Constants and Variables

1
Constants and Variables

• Memory cells used in program
• Called constants and variables
• Identifiers for constants and variables should be
  declared before the constants or variables are
  used
• Declaration is accomplished using declaration
  statements
• Constants
• Values do not change during execution of the
  program
• Variables
• Values can be changed as the program executes.

2
Declarations of symbolic constants in Java

• Value of a constant can not change during the
  execution of the program
• Constant must be initialized when it is declared
• Syntax of constant declaration
• final TYPE identifier value
• OR
• static final TYPE identifier value
• Examples
• final float PI3.141592654F
• static final double CONVERT2.21

3
Declarations of variables in Java

• A variable declaration assigns names for
  variables (memory cells used in program)
• Syntax of variable declaration
• TYPE var_identifier
• or
• TYPE var_identifier1value, var_identifier2,
• multiple variables of the same type may be
  declared on the same line
• Variable may be initialized when it is declared

4
Examples of Declaration of Variable Objects

• int my_integer_value
• char middle_initial, first_initial
• byte counter
• double ScaleFactor
• float temperature
• Boolean TestResult

5
Initialization

• Memory locations associated with defined
  constants must be initialized when the constants
  are defined
• Memory locations associated with variables may be
  initialized anywhere in the program
• Initialization may occur in the declaration
  statement
• Initialization may be done after declaration
  using an assignment statement
• Memory locations associated with variables should
  have their values defined before they are used.
• It is good programming practice to initialize
  variables at the start of your program

6
Examples Initialization and Declaration

• long my_integer_value -12L
• char myinitial 'a'
• byte counter 31
• short AgeInYears 22
• double ScaleFactor 17.346798574
• float temperature 3.475F
• boolean done_flagfalse

7
Data Types and Expressions

• Data Type
• A set of values plus a set of operations on those
  values
• A crucial concept on modern programming
• Data Type of a variable determines how its memory
  cells are interpreted
• In Java every variable or constant object has a
  type
• Only values of that type can be directly assigned
  to the variable.
• Operators combine variables of the same type
• Methods for implicit and explicit conversions
  between types exist for specific conversions

8
Expressions

• An expression can be a single variable, or can
  include a series of variables. If an expression
  includes multiple variables they are combined
  using operators
• Arithmetic expressions manipulate numeric
  variables following the rules of algebra and have
  numeric values.
• Integral expressions manipulate integer values
• Floating point or decimal expressions manipulate
  floating point numbers
• Relational expressions compare numerical values
  and have logical (boolean) values
• Logical expressions combine boolean variables or
  expressions and yield boolean values

9
Expressions

• An expression is a series of variables combined
  using unary and/or binary operations
• An algebraic expression has a numeric value, a
  relational or logical expression has a boolean
  value
• Example
• XY is an algebraic expression
• If the variables X and Y have different types
  then they must be converted to a common type
  before the expression XY is evaluated
• The type of the value of the entire expression is
  the same as the common type the variables are
  converted to

10
Assignment Statements

• Basic statement using the assignment operator to
  set the value of a variable to the value of an
  evaluated expression
• A way to save the results of evaluating an
  expression
• Form resultvariable expression
• Examples
• X X X Y
• root -3
• X Y
• X - 5
• The types of the resultvariable and expression
  must be the same.

11
Assignment operators

• A B assign value of expression B to
  variable A, store result in A
• A B add the value of expression B to
  variable A, store result in A
• A - B subtract the value of expression B
  from variable A, store result in A
• A B multiply the value of expression B
  by variable A, store result in A
• A / B multiply the value of expression B
  by variable A, store result in A

12
Rules for implicit conversion

• The value of the expression in an assignment
  statement may be converted to the type of the
  resultvariable
• The value of one of the operands of a binary
  operation may be converted before the operation
  is performed
• Some conversions are done implicitly. These
  conversions are the widening conversions that
  always have valid results
• byte to int
• float to double
• short to int
• int to long
• int to float

13
Explicit conversion The cast operation

• In Java you can explicitly convert the type of a
  variable or expression within a larger expression
  using a cast operator
• The value of the variable or expression is not
  changed
• The value used in the larger expression is
  converted to the requested type
• Sample expressions including casts
• (int)(FloatoneFloattwo)
• (float)Integerone Float1 Float2
• (double)(int1)double(short2) double2
• (float)(int1)/int2