Instrument.java

public class Instrument {

    private String name;

    private Instrument(Builder builder) {
        this.name = builder.name;
    }

    public static class Builder {
        private String name;
        private ListBuilder listBuilder; // parent builder

        public Builder withName(String name) {
            this.name = name;
            return this;
        }

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

        // setter for parent builder
        public Builder setListBuilder(ListBuilder builder) {
            this.listBuilder = builder;
            return this;
        }

        // build it and get parent builder again
        public ListBuilder toList(){
            this.listBuilder.add(this.build());
            return this.listBuilder;
        }
    }

    public static class ListBuilder {

        private List<Instrument> instruments = new ArrayList<>();
        private Airplane.Builder airplaneBuilder; // parent builder

        public ListBuilder add(Instrument instrument){
            this.instruments.add(instrument);
            return this;
        }

        public List<Instrument> build(){
            return this.instruments;
        }

        // get child builder and hands over parent builder
        public Instrument.Builder add() {
            return new Instrument.Builder().setListBuilder(this);
        }

        // setter for parent builder
        public ListBuilder setAirplaneBuilder(Airplane.Builder builder){
            this.airplaneBuilder = builder;
            return this;
        }

        // build it and get parent builder again
        public Airplane.Builder done() {
            this.airplaneBuilder.addList(this.build());
            return airplaneBuilder;
        }
    }
}