Builder: a builder computes a derived field once at build()

The builder only collects raw inputs; the expensive or derived computation (the total here) happens exactly once, inside the product's constructor when build() calls it. That avoids recomputing the total every time an item is added.

Code

class Invoice {
    final List<Double> items;
    final double total;
    private Invoice(List<Double> items) {
        this.items = items;
        double sum = 0;
        for (double v : items) sum += v;
        this.total = sum;
    }
    static class Builder {
        private final List<Double> items = new ArrayList<>();
        Builder addItem(double price) { items.add(price); return this; }
        Invoice build() { return new Invoice(List.copyOf(items)); }
    }
}
Invoice invoice = new Invoice.Builder().addItem(9.5).addItem(20.0).addItem(4.5).build();
System.out.println("Total computed once at build: " + invoice.total);

Output

Total computed once at build: 34.0