code: /*
   * Copyright (c) 2016-present, RxJava Contributors.
   *
   * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in
   * compliance with the License. You may obtain a copy of the License at
   *
   * http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software distributed under the License is
   * distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See
   * the License for the specific language governing permissions and limitations under the License.
   */
  
  package io.reactivex.rxjava3.internal.operators.observable;
  
  import java.util.NoSuchElementException;
  
  import io.reactivex.rxjava3.core.*;
  import io.reactivex.rxjava3.disposables.Disposable;
  import io.reactivex.rxjava3.internal.disposables.DisposableHelper;
  import io.reactivex.rxjava3.plugins.RxJavaPlugins;
  
  public final class ObservableSingleSingle<T> extends Single<T> {
  
      final ObservableSource<? extends T> source;
  
      final T defaultValue;
  
      public ObservableSingleSingle(ObservableSource<? extends T> source, T defaultValue) {
          this.source = source;
          this.defaultValue = defaultValue;
      }
  
      @Override
      public void subscribeActual(SingleObserver<? super T> t) {
          source.subscribe(new SingleElementObserver<>(t, defaultValue));
      }
  
      static final class SingleElementObserver<T> implements Observer<T>, Disposable {
          final SingleObserver<? super T> downstream;
  
          final T defaultValue;
  
          Disposable upstream;
  
          T value;
  
          boolean done;
  
          SingleElementObserver(SingleObserver<? super T> actual, T defaultValue) {
              this.downstream = actual;
              this.defaultValue = defaultValue;
          }
  
          @Override
          public void onSubscribe(Disposable d) {
              if (DisposableHelper.validate(this.upstream, d)) {
                  this.upstream = d;
                  downstream.onSubscribe(this);
              }
          }
  
          @Override
          public void dispose() {
              upstream.dispose();
          }
  
          @Override
          public boolean isDisposed() {
              return upstream.isDisposed();
          }
  
          @Override
          public void onNext(T t) {
              if (done) {
                  return;
              }
              if (value != null) {
                  done = true;
                  upstream.dispose();
                  downstream.onError(new IllegalArgumentException("Sequence contains more than one element!"));
                  return;
              }
              value = t;
          }
  
          @Override
          public void onError(Throwable t) {
              if (done) {
                  RxJavaPlugins.onError(t);
                  return;
              }
              done = true;
              downstream.onError(t);
          }
  
          @Override
          public void onComplete() {
              if (done) {
                  return;
              }
              done = true;
              T v = value;
              value = null;
              if (v == null) {
                  v = defaultValue;
              }
  
              if (v != null) {
                  downstream.onSuccess(v);
              } else {
                  downstream.onError(new NoSuchElementException());
              }
          }
      }
  }
  