Thursday, 1 October 2015

Calling super constructors

So, a constructor in a subclass, if it needs to call the constructor in the super class, is required to do this in the first line of the body of the constructor1.

If it does not, Java will insert a call to the default super constructor. If it does not exist, you get a compile time error.

So, what if you need to do things, before the super call takes place?

As the object is in the process of being constructed (that's why it's called a constructor), the object is in a non-valid state. This likely explains why the language designers felt that a call to another constructor should be the first part of the body of a constructor.

Well, apparently the only solution is to do everything you need inside the expressions that take the place of the super constructor arguments.

I was forced to do just that in my MudWebException.

So, there's an easy workaround, but it feels clumsy.

Take care not to access any of the methods or properties of the object that is being constructed, as it is in a non-valid state. As a matter of fact, I think it's a really bad idea to call methods in the object from within your constructor.


[1] JLS 8.0 Section 8.8.7. Constructor Body
StackOverflow - Why does this and super have to be the first statement in a constructor?

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