Question

Consider the following case:

class A {
  int x;
  int y;
}

class B extends A {
  int z;
}

Now, somewhere in the code this classes are used like this:

A objA = getAFromSomewhere();
B objB = null;

And in a certain situation I want to do something like

objB = objA; // can't do this
objB.z = someZ;

Of course the real objects are a bit more complicated, so it’s not just about copying two ints. But they aren’t overly complex either.

I know I can write a constructor for B like this:

public B(A anA) {
  this.a = anA.a;
  this.b = anA.b;

  this.z = 0;
}

But if that’s really the only way, I prefer merging the additional members of B into A.

update considering the answers

My question was not clear enough. I understand that objB = objA; can’t work (thus I asked for “something like”, meaning something with comparable code complexity) and I know about the issues with shallow vs deep copies.
What I was looking for is a possibility to copy the members of a base class (let’s say using clone()). You may understand that copying every member manually is a bad solution as it adds complexity and redundancy to the code. Thanks for your replies anyway!

Answer 1

There’s no trivial solution to this because there’s no one-size-fits-all solution. Basically you don’t have all the information within a B, so you can’t guarantee you would have a "sensible" B object.

You probably just want to create a constructor in B which takes an A and copies all the A data into the new B.