Java Class Attributes (Fields / Properties)

Class attributes are variables declared inside a class but outside any method, constructor, or block. They define the state or properties of an object.

1. Types of Class Attributes

Instance Variables
- Belong to an object.
- Each object has its own copy.
- Declared without static keyword.
Class Variables (Static Variables)
- Belong to the class, not the object.
- Shared by all objects.
- Declared using static keyword.
Final Variables (Constants)
- Value cannot be changed after initialization.
- Declared using final keyword.

2. Example 1: Instance Variables

class Car {
String brand; // instance variable
int year;

void displayInfo() {
System.out.println("Brand: " + brand + ", Year: " + year);
}
}

public class Main {
public static void main(String[] args) {
Car car1 = new Car();
car1.brand = "Toyota";
car1.year = 2022;
car1.displayInfo();

Car car2 = new Car();
car2.brand = "Honda";
car2.year = 2023;
car2.displayInfo();
}
}

class Car {

String brand; // instance variable

int year;

void displayInfo() {

System.out.println("Brand: " + brand + ", Year: " + year);

}

public class Main {

public static void main(String[] args) {

Car car1 = new Car();

car1.brand = "Toyota";

car1.year = 2022;

car1.displayInfo();

Car car2 = new Car();

car2.brand = "Honda";

car2.year = 2023;

car2.displayInfo();

}

📌 Output:

Each object has its own copy of instance variables.

3. Example 2: Class (Static) Variables

class Counter {
static int count = 0; // static variable

Counter() {
count++;
}
}

public class Main {
public static void main(String[] args) {
Counter c1 = new Counter();
Counter c2 = new Counter();
Counter c3 = new Counter();

System.out.println("Total objects created: " + Counter.count);
}
}

class Counter {

static int count = 0; // static variable

Counter() {

count++;

}

public class Main {

public static void main(String[] args) {

Counter c1 = new Counter();

Counter c2 = new Counter();

Counter c3 = new Counter();

System.out.println("Total objects created: " + Counter.count);

}

📌 Output:

Static variables are shared among all objects of the class.

4. Example 3: Final Variables (Constants)

class Circle {
final double PI = 3.14159; // constant
double radius;

double area() {
return PI * radius * radius;
}
}

public class Main {
public static void main(String[] args) {
Circle c = new Circle();
c.radius = 5;
System.out.println("Area = " + c.area());
}
}

class Circle {

final double PI = 3.14159; // constant

double radius;

double area() {

return PI * radius * radius;

}

public class Main {

public static void main(String[] args) {

Circle c = new Circle();

c.radius = 5;

System.out.println("Area = " + c.area());

}

📌 Output:

❗ Cannot modify PI because it is final.

5. Key Points

Attribute Type	Belongs To	Shared Across Objects?	Modifiable?
Instance	Object	No	Yes
Static	Class	Yes	Yes
Final	Object/Class	Depends (with static)	No

Instance variables → Object-specific data
Static variables → Class-wide shared data
Final variables → Constants that cannot change

Java Class Attributes (Fields / Properties)