Java Lesson 2: Variables and Numbers

• Variables and Comments
  • Variables are statically typed.
  • A variable can start with any letter, an underscore _ or a dollar sign $ .
  • Declare a variable by giving the type, then the variable name:
    int aNumber
  • Line comments begin with //
  • Block comments are enclosed between /* and */
• Numerical Data Types
  • The range of values stored in each type is independent of your computer platform and architecture.
  • If you put a zero at the beginning of a number, Java will interepret it as being an octal value instead of an integer. Don't do that.
  • byte : A 1-byte number (+- 127)
  • short : A 2-byte number (+- 32,767)
  • int : A 4-byte number (+- 2 billion)
  • long : An 8-byte number (+- 9 quintillion). Requires "L" at end of number, ie long bigNum = 123465789L
  • float : A 4-byte decimal number (+- 3.4E38). Requires "F" at end of number, ie float aNum = 1234.56F
  • double : An 8-byte decimal number (+- 1.7E308)
• Working with Numerical Types
  • You can declare multiple variables of the same type on a single line by separating the declarations with commas.
    int numDogs = 5, numCats = 10, numSnakes = 12
  • Numercial calculations use the following operators:
    • + Add
    • - Subtract
    • * Multiply
    • / Divide
    • % Modulus (remainder after division)
    • ++num : Increment num before calculating the expression
    • --num : Decrement num before calculating the expression
    • num++ : Increment num after calculating the expression
    • num-- : Decrement num after caluculating the expression
  • Calculations using short or byte numbers will result in int numbers.
  • Calculations involving a long will result in a long.
  • Calculations involving a float or double will result in float or double.
  • In order for division to return a decimal value, you must use at least one float or double in the calculation.
• Sample Code

              public class FruitCalc
              {
                  public static void main(String[] args)
                  {
                      // Setting up the numbers
                      int apples = 12; // we have 12 apples
                      float costApples = 1.25F; // each apple is $1.25
                      int oranges = 25; // we have 25 oranges
                      float costOranges = .49F; // each orange is $.49
  
                      // Display inventory
                      System.out.println("Inventory: ");
                      System.out.println("- Apples" + apples);
                      System.out.println("- Oranges" + oranges);
                      System.out.println("- Total Fruit: " + (apples+oranges));
                      
                      System.out.println("Total Prices: ");
                      System.out.println("-Apples: $" + (apples*costApples));
                      System.out.println("-Oranges: $" + (oranges*costOranges));
                  }
              }