代码示例：

//map 1 has 3 keys
HashMap<Integer, String> map1 = new HashMap<>();
map1.put(1, "A");
map1.put(2, "B");
map1.put(3, "C");
//map 2 has 4 keys
HashMap<Integer, String> map2 = new HashMap<>();
map2.put(1, "A");
map2.put(2, "B");
map2.put(3, "C");
map2.put(4, "C");
// 先保存所有key，再移除其中一个的
HashSet<Integer> unionKeys = new HashSet<>(map1.keySet());
unionKeys.addAll(map2.keySet());
unionKeys.removeAll(map1.keySet());
System.out.println(unionKeys);

输出:

[4]