Variables in Java

Objectives

Distinguish between primitive and reference variables
Declare and use primitive variables
Declare, create, and use reference variables
Know sizes of primitive types
Understand automatic promotion of primitive variables
Understand type casting of primitive variables

Declaring a Variable

Before using any variable in a Java program, you must declare that variable.
Syntax:
type name1, name2, ..., nameN;
Examples:
```
double sum, avg;
int i, j;
String s;
```

Assigning a Value to Variable

Storing a value in a variable is called assigning or assignment.
Syntax:
name = value;

Examples:

double price;
String s;

price = 12.95;
s = "Amount:";

Initializing a Variable

It is possible to both declare and assign a value to a variable on a single line.
This is called initializing or initialization.
Syntax:
type name = value;

Examples:

double price = 12.95;
int i = 0, j = 2;
String s = "Amount due:";

Primitive or Reference

Variables may be categorized by their type as either primitive or reference.
Primitive variables store only one value at a time and have no methods.
Reference variables refer to arrays or objects which may store many values at once and may have methods.
There are eight primitive types in Java.
All other types are reference types.

Primitive Reference

boolean Everything else, such as:

char Object

byte String

short StringBuilder

int TreeSet

long Integer

float Long

double Float

Primitive	Reference
boolean	Everything else, such as:
char	Object
byte	String
short	StringBuilder
int	TreeSet
long	Integer
float	Long
double	Float

Primitive Types

Type	Purpose	Size (bytes)	Examples
boolean	Store true or false	1	`boolean b = true;`
char	Store a single unicode character	2	`char c = 'A';`
byte	Store a number from this range [-2⁷, 2⁷) = [-128, 128) = [-128, 127]	1	`byte b = 12;`
short	[-2¹⁵, 2¹⁵) = [-32768, 32768) = [-32768, 32767]	2	`short s = -25000;`
int	[-2³¹, 2³¹) = [-2147483648, 2147483648)	4	`int i = 4013;`
long	[-2⁶³, 2⁶³) = [-9223372036854775808, 9223372036854775808)	8	`long n = 3;`
float	Store a floating point number (a number with digits after the decimal separator) with approx. 7 decimal digits of precision	4	`float f = 4.15f;`
double	More precise than float, approx. 15 decimal digits of precision	8	`double d = 76.178032;`

Automatic Promotion of Primitive Variables

Generally speaking, smaller primitive variables can be promoted to larger variable types.
boolean cannot be promoted to any other primitive type.
Possible promotions:

boolean

char

↓

byte → short → int → long → float → double

Example:

int i = 7;
float f = i;  // Correct.  Promotion from int to float.

Type Casting Primitive Variables

Larger variables cannot be "promoted" to smaller variable types.
Converting from larger to smaller is a "demotion" and may result in lost data (possible loss of precision).
The programmer must explicitly write a type cast to cause the computer to convert from a larger variable to a smaller one.

Examples:

double d = 6.7;
int i = d;  // Wrong!  Results in a compile error.

double d = 6.7;
int i = (int)d;  // Correct type cast from double to int.
// After the type cast and assignment, i
// holds the value 6, d holds the value 6.7