Java doesn’t have anything like C++
const. You might think that
final is like
const, but it’s not:
- A
final variable in Java can be assigned to only once, but if the variable is a reference-type, you can still change what it refers to. Fun! - A
const variable in C++ can be assigned to only once, where it’s declared, and nothing is allowed to change about the value, whether it’s an object or not. Now that is a nice feature!
This is what Java Language Specification talks about the final variable.
A variable can be declared
final. A final variable may only be assigned to once. It is a compile time error if a final variable is assigned to unless it is definitely unassigned
(§16) immediately prior to the assignment.
A blank final is a final variable whose declaration lacks an initializer.
Once a final variable has been assigned, it always contains the same value. If a final variable holds a reference to an object, then the state of the object may be changed by operations on the object, but the variable will always refer to the same object. This applies also to arrays, because arrays are objects; if a final variable holds a reference to an array, then the components of the array may be changed by operations on the array, but the variable will always refer to the same array.
Declaring a variable final can serve as useful documentation that its value will not change and can help avoid programming errors.
In the example:
class Point {
int x, y;
int useCount;
Point(int x, int y) { this.x = x; this.y = y; }
final static Point origin = new Point(0, 0);
}
the class
Point declares a
final class variable
origin. The
origin variable holds a reference to an object that is an instance of class
Point whose coordinates are (0, 0). The value of the variable
Point.origin can never change, so it always refers to the same
Point object, the one created by its initializer. However, an operation on this
Point object might change its state-for example, modifying its
useCount or even, misleadingly, its
x or
y coordinate.
