Helpers and constructor traps

Helpers are the rubric's favorite. A private helper that normalizes, validates, or classifies turns a sprawling method into three clean calls — and the FRQ rubric names helper usage as a point. Design rule: if a piece of logic would be repeated in two methods, it becomes a helper.

private boolean isValid(String code) {
    return code.length() == 4 && code.charAt(0) == 'X';
}

Watch the constructor traps: shadowing (price = price), a field initialized twice, or a constructor that forgets one field — leaving it at a silent default that poisons every later method call. Trace constructors like any other code: list every field and its value after the constructor returns.

Object interaction: when one object holds another (a Register holding a Drawer), the outer object's method calls the inner one's — and null is always a state. "Nothing registered yet" usually means "a null inner object," and the first transition must guard it.