title: HashMap get/put complexity
body: <p>We are used to saying that <code>HashMap</code> <code>get/put</code> operations are O(1). However it depends on the hash implementation. The default object hash is actually the internal address in the JVM heap. Are we sure it is good enough to claim that the <code>get/put</code> are O(1)?</p>
  <p>Available memory is another issue. As I understand from the javadocs, the <code>HashMap</code> load factor should be 0.75. What if we do not have enough memory in JVM and the load factor exceeds the limit?</p>
  <p>So, it looks like O(1) is not guaranteed. Does it make sense or am I missing something?</p>