1	CS110 Lecture 4 Thursday, February 5, 2004 Announcements hw2 due Thursday, February 12, when labs close Read Chapter 2 of JOI Questions (please!) Agenda where does the world begin? tokens, keywords and identifiers arithmetic directory/folder structure (time permitting)
2	Where does the world begin? %> java Foo starts execution at main method in class Foo There are no objects yet main (and other methods) create objects with new, then messages can begin (Bank) Most classes don’t have main (BankAccount) Some classes have only main (Hello)
3	Reading Java Token: smallest sequence of characters that makes sense by itself (like a word in English) atm.println(“sorry... ”); The seven tokens on this line are atm . println ( “sorry... ” ) ; White space (blank, tab, newline) may (but need not) separate tokens: 2+3 and 2 + 3 are the same (but the last is bad form)
4	Keywords Words whose meaning is fixed by Java Can’t be reused by the programmer Only 48 of them if class int new while public … Listed in JOI and in Sun’s Java tutorial (link from course home page) true, false and null also have fixed meanings, although they are not keywords Punctuation tokens also have fixed meanings
5	Identifiers Tokens (words) we make up to use in code: names for classes, fields, methods, variables Start with letter, then letters and numbers and some other characters Terminal atm CS110 String whichAccount Bank MAXVALUE Conventions ...
6	Identifier conventions Class names start with upper case: Bank, BankAccount, Terminal, String Some classes, like String, come with Java, but make String is not a keyword. We could write our own String class if we wished Field, method and variable names start lower case: account1, getBalance, moreTransactions Class, field and variable names should be nouns, method names should be verbs Internal words capitalized JOI has more detail …
7	Exam question What are the tokens in the following line of Java code? Which of these tokens are keywords? Which of these tokens are identifiers? Which of these tokens name classes? Which of these tokens name objects? Practice on the code you read every day Quiz your friends
8	Fields and variables Variables store values A field is one kind of variable Fields in an object keep track of object’s state String bankName // Bank.java int balance; // BankAccount.java Variables in a method store values needed while that method runs – for example, in Bank.java boolean moreTransactions; // line 105 int amount; // line 119
9	Declaring variables You must tell the compiler the kind of value you want to store, and the name of the place you will store it Declaration syntax: type name; type tells the compiler what kind of value you want to store name is the identifier you choose so that you can refer to the variable elsewhere in the program Declaration may (but need not) initialize value
10	Fields are declared … in the class description, outside any methods at the top of the class (by convention) 20 public class Bank 21 { 22 private String bankName; 24 private Terminal atm; 26 private BankAccount account1; private BankAccount account2; 29 private static final int INITIAL_BALANCE = 200; for now, ignore public,private, static, final
11	A method’s variables are declared … inside the method where they are used near where they are first used (our convention) or at the top of the method (another convention) 57 public void open() 58 { atm.println( "Welcome to “ + bankName ); 60 boolean bankIsOpen = true; no public/private for method variables
12	Types of variables primitive (int, boolean, ...) just eight of them, built into language names are Java keywords in box-and-arrow pictures, values are in the box reference (Bank, Terminal, ...) these are the objects in OOP names are the names of classes there are as many classes as you care to invent some come shipped with Java (String, …) in box-and-arrow pictures, values are arrows (references) in the box
13	Box-and-arrow pictures
14	= x = y means “make the value of x the value of y !” Example (primitive types) int y = 6; // declare y, initialize value int x; // declare x x = y; // copy content of box y into box x For reference types, x = y means arrow in x now points to the same place as arrow in y. x = y is not the same as y = x This makes mathematicians unhappy
15	== x == y means “is the value of x the same as the value of y?” Bank.java line 82: if (accountNumber == 1 ) x == y is the same as y == x
16	Integer arithmetic primitive type int (also short, long) there are maximum and minimum values +, - , * do what you expect them to so do >, >= , <, <=, == use () to alter order of operations / truncates: 20/6 is 3 % (modulus operator) gives remainder: 20 % 6 is 2
17	Decimal arithmetic primitive type double (also float) there are maximum and minimum values 6.023E23 is acceptable input +, - , *, >, >= , <, <=, == do what you expect them to use () to alter order of operations you can mix int and double in formulas / works properly (20.0/6 is 3.33333…)
18	True or false? Primitive type boolean Just two values true and false Use in tests (if and while)
19	Strings Class String predefined in Java A string is an object, not a primitive type Can use = for assignment String greeting = “Hello, world”; Can’t use == for equality, since Strings are objects Test equality by sending an equals message: if (command.equals(“exit”)) {
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