Java Chapter 3 - Variables

3.1 Variable Types

Below are the primitive variable types in Java. There are also wrapper classes (Chapter 6) that allow primitive variables be "wrapped" into a class.

byte    (1 byte)   -128 to 127
short   (2 bytes)  -32,768 to 32,767
int     (4 bytes)  -2.1 billion to 2.1 billion
long    (8 bytes)  -9 quintillion to 9 quintillion
float   (4 bytes)  range depends on number of places past decimal
double  (8 bytes)  range depends on number of places past decimal
boolean (1 byte)   1 or 0 (true or false)
char    (2 bytes)  any ASCII or Unicode character

3.2 Declaring and Using Variables

Below are examples of declaring and using variables.

short Value1 = 5, Value2 = 7;
int Sum = Value1 + Value2;
char MyGrade = 'A';
float Money = 5.75;
double Pi = 3.1416;
boolean Married = false;

The program below uses some of the primitive variable types.

// This Java program demonstrates some variable types

public class variables
{
   public static void main (String[] args)
   {
      double Wage = 12.25, Hours = 5.5, Pay;
      char Initial = 'D';
      Pay = Wage * Hours;

      System.out.println(Initial + " gets paid $" + Pay);
   }
}

3.3 Mathematical Operations

The basic mathematical operations are addition (+), subtraction (-), multiplication (*), division (/), and modulus (%).  The table below demonstrates these operations.

3.4 String Class

In Java, strings are represented as an instance of a class.  Below is an example of declaring and using a string.

String Name = "Joshua";
System.out.println("My name is " + Name);

There are many methods for the string class.  Some of these are demonstrated in the program below.

// This Java program demonstrates some of the string methods

public class stringdemo
{
   public static void main(String[] args)
   {
      String S1 = "lemon", S2 = "lime", S3 = " CHIPS ";

      System.out.println("1. Length of S1 = " + S1.length());
      System.out.println("2. Character at position 3 of S1 = " + S1.charAt(3));
      System.out.println("3. " + S2.toUpperCase());
      System.out.println("4. I like " + S3.trim().toLowerCase() + " and salsa");
      System.out.println("5. Location of 'o' in S1 = " + S1.indexOf('o'));
      System.out.println("6. Substring Example: " + S3.substring(3,5));
      System.out.println("7. Replace Example: " + S1.replace("emo","iste"));
   }
}

Below is a list of the string methods.  Assume the following declaration:

String A = "apple", B = "orange", C = "pear,grape", Age="21";

3.5 Arrays

Arrays are used to store multiple values in a single variable.  To declare and initialize an array in one line, you can do the following:

double[] HourlyWage = {17.25, 20.75, 22.0, 18.55, 20.0};

To access individual elements in the array:

HourlyWage[0] += 1.0;              // Makes first element 18.25
HourlyWage[1] = HourlyWage[4];     // Makes second element 20.0

To declare a 10 element array without initializing:

int[] SomeNumber = new int[10];    // Besides variables, it can be an array of objects

To declare a 10x10 multidimensional array:

int[][] World = new int[10][10];   // 100 total integer variables declared in 2-diminsional array

Below is a program that steps through each element of an array using a loop.  Notice use of the array.length variable.

public class array
{
   public static void main (String[] args)
   {
      double[] HourlyWage = {17.25, 20.75, 22.0, 18.5, 20.0};
      for(int i=0; i<HourlyWage.length; i++)
      {
         HourlyWage[i]++;
         System.out.println(HourlyWage[i]);
      }
   }
}