How do I initialize a final field in constructor's body?

It's not possible to assign a final field in a constructor body. The final field needs to be assigned before the constructor body, in the initializer list or on declaration:

class ClassName
{
    final OtherClass field = new OtherClass(); // Here

    ClassName()
        : field = new OtherClass() // or here 
    {
     
    }
}

As you can't use this in the initializer list or on the declaration, you can't do what you plan to do.

There are 3 ways to initialise a final field in a class.

• Initialise it in constructor parameter.

  class Foo {
    final int bar;
  
    // Initialising in constructor parameter.
    Foo(this.bar);
  }
  

• Use initialiser list.

  class Foo {
    final int bar;
  
    // Initialiser list
    Foo() : bar = 1;
  }    
  

• Combination of both.

  class Foo {
    final int bar;
  
    Foo(int value) : bar = value;
  }