by calling a constructor

s1 = new String();

“”

• Do both togethers2

String s2 = new String(“white”);

“white”

• Assign one to another (swing pointer)

s1 = s2;

X


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


Equality on Strings

• Two notions of equality: == and .equals

Same object Different 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)


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?


Scheme vs. Java: DifferencesScheme Java

PunctuationComments ; // or /*

*/Separators

( ) ,;.{ } ( )

Arithmetic prefix infixAssignment

set! =

TypesImplicit:list, atom, function

Explicitint, char, boolean, …


Scheme to JavaScheme Java

Kinds of ValuesPrimitive atom int, char,

boolean …Non-primitive built-in

Lists String,URL,…

Non-primitive user defined

Trees, etc. NewTree,…

Invoking FunctionsWith objects (ask o ‘m arg …) o.m( arg …)

Without objects

(f arg …) f( arg …)


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


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/–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


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


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, …)


Code for Accessing the Webclass 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()); }}


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!

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