What is the difference between the “const” and “final” keywords in Dart?

Question

What is the difference between const and final keyword in Dart?

Answer 1

my understand

const means its initial value is must be fixed, can not be a dynamic value;

final means its initial value is must be fixed but can be a dynamic value, equal to the var with a fixed value.

code demos

void main() {
  const sum = 1 + 2;
  // const can not change its value
  print("sum = ${sum}");
  // Const variables must be initialized with a constant value.
  const time = new DateTime.now();
  // Error: New expression is not a constant expression.
  print("time = ${time}");
}

// new DateTime.now();
// dynamic timestamp

void main() {
  final sum = 1 + 2;
  // final can not change its value
  print("sum = ${sum}");
  final time = new DateTime.now();
  // final === var with fixed value
  print("time = ${time}");
}

refs

https://dart.dev/guides/language/language-tour#final-and-const

Answer 2

Const

Value must be known at compile-time, const birthday = "2008/12/25"
Can't be changed after initialized.

---

Final

Value must be known at run-time, final birthday = getBirthDateFromDB()
Can't be changed after initialized.

Answer 3

If you are coming from C++ then const in Dart is constexpr in C++ and final in Dart is const in C++.

The above applies to primitive types only. However in Dart objects marked final are mutable in terms of it's members.

Answer 4

You can't initialise a const using a final. For example :

  final myConst = 1;
  const myFinal = 2;

  final a = myConst; // possible
  final b = myFinal; // possible
  const c = myConst; // this is not possible
  const d = myFinal; // possible

Answer 5

Consolidated @Meyi and @faisal-naseer answers and Comparing with little programming.

const:

const keyword used to make a variable to store a compile time constant value. Compile time constant value is a value which will be constant while compiling :-)

For example 5 is a compile time constant. While DateTime.now() which is not compile time constant. Because this method will return the time when the line is getting executed at runtime. So we can't assign the DateTime.now() to a const variable.

const a = 5;
// Uncommenting below statement will cause compile time error.
// Because we can't able to assign a runtime value to a const variable
// const b = DateTime.now();

Should be initialized at the same line.

const a = 5;
// Uncommenting below 2 statement will cause compilation error.
// Because const variable must be initialized at the same line.
// const b;
// b = 6;

All statements mentioned below are acceptable.

// Without type or var
const a = 5;
// With a type
const int b = 5;
// With var
const var c = 6;

Class level const variable should be initialized like below.

Class A {
    static const a = 5;
}

Instance level const variable is not possible.

Class A {
    // Uncommenting below statement will give compilation error.
    // Because const is not possible to be used with instance level 
    // variable.
    // const a = 5;
}

The another major use of const is used to make the object immutable. To make a class object immutable we need to use the const keyword with constructor and make all the fields as final like mentioned below.

Class A {
    final a, b;
    const A(this.a, this.b);
}

void main () {
    // There is no way to change a field of object once it's 
    // initialized.
    const immutableObja = const A(5, 6);
    // Uncommenting below statement will give compilation error.
    // Because you are trying to reinitialize a const variable
    // with other value
    // immutableObja = const A(7, 9);

    // But the below one is not the same. Because we are mentioning objA 
    // is a variable of a class A. Not const. So we can able to assign
    // another object of class A to objA.
    A objA = const A(8, 9);
    // Below statement is acceptable.
    objA = const A(10, 11);
}

we can use const keyword to a list.

const a = const [] - A variable a initialized as const which contains a list of const objects(i.e., The list should contain only compile time constant and immutable objects). So we can't able to assign a with another list.

var a = const [] - A variable a initialized as var which contains a list const objects. So we can able to assign another list to the variable a.

Class A {
    final a, b;
    const A(this.a, this.b);
}

class B {
    B(){ // Doing something }
}

void main() {
    const constantListOfInt = const [5, 6, 7,
                 // Uncommenting below statement give compilation error.
                 // Because we are trying to add a runtime value
                 // to a constant list
                 // DateTime.now().millisecondsSinceEpoch
              ];
    const constantListOfConstantObjA = const [
        A(5, 6),
        A(55, 88),
        A(100, 9),
    ];
    // Uncommenting below 2 statements will give compilation error.
    // Because we are trying to reinitialize with a new list.
    // constantListOfInt = [8, 9, 10];
    // constantListOfConstantObjA = const[A(55, 77)];

    // But the following lines are little different. Because we are just
    // trying to assign a list of constant values to a variable. Which 
    // is acceptable
    var variableWithConstantList = const [5, 6, 7];
    variableWithConstantList = const [10, 11, 15];
    var variableOfConstantListOfObjA = const [A(5, 8), A(7, 9), A(10, 4)];
    variableWithConstantList = const [A(9, 10)];
}

---

final:

final keyword also used to make the variable to hold a constant value. Once initialized we can't able to change the value.

final a = 5;
// Uncommenting below statement will give compilation error.
// Because a is declared as final.
// a = 6;

All statements mentioned below are acceptable.

// Without type or var
final a = 5;
// With a type
final int b = 5;
// With var
final var c = 6;

Can able to assign a runtime value.

// DateTime.now() will return the time when the line is getting
// executed. Which is a runtime value.
final a = DateTime.now();
var b = 5;
final c = b;

Class level final variable must be initialized in the same line.

Class A {
    static final a = 5;
    static final b = DateTime.now();
}

Instance level final variable must be initialized in the same line or in the constructor initialization. The value will be put into memory when the object is created.

Class A {
    final a = 5;
}

// Constructor with a parameter.
Class B {
    final b;
    B(this.b);
}

// Constructor with multiple parameter.
Class C {
    final c;
    C(this.c, int d) {
        // Do something with d
    }
}

void main() {
    A objA = new A();
    B objB = new B(5);
    C objC = new C(5, 6);
}

Assigning a list.

final a = [5, 6, 7, 5.6, A()];
// Uncommenting Below statement will give compilation error.
// Because we are trying to reinitialize the object with another list.
// a = [9.9, 10, B()];

Answer 6

Both final and const prevent a variable from being reassigned (similar to how final works in Java or how const works in JavaScript).

The difference has to do with how memory is allocated. Memory is allocated for a final variable at runtime, and for a const variable at compile-time. The final modifier should be the more commonly used, because many program variables won't need any memory since the program logic won't call for them to be initialized. With a const variable you are basically telling the computer, "Hey, I need memory for this variable up front because I know I'm going to need it."

Thinking of them in this way makes it easier to understand differences in their syntactical usage. Mainly that a final variable may be an instance variable, but a const must be a static variable on a class. This is because instance variables are created at runtime, and const variables--by definition--are not. Thus, const variables on a class must be static, which means simply that a single copy of that variable exists on a class, regardless of whether that class is instantiated.

This video breaks it down fairly simply: https://www.youtube.com/watch?v=9ZZL3iyf4Vk

This article goes into more depth and explains a very important semantic difference between the two, i.e. final modifies variables and const modifies values, which essentially boils down to only being able to initialize const values which are derivable at compile-time.

https://news.dartlang.org/2012/06/const-static-final-oh-my.html