core/src/main/java/io/grpc/internal/InUseStateAggregator2.java

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package io.grpc.internal;

import java.util.HashSet;

import javax.annotation.concurrent.NotThreadSafe;

/**
 * Aggregates the in-use state of a set of objects.
 */
@NotThreadSafe
abstract class InUseStateAggregator2<T> {

  private final HashSet<T> inUseObjects = new HashSet<T>();

  /**
   * Update the in-use state of an object. Initially no object is in use.
   *
   * <p>This may call into {@link #handleInUse} or {@link #handleNotInUse} when appropriate.
   */
  final void updateObjectInUse(T object, boolean inUse) {
    int origSize = inUseObjects.size();
    if (inUse) {
      inUseObjects.add(object);
      if (origSize == 0) {
        handleInUse();
      }
    } else {
      boolean removed = inUseObjects.remove(object);
      if (removed && origSize == 1) {
        handleNotInUse();
      }
    }
  }

  final boolean isInUse() {
    return !inUseObjects.isEmpty();
  }

  /**
   * Called when the aggregated in-use state has changed to true, which means at least one object is
   * in use.
   */
  abstract void handleInUse();

  /**
   * Called when the aggregated in-use state has changed to false, which means no object is in use.
   */
  abstract void handleNotInUse();
}