在Hashmap中处理键冲突

Question

The code I found for Hashmap's put(K key, V value) . This method replaces the value if collision occured, it doesn't add the Entry to the LinkedList of the bucket.
What I am missing here?

Answer 1

You aren't missing a thing. The general contract for Java's Map s is to have a single value per key. If you want to hold multiple values per key, you'd have to use something like Apache Commons' MultiMap , or implement something similar on your own by having a Map<K, List<V>> .

Answer 2

Why do you think? The relevant lines are as follows:

Calculate index in internal table:

390 int i = indexFor(hash, table.length);

Iterate over the linked list at table[i] and search if the key is already contained:

391 for (Entry<K,V> e = table[i]; e != null; e = e.next) {
392     Object k;
393     if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {

Key is already contained => replace value...

394         V oldValue = e.value;
395         e.value = value;
396         e.recordAccess(this);

...and return old value

397         return oldValue;
398     }

otherwise continue the loop

399 }

We're at the end of the linked list and we haven't found the key yet, so it's not already contained. So let's add a new entry:

402 addEntry(hash, key, value, i);

Answer 3

You cannot have the same key several times in an HashMap . That is why the value gets replaced rather than just creating a new Map.Entry<K,V>