Java Class Attributes

Declaring attributes

An attribute is simply a variable declared directly inside a class, outside of any method. Each attribute has a type and a name. Once the class defines an attribute, every object created from that class gets its own copy of it.

public class Person {
  String name;
  int age;
  boolean isStudent;
}

Reading and changing attribute values

After creating an object you access its attributes with the dot operator. You can read a value to use it, or assign a new value to change it. Because each object is independent, you set the attributes of one object without affecting any other.

public class Main {
  public static void main(String[] args) {
    Person p = new Person();
    p.name = "Aisha";
    p.age = 27;
    p.isStudent = false;

    System.out.println(p.name + " is " + p.age);  // Aisha is 27

    p.age = 28;  // change the value
    System.out.println(p.age);  // 28
  }
}

Default and initial values

If you create an object without setting its attributes, Java does not leave them as garbage. Every attribute gets a sensible default: numbers become 0, booleans become false, and object references (including String) become null. You can also give an attribute an initial value right where you declare it, and that value applies to every new object unless it is changed afterwards.

public class Account {
  String owner;          // defaults to null
  double balance = 0.0;  // initial value for every new Account
  String currency = "USD";
}

Constant attributes with final

Sometimes an attribute should never change once it has been set - a maximum limit, a fixed rate, or an ID. Marking it with the 'final' keyword makes it a constant: the compiler will reject any attempt to reassign it after its initial value is set. Final attributes that are also shared across all objects are usually combined with 'static' and written in UPPER_CASE by convention.

public class Circle {
  final double PI = 3.14159;  // cannot be reassigned
  double radius;
}

public class Main {
  public static void main(String[] args) {
    Circle c = new Circle();
    c.radius = 5;
    System.out.println(c.PI * c.radius * c.radius);  // 78.53975
    // c.PI = 3.14;  // ERROR: cannot assign a value to final variable
  }
}

• An attribute is a variable declared inside a class but outside any method.
• Access attributes through an object with the dot operator.
• Unset attributes take a default value based on their type, never a random one.
• Use 'final' to make an attribute a constant that cannot be changed after it is set.