Airplane.java

public final class Airplane {

    private final int seats;
    private final int engine;
    private final int rescue;
    private final List<Instrument> instruments;

    private Airplane(Builder builder){
        this.seats = builder.seats;
        this.engine = builder.engine;
        this.rescue = builder.rescue;
        this.instruments = builder.instruments;
    }

    public static class Builder {

        private final int seats;
        private int engine;
        private int rescue;
        private List<Instrument> instruments;

        public Builder(int seats){
            this.seats = seats;
        }

        public Builder withEngine(int engine) {
            this.engine = engine;
            return this;
        }

        public Builder withRescue(int rescue) {
            this.rescue = rescue;
            return this;
        }

        // get child builder and hands over parent builder
        public Instrument.ListBuilder addList(){
            return new Instrument.ListBuilder().setAirplaneBuilder(this);
        }

        public void addList(List<Instrument> instruments){
            this.instruments = instruments;
        }

        public Airplane build(){
            return new Airplane(this);
        }
    }
}