Variables

Variables are a way of storing data that can be used and changed throughout the program.

Variable Declaration

Before a variable may be used it must be declared. This tells the compiler to allocate memory for the variable. The format for declaring a variable is shown below.

dataType variableName;

  • dataType is the data type that the variable holds.

  • variableName is the name of the variable you wish to give.

Some examples of real world variables:

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int number;
double area;
char letter;

Variables of the same data type can be declared together.

Instead of:

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int a;
int b;
int c;

Use:

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int a, b, c;

Variables should have initial values. It is easy to do this when declaring them.

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int number = 1;
double area = 42.5;
char letter = 'B';

int a = 1, b = 2, c = 3;

Variable Naming

Variables follow a camel case naming structure. This means that the first word is lower case and any preceding word in the variable has a capital first letter.

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// Bad naming of variables ie. no camelCase
int Hellothere;
double thatsreallycool;

// Good naming of variables using camelCase
int helloThere;
double thatsReallyCool;

If you notice it becomes easier to read the different words in the variable.

Variables containing more than one word should be joined together.

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// Bad variable naming
int hello_There;
double thats-really-cool;

// Good variable naming
int helloThere;
double thatsReallyCool;

Variables must always start with a lowercase letter, an underscore _ or a $ sign. Variables cannot start with a number or any other symbol.

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// Acceptable starts of variables
int hello;
double _variable;
long $money;

Variables should always be descriptive but not to long and match the function.

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// Good Example

// Variables for calculating Pythagorean theorem
double a, b, c;

// Bad Example

// Variables for calculating Pythagorean theorem
double edge, longerEdge, reallyLongEdge;

Constants

Constants are a special type of variable that cannot change during the operation of the program. Constants are useful for values that wont change or don’t need to change.

To declare a constant we use:

final dataType CONSTANT_NAME = valueOfConstant;

final tells the compiler that this variable cannot be changed.

Note

Unlike variables constants use all caps for naming. Also if more than one word is in the constant name we use an underscore _ to separate them.

Some examples

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final int CONTROLLER_AXIS = 1;
final double PI = 3.14159265358979;

Using Variables in code

Variables make programing easy lets go through some examples.

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public class Variables
{
   public static void main(String[] args)
   {
      int x = 1;
      System.out.println(x);
   }
}

Output

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Line 5 holds the variable and its initial value. The variable is x, the data type is int and the value of the variable is 1.

Line 6 is the output of the variable. When printing a variable the quotations " " are not used.

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public class Variables
{
   public static void main(String[] args)
   {
      final double PI = 3.14159265358979;
      System.out.println("The Value of pi to 15 decimal places is: " + PI);
   }
}

Output

The Value of pi to 15 decimal places is: 3.14159265358979

Line 5 holds the variable which is being used as a constant.

Line 6 is the output. Notice how this time we are mixing a String with a variable. The String “The Value of pi to 15 decimal places is: ” and variable PI are joined by using +.

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public class Variables
{
   public static void main(String[] args)
   {
      double x = 6.84;
      System.out.println("Original Variable x: " + x);
      x = 10.8;
      System.out.println("Changed Variable x: " + x);
   }
}

Output

Original Variable x: 6.84
Changed Variable x: 10.8

In this example we define the variable x and give it the value of 6.84 on line 5. On line 7 we assign x a new value of 10.8.