java beginner: initializing class variables

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 1

These are instance variables, not class variables. Instance variables belong to a specific object, class variables don't (sorry to nitpick).

I think initializing the variable where it's declared is simpler and easier to read.

The jvm starts initializing instance variables and instance initializer blocks at the top of the file and works its way down, only after it has initialized all the variables and run the initializer blocks does it execute the constructor.

Answer 2

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.