Variables

Key Points

• Every variable must be associated with a data type, which can be either a primitive type or a reference type.
• The variable name must be a valid identifier.
• A variable declaration is a complete statement, so it must always end with a semicolon (;).

Scope

Class Variables (Static)

Syntax:

static DataType variableName; // Initialization is optional

Default Values: If not explicitly initialized, primitive types default to 0 or 0.0 (and \u0000 for char). Boolean defaults to false. All non-primitive reference types default to null.

Class variables can be acessed directly by other methods within the class.

Instance Variables

Syntax:

DataType variableName; // Initialization is optional

Default Values: The default values are identical to those of class variables.

Note: To access instance variibles from other methods, you must instantiate the class containing the variable using the new keyword.

Example:

public class Example {
    float balance;

    public static void main(String[] args) {
        Example instance = new Example();
        System.out.println(instance.balance);
    }
}

The output will be 0.0 since float is a numeric floating-point type.

Local Variables

• Local variables must be declared and initialized with a value.
• They are only accessible within the block or method where they are defined. Attempting to use them outside their scope will result in a compilation error.

Naming Conventions

• General: Choose names that clearly describe the purpose (meaningful names).
• Classes: Use PascalCase (every word starts with a capital letter).

  public class UserAccount {}
  

• Variables and Methods: Use camelCase (start with a lowercase letter, then capitalize subsequent words).

  double totalAmount;
  void calculateInterest() {}
  

Constants

Constants represent immutable data. Once a constant is assigned a value, it cannot be modified during the program's execution. They are essentially special variables with fixed values.

Syntax:

final DataType CONSTANT_NAME = value;

By convention, constant names are written in uppercase letters using underscores to separate words.

Example:

final double MAX_SPEED = 120.50;