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. There are eight primitive variable types in Java:
- boolean
- char
- byte
- short
- int
- long
- float
- double
Reference variables refer to arrays or objects which may store many values at once and may have methods. All variable types that are not primitive types are reference types, such as:
- Object
- String
- StringBuilder
- Integer
- Long
- Float
- TreeSet

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 within this range: [−2⁷, 2⁷) = [−128, 128) = [−128, 127]	1	`byte b = 12;`
short	Store a number within this range: [−2¹⁵, 2¹⁵) = [−32,768, 32,768) = [−32,768, 32,767]	2	`short s = -25000;`
int	Store a number within this range: [−2³¹, 2³¹) = [−2,147,483,648, 2,147,483,647]	4	`int i = 4013;`
long	Store a number within this range: [−2⁶³, 2⁶³) = [−9,223,372,036,854,775,808, 9,223,372,036,854,775,807]	8	`long n = 3;`
float	A floating-point number is a number with digits after the decimal separator. Store a floating-point number with approximately 7 decimal digits of precision within the following range: [−3.4028235×10³⁸, 3.4028235×10³⁸] `float` can store exact integers within the following range without approximation: [−16,777,216, 16,777,216] Integers that are more negative or more positive than the previous range are approximated in a `float`.	4	`float f = 4.15f;`
double	More precise than `float`. Store a floating-point number with approximately 15 decimal digits of precision within the following range: [−1.7976931348623157×10³⁰⁸, 1.7976931348623157×10³⁰⁸] `double` can store exact integers within the following range without approximation: [−9,007,199,254,740,992, 9,007,199,254,740,992] Integers that are more negative or more positive than the previous range are approximated in a `double`.	8	`double d = 76.178032;`

Automatic Promotion of Primitive Variables

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

char

↓

byte → short → int → long → float → double

Example:

int i = 7;
float f = i;  // Automatic promotion from int to float.

Type Casting Primitive Variables

Larger variable types 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;  // Type cast from double to int.
// After the type cast and assignment,
// d will store the value 6.7 and
// i will store the value 6