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

Variable Declaration

• 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;

Variable Assignment

• Storing a value in a variable is called assigning or assignment.
• Syntax:
  name = value;
• Examples:
  double price;
String s;

price = 12.95;
s = "Amount:";

Variable Initialization

• 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 Types

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