Java Class Attributes
Variables that store data in classes
📊 What are Class Attributes?
Class attributes are variables declared inside a class that store data for objects. Each object has its own copy of these attributes with different values, defining the object's state and characteristics.
class Phone {
String brand; // Attribute
String model; // Attribute
int storage; // Attribute
boolean isOn; // Attribute
}
Types of Attributes
String
Text data
String name = "John";int
Whole numbers
int age = 25;double
Decimal numbers
double price = 99.99;boolean
True or false values
boolean isActive = true;🔹 Declaring Attributes
Attributes are declared inside the class but outside any methods:
// Student.java
public class Student {
// Class attributes (instance variables)
String firstName;
String lastName;
int studentId;
double gpa;
boolean isEnrolled;
char grade;
// Methods will go here...
void displayInfo() {
System.out.println("Student: " + firstName + " " + lastName);
System.out.println("ID: " + studentId);
System.out.println("GPA: " + gpa);
System.out.println("Enrolled: " + isEnrolled);
System.out.println("Grade: " + grade);
}
}Key Points:
- Attributes are declared at the class level
- Each object gets its own copy of attributes
- Attributes can be different data types
- Default values are assigned if not initialized
🔹 Accessing and Modifying Attributes
You can access and modify attributes using the dot (.) operator:
// StudentExample.java
public class StudentExample {
public static void main(String[] args) {
// Create a student object
Student student1 = new Student();
// Set attribute values
student1.firstName = "Emma";
student1.lastName = "Johnson";
student1.studentId = 12345;
student1.gpa = 3.8;
student1.isEnrolled = true;
student1.grade = 'A';
// Display student info
student1.displayInfo();
System.out.println("---");
// Modify attributes
student1.gpa = 3.9;
student1.grade = 'A';
System.out.println("After grade update:");
student1.displayInfo();
}
}Output:
Student: Emma Johnson ID: 12345 GPA: 3.8 Enrolled: true Grade: A --- After grade update: Student: Emma Johnson ID: 12345 GPA: 3.9 Enrolled: true Grade: A
🔹 Default Values
If you don't initialize attributes, Java assigns default values:
// DefaultValues.java
class DefaultExample {
String text; // Default: null
int number; // Default: 0
double decimal; // Default: 0.0
boolean flag; // Default: false
char character; // Default: '\u0000' (null character)
void showDefaults() {
System.out.println("String: " + text);
System.out.println("int: " + number);
System.out.println("double: " + decimal);
System.out.println("boolean: " + flag);
System.out.println("char: '" + character + "'");
}
}
public class DefaultValues {
public static void main(String[] args) {
DefaultExample obj = new DefaultExample();
obj.showDefaults();
}
}Output:
String: null int: 0 double: 0.0 boolean: false char: ''
🔹 Initializing Attributes
You can initialize attributes when declaring them:
// Car.java
class Car {
// Initialized attributes
String brand = "Unknown";
String color = "White";
int year = 2023;
double price = 0.0;
boolean isElectric = false;
int mileage = 0;
void displayCar() {
System.out.println("=== Car Details ===");
System.out.println("Brand: " + brand);
System.out.println("Color: " + color);
System.out.println("Year: " + year);
System.out.println("Price: $" + price);
System.out.println("Electric: " + isElectric);
System.out.println("Mileage: " + mileage + " miles");
}
}
// CarExample.java
public class CarExample {
public static void main(String[] args) {
// Create car with default values
Car car1 = new Car();
System.out.println("Car 1 (default values):");
car1.displayCar();
// Create another car and customize
Car car2 = new Car();
car2.brand = "Tesla";
car2.color = "Red";
car2.year = 2024;
car2.price = 45000.0;
car2.isElectric = true;
car2.mileage = 1500;
System.out.println("\nCar 2 (customized):");
car2.displayCar();
}
}Output:
Car 1 (default values): === Car Details === Brand: Unknown Color: White Year: 2023 Price: $0.0 Electric: false Mileage: 0 miles Car 2 (customized): === Car Details === Brand: Tesla Color: Red Year: 2024 Price: $45000.0 Electric: true Mileage: 1500 miles
🔹 Best Practices
Naming Conventions:
- camelCase: firstName, lastName, studentId
- Descriptive: Use meaningful names
- No spaces: Use camelCase instead
Good Practices:
- Initialize attributes with sensible default values
- Use appropriate data types
- Group related attributes together
- Consider using private attributes with getter/setter methods