The document discusses identifiers, variables, and constants in Java. It defines identifiers as names given to variables, constants, classes and methods. Variables are names for locations in memory that store values, and must be declared before use. Constants hold fixed values. The document outlines Java's rules for identifiers, describes how to declare and initialize variables, and lists common Java keywords.

1. DECLARING AND USING
IDENTIFIERS, VARIABLES AND
CONSTANT
Chapter 3.1:

2.  Name of things such as variables,
constants, name of classes and
methods, that appears in programs.
 Example:
 Variable: num1
 Constant: PIE
 Method: countNum()
Identifiers

3. Java’s Rules for Identifiers
 Can be made of ONLY letters, digits, (_)
and $.
 Cannot start with a digit.
 Spaces are not permitted inside identifiers.
 Cannot use reserved words.
 e.g: int,public, throws,return
 An identifier cannot be true, false, or
null.
 They are case sensitive.
 An identifier can be of any length.

4. Variables
 A variable is a name for a location in memory that
holds a value.
 Is used to store data input, data output, or
intermediate data
 Representing data of a certain type
 Content may change during program execution.
 Must be declared before it can be used.
 May not be automatically initialized.
 If new value is assigned, old one is destroyed.
 Value can only be changed by an assignment
statement or an input (read) statement.

5. Java Keywords
 abstract
 assert
 boolean
 break
 byte
 case
 catch
 char
 class
 continue
 default
 do
 super
 switch
 synchronized
 this
 throws
 transient
 try
 void
 volatile
 while
• double
• else
• enum
• extends
• for
• final
• finally
• float
• if
• implements
• import
• instanceof
• int
• interface
• long
• native
• new
• package
• private
• protected
• public
• return
• short
• static
• strictfp*

6. Declaring Variables
Syntax: datatype variableName
Example:
int i,j; //Declare i and j to
//be an integer variable;
double sale; //Declare sale to
// be a double variable;
char first; // Declare first to be a
// character variable;
String str; // Declare str to be a
//String variable;

7. Declaring Variables
 A variable declaration specifies the variable's
name and the type of information that it will hold
int total;
int count, temp, result;
Multiple variables can be created in one declaration
data type variable name

8. Variable Initialization
 A variable can be given an initial value in the
declaration
int sum = 0;
int base = 32, max = 149;
 When a variable is referenced in a program, its
current value is used
 See PianoKeys.java

9. Variable Initialization
//********************************************************************
// PianoKeys.java Author: Lewis/Loftus
//
// Demonstrates the declaration, initialization, and use of an
// integer variable.
//********************************************************************
public class PianoKeys
{
//-----------------------------------------------------------------
// Prints the number of keys on a piano.
//-----------------------------------------------------------------
public static void main (String[] args)
{
int keys = 88;
System.out.println ("A piano has " + keys + " keys.");
}
}

10. Variable Initialization
//********************************************************************
// PianoKeys.java Author: Lewis/Loftus
//
// Demonstrates the declaration, initialization, and use of an
// integer variable.
//********************************************************************
public class PianoKeys
{
//-----------------------------------------------------------------
// Prints the number of keys on a piano.
//-----------------------------------------------------------------
public static void main (String[] args)
{
int keys = 88;
System.out.println ("A piano has " + keys + " keys.");
}
}
Output
A piano has 88 keys.