composite inheritance: how to assign a final field at sub-class constructor which depends on 'this' value (backward reference)?

问题

I use composite classes to group functionalities.
But, the class A (with composite A1), got inherited by B (with composite B1), and a behavior existent at A1 is going to be adapted at B1, but the final a1 must be a B1 instance for this to work.

Obs.: I have ways to make sure the composite instantiation happens properly (only by its composite partner).

Unable to assign a B1 object to a1 final field:

class finalFieldTestFails{
    class A1{
      A1(A a){}
    }

    class A{
      protected final A1 a1;
      A(){
        this.a1 = new A1(this);
      }

      A(A1 a1){
        this.a1 = a1;
      }
    }

    class B1 extends A1{
      B1(B b){
        super(b);
      }
    }

    class B extends A{
        //B(){ super.a1=new B1(this); } //FAIL: cant change final value
        //B(){super(new B1(this));} //FAIL: cant use 'this' or 'super'
    }
}

PS.: the answer shall not involve reflection security tricks if possible.

回答1:

B(){ super.a1=new B1(this); } //FAIL: cant change final value

You can not assign values to already assigned final variables, You can instantiate it within the constructor but not as following.

B(){super(new B1(this));} //FAIL: cant use 'this' or 'super'

You can use this once the instance is created only. Since there is a inheritance tree, it is not completely instantiated, So the compiler complains,

cannot reference this before supertype constructor has been called

回答2:

With this tip, given by @KevinHooke, I found this answer and I came with this code below.

But be aware that this option I chose may cause trouble.

So I ended up using an overriden method to instantiate the "1" kind, letting the super class take care of the assignment.

class finalFieldTestWorks{
    class A1{A1(A a){}}

    class A{
      protected final A1 a1;
      A(){
        this.a1=new1();
      }
      protected A1 new1(){return new A1(this);}
    }

    class B1 extends A1{B1(B b){super(b);}}

    class B extends A{
      B(){
        super();
      }
      @Override
      protected B1 new1(){return new B1(this);}
    }
}

PS.: Other tips/workarounds will be appreciated.

来源：https://stackoverflow.com/questions/34822434/composite-inheritance-how-to-assign-a-final-field-at-sub-class-constructor-whic

标签

java

inheritance

variable-assignment

final

composite