Question

From the Sun docs

Normally, you would put code to
initialize an instance variable in a
constructor.
There are two
alternatives to using a constructor to
initialize instance variables:
initialize blocks and final methods.

I could understand the use of initialize blocks. Can anyone please explain the use of final methods for instance var initialization? A non-final public setter can do this job. Why not just use them ?

Answer 1

The advantage is already described in the very same Sun tutorial you linked to:

A final method cannot be overridden in a subclass. This is discussed in the lesson on interfaces and inheritance.

This is especially useful if subclasses might want to reuse the initialization method. The method is final because calling non-final methods during instance initialization can cause problems. Joshua Bloch describes this in more detail in Effective Java(item 17 Design and document for inheritance).

The reason a non-final method is dangerous in initialization is because the instance initialization of the superclass executes before the sub class is initialized. Therefore if the non-final method is overriden in the sub class and is executed during the initialization of the superclass it may be accessing uninitialized fields of the subclass giving erroneous results.

The general rule is(quoting from Effective Java): Constructors must not invoke overridable methods, directly or indirectly.