Understand the idea

Fields store object or class state. An instance field belongs to each object; a static field is associated with the class. Give fields sensible defaults and avoid exposing every field publicly in real application models.

Explore the details

Java Class Attributes

Attributes (also called fields) are variables declared inside a class. Every object created from that class gets its own copy of those attributes.

Declaring and Accessing Attributes

Independent Copies Per Object

Final Fields (Constants)

Use the final keyword to make an attribute a constant — its value is set once and can never be changed after that.

class Circle {
    final double PI = 3.14159;  // constant — cannot be changed
    double radius;

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

Circle c = new Circle();
c.radius = 5;
System.out.println(c.area());  // 78.53975

// c.PI = 3.0;  // ERROR: cannot assign a value to final variable PI

A complete runnable example

This complete Main.java program focuses on the lesson idea. Reference fragments above may demonstrate individual statements; this example includes the imports, entry point, and supporting types it needs.

public class Main {
    public static void main(String[] args) throws Exception {
        Lamp desk = new Lamp();
        Lamp floor = new Lamp();
        desk.on = true;
        System.out.println(desk.on);
        System.out.println(floor.on);
    }
}
class Lamp { boolean on; }

true
false

Change it. Run it. Understand it.

Load this lesson example into the editor. Change one value or statement, predict the result, then press RUN to execute Java.

Make it your own

Your learning checkpoint

• Explain the example without reading the output first.
• Make the change requested in the practice task.
• Describe why the result changed—or why compilation failed.

Ready to move on? Follow the next lesson in course order, or return to the curriculum to review a prerequisite.