Variables in Java are categorized based on their scope, lifetime, and association with classes or instances.

• Local Variables: Declared within methods, constructors, or code blocks. They are destroyed when the enclosing execution context exits and must be explicitly initialized before use.
• Instance Variables: Declared in a class but outside methods/constructors. Each class instance maintains its own copy. If uninitialized, they receive default values (0 for numerics, false for booleans, null to references).
• Static Variables (Class Variables): Declared with the static keyword. Shared across all class instances, initialized once during class loading. Accessed via ClassName.variableName.
• Constants: Member variables marked static final. Immutable, single copy per class.
• Parameter Variables: Defined in method/constructor signatures to accept input values. Scoped to the method body. Example:

// beverageType is a parameter variable
public void drink(String beverageType) {
    System.out.println("Today I drank " + beverageType);
}

In summary, variables fall into three groups: local, member (instance/static/constant), and parameter variables.

Member Variables and Field-Property Distinction

Member Variables: Scope spans the entire class (similar to C globals), declared outside methods. Encludes instance variables, static variables, and constants.

Fields vs. Properties:

• Fields: Synonymous with member variables (instance/static/connstants) declared in a class.
• Properties: Defined by accessor methods (getXxx()/setXxx()), independent of direct field storage. A property exists if at least one accessor is present.

Example with property accessors:

public class Person {
    private int age;  // Field

    // Readable age property
    public int getAge() {
        return age;
    }

    // Writable age property
    public void setAge(int newAge) {
        age = newAge;
    }
}

A class with both accessors has a read-write property; only a getter implies read-only, only a setter implies write-only. Properties may lack backing fields (computed properties):

public class Rectangle {
    private double width;   // Backing field
    private double height;  // Backing field

    // Computed area property (no dedicated field)
    public double getArea() {
        return width * height;
    }
}

Default Values

Local variables require explicit initialization. Member variables use implicit defaults:

Variable Type	Default Value
int, byte, short, long	0
float, double	0.0
char	'\u0000'
Reference types	null