[java] Initialize class fields in constructor or at declaration?

Not a direct answer to your question about the best practice but an important and related refresher point is that in the case of a generic class definition, either leave it on compiler to initialize with default values or we have to use a special method to initialize fields to their default values (if that is absolute necessary for code readability).

class MyGeneric<T>
{
    T data;
    //T data = ""; // <-- ERROR
    //T data = 0; // <-- ERROR
    //T data = null; // <-- ERROR        

    public MyGeneric()
    {
        // All of the above errors would be errors here in constructor as well
    }
}

And the special method to initialize a generic field to its default value is the following:

class MyGeneric<T>
{
    T data = default(T);

    public MyGeneric()
    {           
        // The same method can be used here in constructor
    }
}