从哈希图提取元素并更新它

Question

If I extract an element from a hash map through the method get(<key>) and update the extracted element, will these updates persist in the map? Or do I have to re-insert the element back to the hash map?

Answer 1

If you change fields of the object you got out, like this...

Thing thing = map.get(key);
thing.setOtherThing(yetAnotherThing);

then that'll update the value in the map.

On the other hand, if you modify the reference that you obtained by getting a value out of the map...

Thing thing = map.get(key);
thing = doSomethingWith(thing);

then you need to put it back into the map.

Answer 2

If you modify the object obtained by the Map.get(K) method, the object does not need to be re-inserted. However, if you change a key in a way that the hashCode() function is affected, then you need to remove the map entry before modifying the key and then you can put back your value using your new key.

Consider the following map corruption case:

Map<List<String>, String> map = new HashMap<List<String>, String>();

List<String> key1 = new ArrayList<String>();
key1.add("key1");

map.put(key1, "value1");
System.out.println(map.get(key1)); //prints "value1"

key1.add("buzz2");
System.out.println(map.get(key1)); //prints "null"

List<String> k = map.keySet().iterator().next();
System.out.println(map.get(k)); //prints "null"

Morale of the story: for maps, always use immutable keys like String or int.