I am looking at the source code for HashMap in Java 7, and I see that the put method will check if any entry is already present and if it is present then it will replace the old value with the new value.
for (Entry<K,V> e = table[i]; e != null; e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k))) {
V oldValue = e.value;
e.value = value;
e.recordAccess(this);
return oldValue;
}
}
So, basically it means that there would always be only one entry for the given key, I have seen this by debugging as well, but if I am wrong then please correct me.
Now, since there is only one entry for a given key, why does the get method have a FOR loop, since it could have simply returned the value directly?
for (Entry<K,V> e = table[indexFor(hash, table.length)];
e != null;
e = e.next) {
Object k;
if (e.hash == hash && ((k = e.key) == key || key.equals(k)))
return e.value;
}
I feel the above loop is unnecessary. Please help me understand if I am wrong.
table[indexFor(hash, table.length)] is the bucket of the HashMap that may contain the key we are looking for (if it is present in the Map).
However, each bucket may contain multiple entries (either different keys having the same hashCode(), or different keys with different hashCode() that still got mapped to the same bucket), so you must iterate over these entries until you find the key you are looking for.
Since the expected number of entries in each bucket should be very small, this loop is still executed in expected O(1) time.