in java hashmap implementation key is first assigned to object and then compared

Question

Looking java's hashmap implementation, not able to understand the reason behind some lines. In below code copied from here, in line 365-367, I am not able to understand why have they done the assignment of e.key to k first and then compared == with key [ (k = e.key) == key ]. Why not directly do (e.key == key) . This pattern appears several times in code.

359 
360     final Entry<K,V> getEntry(Object key) {
361         int hash = (key == null) ? 0 : hash(key.hashCode());
362         for (Entry<K,V> e = table[indexFor(hash, table.length)];
363              e != null;
364              e = e.next) {
365             Object k;
366             if (e.hash == hash &&
367                 ((k = e.key) == key || (key != null && key.equals(k))))
368                 return e;
369         }
370         return null;
371     }

Answer 1

This is probably a matter of optimization. Calling e.key adds a level of indirection (as you use the reference on e to get a reference on key). The variable k allows to have a shortcut for e.key and avoids using this unnecessary indirection twice. The implementation also directly use the result of the assignation k = e.key instead of assigning the value to k and then compare k to key.

I do not know if the impact of such an optimization is significant (assigning a new variable vs indirect access). It is probably tricky to evaluate, and it may be dependent on the JVM (as different JVM may perform different optimization on the code).

As HashMap is widely used in Java, I guess the implementation aims to offer maximum performance without expecting any optimization from the execution environment; hence, the common use of this pattern.