java - 为什么在将重复项目添加到 Set 时不会出现错误？

Question

我将相同的整数两次添加到 Set，但它不会给出任何错误，尽管 Set 不允许重复。这是为什么？

 Set<Integer> set = new HashSet<Integer>();
 set.add(1);
 set.add(1);

Answer 1

Set:add 不应在您尝试添加已在 Set 中的值时给您一个错误。它只会返回 false 而不会将值添加到 Set。

检查 JavaDoc :

boolean add(E e)

Adds the specified element to this set if it is not already present (optional operation). More formally, adds the specified element e to this set if the set contains no element e2 such that (e==null ? e2==null : e.equals(e2)). If this set already contains the element, the call leaves the set unchanged and returns false. In combination with the restriction on constructors, this ensures that sets never contain duplicate elements.