final field guarantees

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.

Exercise 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.

Show example 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);
    }
}

Test yourself

What visibility does a final field guarantee?
- Variable visibility
- A single instance
- Post-constructor visibility
- Stack placement
Show answer

Post-constructor visibility — A final field becomes visible when the constructor finishes.
Declare a final field
```
public ____ int x;
```
Show answer
```
public final int x;
```
Match the final field concepts

final field

Visible after constructor
volatile

Visibility
synchronized

Locking
local variable

On the stack

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