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.subjects;
  
  import io.reactivex.rxjava3.annotations.*;
  import io.reactivex.rxjava3.core.*;
  
  /**
   * Represents an {@link Observer} and an {@link Observable} at the same time, allowing
   * multicasting events from a single source to multiple child {@code Observer}s.
   * <p>
   * All methods except the {@link #onSubscribe(io.reactivex.rxjava3.disposables.Disposable)}, {@link #onNext(Object)},
   * {@link #onError(Throwable)} and {@link #onComplete()} are thread-safe.
   * Use {@link #toSerialized()} to make these methods thread-safe as well.
   *
   * @param <T> the item value type
   */
  public abstract class Subject<T> extends Observable<T> implements Observer<T> {
      /**
       * Returns true if the subject has any Observers.
       * <p>The method is thread-safe.
       * @return true if the subject has any Observers
       */
      @CheckReturnValue
      public abstract boolean hasObservers();
  
      /**
       * Returns true if the subject has reached a terminal state through an error event.
       * <p>The method is thread-safe.
       * @return true if the subject has reached a terminal state through an error event
       * @see #getThrowable()
       * @see #hasComplete()
       */
      @CheckReturnValue
      public abstract boolean hasThrowable();
  
      /**
       * Returns true if the subject has reached a terminal state through a complete event.
       * <p>The method is thread-safe.
       * @return true if the subject has reached a terminal state through a complete event
       * @see #hasThrowable()
       */
      @CheckReturnValue
      public abstract boolean hasComplete();
  
      /**
       * Returns the error that caused the Subject to terminate or null if the Subject
       * hasn't terminated yet.
       * <p>The method is thread-safe.
       * @return the error that caused the Subject to terminate or null if the Subject
       * hasn't terminated yet
       */
      @Nullable
      @CheckReturnValue
      public abstract Throwable getThrowable();
  
      /**
       * Wraps this Subject and serializes the calls to the onSubscribe, onNext, onError and
       * onComplete methods, making them thread-safe.
       * <p>The method is thread-safe.
       * @return the wrapped and serialized subject
       */
      @NonNull
      @CheckReturnValue
      public final Subject<T> toSerialized() {
          if (this instanceof SerializedSubject) {
              return this;
          }
          return new SerializedSubject<>(this);
      }
  }
  