java - 包含自身作为值的 map ；

Question

直接来自 this java文档:

A special case of this prohibition is that it is not permissible for a map to contain itself as a key. While it is permissible for a map to contain itself as a value, extreme caution is advised: the equals and hashCode methods are no longer well defined on such a map.

为什么 hashcode 和 equals 在这样的 map 上不再明确定义？

Answer 1

大多数 Map 实现使用的相关部分形成 AbstractMap.equals:

            Iterator<Entry<K,V>> i = entrySet().iterator();
            while (i.hasNext()) {
                Entry<K,V> e = i.next();
                K key = e.getKey();
                V value = e.getValue();
                if (value == null) {
                    if (!(m.get(key)==null && m.containsKey(key)))
                        return false;
                } else {
                    if (!value.equals(m.get(key))) // would call equals on itself.
                        return false;
                }
            }

将 map 添加为值会导致无限循环。