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

    class MyClass extends FooClass {
    String a = null;

    public MyClass() {
        super();     // Superclass calls init();
    }

    @Override
    protected void init() {
        super.init();
        if (something)
            a = getStringYadaYada();
    }
}

Regarding the above,

String a = null;

null init could be avoided since anyway it's the default. However, if you were to need another default value, then, because of the uncontrolled initialization order, I would fix as follow:

class MyClass extends FooClass 
{
    String a;
    {
        if( a==null ) a="my custom default value";
    }
    ...