Identifiers, Variables and Constants

Objectives

1. Define identifier, variables and constants.
2. Understand where constants and variables are used.
3. Understand the syntax rules for declaring constants/variables.
4. Understand the style rules for declaring constants/variables.
5. Understand variable initialization.

Notes

• This information is from
  • Chapter 2.2.1 (variables) and 2.2.5 (variables in programs)
  • He does not discuss constants, so the javapoint section on constants will do.
• When we write programs we need to store data.
• Two Program examples for now:
  1. Write a program that will calculate the cost of a tank of gas.

     • ask for the volume of the tank of gas (tankSize)
       ask for the price of gas              (pricePerGallon)
       cost = tankSize * pricePerGallon + tankSize * TAX_PER_GALLON
       print the cost 

  2. If our store sells one type of ice cream cone compute the number of gallons of ice cream sold on a given day

     • ask for the number of cones sold   (conesSold)
       ozSold = cones * OZ_PER_CONE
       gallonsSold = ozSold / OZ_PER_GALLON
       print gallons sold 

• Note, in each of these we have some data we need to deal with.
  • Some of it will change every time we run the program.
    • Gallons of gas per tank.
    • Price per gallon
    • Cones sold
    • Oz of ice cream sold
    • Gallons of ice cream sold
  • We call these variables
  • Some of it will not change.
    • Tax per gallon of gas
    • oz to gallons
  • We call these constants.
• Variables are named locations in memory with an associated type and value where the value can be changed.
• Constants are named locations in memory with an associated type and value where the value can not be changed.
• Variables and constants must be declared.
  • This is a statement that provides
    • A data type
    • A name (or identifier)
    • a value, but this is optional for variables.
  • Other things can added (public, static, ...) but are not required.
  • We will declare each in a bit.
• Identifiers
  • These are names in a program.
    • associated with data, and other things.
    • Made up by the user.
  • There are syntax rules:
    1. Contain only letters, digits, the underscore _
    2. Start with a letter or underscore _
    3. It can not be a reserved word or keyword.
  • There are style rules as well
    • They should have a meaning or describe the data.
    • Variables
      • start with a lower case letter.
      • In multi-word variables each additional word starts with an upper case letter.
        • gallonsPerTank
        • This is called CammelCase
      • We will use identifiers starting with an upper case letter to name methods or functions.
    • Constants
      • Are all upper case
      • Words are separated with an _
      • OZ_PER_CONE
      • TAX_PER_GALLON
• Variable syntax
  • Declared inside a code block of a function or class
  • [public private] [static] type identifier [= value] [, identifier = value] ;

  • public static void main(String[] args) {
        int numberOfCones = 123;
        float pricePerGallon = 3.45f;
        char letter = 'z';
        boolean isFinished = false;
    
        int x,y;
        int day = 1,
            month = 2,
            year = 2025;
       
        double bigNumber = 1.65E23;
        long largeNumber = 1_000_000_000L;
        long largerNumber = 2000000000L; 

• Constant syntax
  • Declared inside the class
  • Same as variable but
    • public static final type identifier = value

  • class VariablesDemo{
        public static final int OZ_PER_CONE = 8;
        public static final float TAX_PER_GALLON = 0.576f;
        ...
    }