final field guarantees
- Java course
- Expert
- Lesson 10/46
- 5 min
Learn initialization guarantees of final fields.
The guarantee of final fields
A final field is assigned exactly once, in the constructor, and never changes after. The Java Memory Model guarantees that when the constructor completes, the field is visible to all threads; this is called safe publication.
Example: final int x. When an object is published, another thread reading x sees the value assigned in the constructor; there is no risk of seeing a half-constructed object. So immutable objects are designed with final fields; this is both safe and efficient for the garbage collector.
Immutable objects
An object whose fields are all final and whose methods do not change state is immutable. Immutable objects are thread-safe because their state never changes after publication.
Example: class Point { final int x; final int y; }. Once a Point is created, x and y stay constant, so it can be shared without locking. Java's String and LocalDate are immutable, which lets them be cached and shared safely.
Practice An immutable Point class
Define a Point class with final int x and final int y fields; a Point(int x, int y) constructor assigns them. In main, create new Point(3, 4) and print p.x + p.y with System.out.println.
Starter code
// Define the Point class
// Add final x and y fields
// Print the sum in mainSample solution
class Point {
final int x;
final int y;
public Point(int x, int y) { this.x = x; this.y = y; }
}
public class FinalDemo {
public static void main(String[] args) {
Point p = new Point(3, 4);
System.out.println(p.x + p.y);
}
}Expected output
7
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What visibility does a final field guarantee?
- Variable visibility
- A single instance
- Post-constructor visibility Correct answer
- Stack placement
Explanation: A final field becomes visible when the constructor finishes.
2 Declare a final field
public ____ int x;public final int x;Explanation: A final field is assigned only in the constructor.
3 Match the final field concepts
| Term | Meaning |
|---|---|
final field |
Visible after constructor |
volatile |
Visibility |
synchronized |
Locking |
local variable |
On the stack |