Construction and initialisation order

The order is fixed and worth memorising, because a surprising number of initialisation bugs are just this sequence being misread.

1. Static fields and static blocks, once, when the class is first initialised.
2. The superclass instance initialisation, in full.
3. Instance field initialisers and instance blocks, in source order.
4. The constructor body.

The trap: calling an overridable method from a constructor

class Base {
    Base() { init(); }              // runs before Derived's fields exist
    void init() {}
}

class Derived extends Base {
    private final List<String> items = new ArrayList<>();

    @Override void init() { items.add("x"); }   // NullPointerException
}

Step 2 completes before step 3, so items is still null when the overridden init() runs. Constructors should call only private, static or final methods.

Construction guarantee

A constructor that throws leaves no usable reference behind — the object is never published. This is what makes constructor-based validation reliable, and why a final field assigned exactly once in the constructor is safe to publish across threads.