1. Chap 2. Identifiers, Keywords, and Types
Objectives
Comment
Identifier
Keyword
Basic meaning: Class, object, member variable, reference
Create a Java class definition
Declare variables of class type
Constructor
Access member variables of an object using dot notation
reference variable

2. Comments Comments in a program are also called inline documentation
They should be included to explain the purpose of the program and describe processing steps
Java comments can take two forms:
// comment runs to the end of the line
/* comment runs to terminating
symbol, even across line breaks */

3. Semicolon, Blocks, White Space
In java, statements are terminated with a semicolon(;)
A block is marked by enclosing it in brackets({ })
Spaces, blank lines, and tabs are collectively called white space and are used to separate words and symbols in a program
Extra white space is ignored

4. Identifiers Identifiers are the words a programmer uses in a program
Most identifiers have no predefined meaning except as specified by the programmer
An identifier can be made up of letters, digits, the underscore character (_), and the dollar sign
They cannot begin with a digit
Java is case sensitive, therefore Total and total are different identifiers

5. Identifier examples Valid identifier Invalid identifier identifier
userName
User_name
_sys_var1
$change
Invalid identifier
3identifier

6. Reserved Words(Keywords)
Some identifiers, called reserved words, have specific meanings in Java and cannot be used in other ways
abstract
boolean
break
byte
byvalue
case
cast
catch
char
class
const
continue
default do
double
else
extends
false
final
finally
float
for
future
generic
goto if
implements
import
inner
instanceof
int
interface
long
native
new
null
operator
outer
package
private
protected
public
rest
return
short
static
super
switch
synchronized
this
throw
throws
transient
true
try
var
void
volatile
while

7. Primitive Data Types
A data type is defined by a set of values and the operators you can perform on them
Each value stored in memory is associated with a particular data type
The Java language has several predefined types, called primitive data types
The following reserved words represent eight different primitive types:
byte, short, int, long, float, double, boolean, char

8. Integers There are four separate integer primitive data types
They differ by the amount of memory used to store them
Type
byte
short
int
long
Storage
8 bits
16 bits
32 bits
64 bits
Min Value
-128
-32,768
-2,147,483,648
< -9 x 1018
Max Value
127
32,767
2,147,483,647
> 9 x 1018

9. Characters
The ASCII character set is still the basis for many other programming languages
ASCII is a subset of Unicode, including:
uppercase letters
lowercase letters
punctuation
digits
special symbols
control characters
A, B, C, …
a, b, c, …
period, semi-colon, …
0, 1, 2, …
&, |, \, …
carriage return, tab, ...

10. Floating Point There are two floating point types:
The float type stores 7 significant digits
The double type stores 15 significant digits
Approximate
Min Value
-3.4 x 1038
-1.7 x 10308
Approximate
Max Value
3.4 x 1038
1.7 x 10308
Type
float
double
Storage
32 bits
64 bits

11. Characters
A char value stores a single character from the Unicode character set
A character set is an ordered list of characters
The Unicode character set uses sixteen bits per character, allowing for 65,536 unique characters
It is an international character set, containing symbols and characters from many world languages

12. Boolean A boolean value represents a true or false condition
They can also be used to represent any two states, such as a light bulb being on or off
The reserved words true and false are the only valid values for a boolean type

13. Wrappers
For each primitive data type there is a corresponding wrapper class. For example:
Wrapper classes are useful in situations where you need an object instead of a primitive type
They also contain some useful methods
Primitive Type
int
double
char
boolean
Wrapper Class
Integer
Double
Character
Boolean

14. "To thine own self be true."
Literals
A literal is an explicit data value used in a program
Integer literals:
Floating point literals:
String literals:
"The result is: "
"To thine own self be true."

15. Variables
A variable is an identifier that represents a location in memory that holds a particular type of data
Variables must be declared before they can be used
The syntax of a variable declaration is:
data-type variable-name;
For example:
int total;

16. Variables Multiple variables can be declared on the same line:
int total, count, sum;
Variables can be initialized (given an initial value) in the declaration:
int total = 0, count = 20;
float unit_price = 57.25;

17. Assignment Statements
An assignment statement takes the following form:
variable-name = expression;
The expression is evaluated and the result is stored in the variable, overwriting the value currently stored in the variable
The expression can be a single value or a more complicated calculation

18. Constants
A constant is similar to a variable except that they keep the same value throughout their existence
They are specified using the reserved word final in the declaration
For example:
final double PI = ;
final int STUDENTS = 25;

19. Aggregate Data Types. public class Date {. int day;. int month;
Aggregate Data Types public class Date { int day; int month; int year; } Date mybirth, yourbirth; mybirth.day=26; mybirth.month=11; yourbirth.year=1960;

20. Java Coding Convention
Class name should be noun(첫문자는 대문자)
Interface name should be capitalized like class names.
Method names should be verb(첫문자는 소문자)
Variable: 소문자로 시작, meaningful
Use comments to explain code segments
Place only a single statement on any line, use 4-space indentation, to make your code readable

21. Objects An object has: For example, a particular bank account
state - descriptive characteristics
behaviors - what it can do (or be done to it)
For example, a particular bank account
has an account number
has a current balance
can be deposited into
can be withdrawn from

22. Classes A class is a blueprint of an object
It is the model or pattern from which objects are created
A class defines the methods and types of data associated with an object
Creating an object from a class is called instantiation; an object is an instance of a particular class
For example, the Date class could describe many bank accounts, but mybirth is a particular bank account with a particular balance

23. Creating Objects The new operator creates an object from a class:
Date mybirth = new Date();
This declaration asserts that mybirth is a variable that refers to an object created from the Date class
It is initialized to the object created by the new operator
The newly created object is set up by a call to a constructor of the class