[Collection] HashSet and ArrayList lookup Contains() time complexity

HashSet和ArrayList的查找Contains()时间复杂度

ArrayListEssence is through the array,Find if an element contains to use traversal,时间复杂度是O(N)

HashSetHashSet的查找是通过HashMap的KeySet来实现的,Determine whether contains an element of true,时间复杂度是O(1)

//ArrayListDetermine whether contains an element of the source code to achieve：

public boolean contains(Object o) {
    
  return indexOf(o) >= 0;
}

public int indexOf(Object o) {
    
  if (o == null) {
    
    for (int i = 0; i < size; i++)
      if (elementData[i]==null)
        return i;
  } else {
    
    for (int i = 0; i < size; i++) //从头遍历
      if (o.equals(elementData[i]))
        return i;
  }
  return -1;
}

//HashSetDetermine whether contains an element of the source code to achieve：
public boolean contains(Object o) {
    
  return map.containsKey(o);
}

public boolean containsKey(Object key) {
    
  return getNode(hash(key), key) != null;
}

final Node<K,V> getNode(int hash, Object key) {
    
  Node<K,V>[] tab; Node<K,V> first, e; int n; K k;
  if ((tab = table) != null && (n = tab.length) > 0 &&
      (first = tab[(n - 1) & hash]) != null) {
     //直接通过hash确定元素位置,不用从头遍历
    if (first.hash == hash && // always check first node
        ((k = first.key) == key || (key != null && key.equals(k))))
      return first;
    if ((e = first.next) != null) {
    
      if (first instanceof TreeNode)
        return ((TreeNode<K,V>)first).getTreeNode(hash, key);
      do {
    
        if (e.hash == hash &&
            ((k = e.key) == key || (key != null && key.equals(k))))//Some cases may continue to traverse the list to locate
          return e;
      } while ((e = e.next) != null);
    }
  }
  return null;
}