如何在方法中将多个可观测对象合并到第一个可观测对象完成并返回为新的可观测对象

import io.reactivex.rxjava3.core.Observable;
import io.reactivex.rxjava3.core.Observer;
import io.reactivex.rxjava3.disposables.Disposable;

public class MergeExample {
    public static void main(String[] args) {
        Observable<Integer> observable1 = Observable.just(1, 2, 3);
        Observable<Integer> observable2 = Observable.just(4, 5, 6);
        Observable<Integer> observable3 = Observable.just(7, 8, 9);

        Observable<Integer> mergedObservable = Observable.merge(observable1, observable2, observable3);

        mergedObservable.subscribe(new Observer<Integer>() {
            @Override
            public void onSubscribe(Disposable d) {
                // 可忽略
            }

            @Override
            public void onNext(Integer integer) {
                System.out.println("onNext: " + integer);
            }

            @Override
            public void onError(Throwable e) {
                // 可忽略
            }

            @Override
            public void onComplete() {
                System.out.println("onComplete");
            }
        });
    }
}

onNext: 1
onNext: 2
onNext: 3
onNext: 4
onNext: 5
onNext: 6
onNext: 7
onNext: 8
onNext: 9
onComplete