Should I instantiate instance variables on declaration or in the constructor?

Question

Is there any advantage for either approach?

Example 1:

class A {
    B b = new B();
}

Example 2:

class A {
    B b;         


        

Answer 1

There is one more subtle reason to initialize outside the constructor that no one has mentioned before (very specific I must say). If you are using UML tools to generate class diagrams from the code (reverse engineering), most of the tools I believe will note the initialization of Example 1 and will transfer it to a diagram (if you prefer it to show the initial values, like I do). They will not take these initial values from Example 2. Again, this is a very specific reason - if you are working with UML tools, but once I learned that, I am trying to take all my default values outside of constructor unless, as was mentioned before, there is an issue of possible exception throwing or complicated logic.