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Dahua ICC Open API SDK for Java
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package com.dahuatech.hutool.core.collection;

import java.util.AbstractSet;
import java.util.Collection;
import java.util.Iterator;
import java.util.concurrent.ConcurrentHashMap;

/**
 * 通过{@link ConcurrentHashMap}实现的线程安全HashSet
 *
 * @author Looly
 * @param  元素类型
 * @since 3.1.0
 */
public class ConcurrentHashSet extends AbstractSet implements java.io.Serializable {
  private static final long serialVersionUID = 7997886765361607470L;

  /** 持有对象。如果值为此对象表示有数据，否则无数据 */
  private static final Boolean PRESENT = true;

  private final ConcurrentHashMap map;

  // ----------------------------------------------------------------------------------- Constructor
  // start
  /**
   * 构造

   * 触发因子为默认的0.75
   */
  public ConcurrentHashSet() {
    map = new ConcurrentHashMap<>();
  }

  /**
   * 构造

   * 触发因子为默认的0.75
   *
   * @param initialCapacity 初始大小
   */
  public ConcurrentHashSet(int initialCapacity) {
    map = new ConcurrentHashMap<>(initialCapacity);
  }

  /**
   * 构造
   *
   * @param initialCapacity 初始大小
   * @param loadFactor 加载因子。此参数决定数据增长时触发的百分比
   */
  public ConcurrentHashSet(int initialCapacity, float loadFactor) {
    map = new ConcurrentHashMap<>(initialCapacity, loadFactor);
  }

  /**
   * 构造
   *
   * @param initialCapacity 初始大小
   * @param loadFactor 触发因子。此参数决定数据增长时触发的百分比
   * @param concurrencyLevel 线程并发度
   */
  public ConcurrentHashSet(int initialCapacity, float loadFactor, int concurrencyLevel) {
    map = new ConcurrentHashMap<>(initialCapacity, loadFactor, concurrencyLevel);
  }

  /**
   * 从已有集合中构造
   *
   * @param iter {@link Iterable}
   */
  public ConcurrentHashSet(Iterable iter) {
    if (iter instanceof Collection) {
      final Collection collection = (Collection) iter;
      map = new ConcurrentHashMap<>((int) (collection.size() / 0.75f));
      this.addAll(collection);
    } else {
      map = new ConcurrentHashMap<>();
      for (E e : iter) {
        this.add(e);
      }
    }
  }
  // ----------------------------------------------------------------------------------- Constructor
  // end

  @Override
  public Iterator iterator() {
    return map.keySet().iterator();
  }

  @Override
  public int size() {
    return map.size();
  }

  @Override
  public boolean isEmpty() {
    return map.isEmpty();
  }

  @Override
  public boolean contains(Object o) {
    return map.containsKey(o);
  }

  @Override
  public boolean add(E e) {
    return map.put(e, PRESENT) == null;
  }

  @Override
  public boolean remove(Object o) {
    return map.remove(o) == PRESENT;
  }

  @Override
  public void clear() {
    map.clear();
  }
}