Question: I've just read the **SUN java code conventions**; very nice document btw. And I've read this

> 
> 6.3
>  Initialization: 
>  Try to initialize local variables where they’re declared. The only reason not to initialize a
>  variable where it’s declared is if the initial value depends on some computation occurring first.
> 
> 
> 

And I was wondering if `Class variables` are having same suggestion or not, for example I have:

```
public class NNmatrix {

    protected ArrayList<ArrayList<Double>> matrix;     // line 1
    public NNmatrix() {
        matrix = new ArrayList<ArrayList<Double>>();     // line 2
    }
    /**
     * 
     * @param otherMtrx
     */
    public NNmatrix(final ArrayList<ArrayList<Double>> otherMtrx) {
        final int rows = otherMtrx.size();
        matrix = new ArrayList<ArrayList<Double>>(rows);  // line3
        for (int i = 0; i < rows; i++) {
            this.matrix.add(new ArrayList<Double>(otherMtrx.get(i)));
        }
    }
}

```

EDITING CODE# If I would initialize the `variable` where it's declared (in class), I would **remove** "line 2" and **leave** "line 3" because performance issue# reserving (rows) in memory as you know. 

The question is: 

1. Is doing that a good practice or initialization matter only apply for local variables inside methods etc only?
2. If it's fine, I want to know which will come first if I did the EDITING CODE# the initialization @ line 3 or initialization @ line 1 ?

Answer: 
The line you quoted refers only to local variables. Because in Java local variables don't need to be declared at the top of the scope but can be declared anywhere before they are used, it makes a lot of sense.

For class and instance variables, my personal preference is to initialize the variable where it is declared. In many cases where I don't have any other constructors than the default this removes the need to have a default constructor written as the compiler will automatically provide one. I find this cleans up and shortens my code.

In the second case constructor you provided you can make a good case for initializing in the constructor.

With class variables I have found few times where I would want to initialize in the instantiation block instead of inline at the declaration.
You have a couple different options that probably all fall under micro-optimizations. Typically these types of optimizations are of little concern unless you are working with static variables in a concurrent environment or doing something funky like pooling classes. 

1. There's no reason why you cant initialize your variables in your constructor or other methods similar to what is outlined [here](http://download.oracle.com/javase/tutorial/java/javaOO/initial.html). Personally, I prefer to initialize my variables at their declaration whenever possible.
2. From my understanding, initialization will occur at roughly the same time if occurring in the constructor vs at declaration. I've never had to know which actually comes first in practice, but you could do a simple System.out test to find out.