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

my personal "rule" (hardly ever broken) is to:

• declare all variables at the start of a block
• make all variables final unless they cannot be
• declare one variable per line
• never initialize a variable where declared
• only initialize something in a constructor when it needs data from the constructor to do the initialization

So I would have code like:

public class X
{
    public static final int USED_AS_A_CASE_LABEL = 1; // only exception - the compiler makes me
    private static final int A;
    private final int b;
    private int c;

    static 
    { 
        A = 42; 
    }

    {
        b = 7;
    }

    public X(final int val)
    {
        c = val;
    }

    public void foo(final boolean f)
    {
        final int d;
        final int e;

        d = 7;

        // I will eat my own eyes before using ?: - personal taste.
        if(f)
        {
            e = 1;
        }
        else
        {
            e = 2;
        }
    }
}

This way I am always 100% certain where to look for variables declarations (at the start of a block), and their assignments (as soon as it makes sense after the declaration). This winds up potentially being more efficient as well since you never initialize a variable with a value that is not used (for example declare and init vars and then throw an exception before half of those vars needed to have a value). You also do not wind up doing pointless initialization (like int i = 0; and then later on, before "i" is used, do i = 5;.

I value consistency very much, so following this "rule" is something I do all the time, and it makes it much easier to work with the code since you don't have to hunt around to find things.

Your mileage may vary.