How ArrayList and HashMap Work
As you arrive
1. Get the handout
2. Snarf the code for today’s class
3. Go online to Piazza and take the “What should
Mike review Wednesday?” poll
After today you will…
• Be able to explain how HashMap and ArrayList
work
• Be able to derive/reason about the runtimes
of ArrayList and HashMap
Derive, don’t
memorize
• “There is no doubt that Marley was dead. This
must be distinctly understood, or nothing
wonderful can come of the story I am going to
relate. If we were not perfectly convinced that
Hamlet's Father died before the play began,
there would be nothing more remarkable in
his taking a stroll at night…”
Accessing the nth element of an array is O(1)
String var = myArray[1];
//THE SAME SPEED?!?
var = myArray[100000];
//Also the same speed
myArray[100000] = “Ninjas”;
Accessing elements of an array is very fast, but some other
parts of arrays are more problematic.
1. You have to decide on their size up front
2. You can only index them by consecutive integers
What if we want to expand an Array
but we have no slots left?
• Look at ArrayGrow.java
• Answer questions 1 and 2 in the handout,
then go on to 3 if you have time
What if we want to add a new element
at the beginning of the list?
Now it makes sense: ArrayList
• Add/Remove at (n-k)th element – O(k) because
you need to move all the elements one to the
right or left
• Remove at end O(1) – so long as we don’t want to
copy to a smaller array
• Add at end O(1) – sort of. 99% of the time, it is
really O(1) because we have empty slot. We
normally ignore the other 1%.
• Get/set kth element O(1) – because it’s an array!
On to hashMap
• Silly analogy
• Apartment that assigns rooms based on
phone number
Ok, on to ArrayListHash
• Like hashmap
• Answer questions 4, 4 (numbering mishap),
and 5
Now it makes sense: HashMap
• Get an element based on key O(1): Just a
lookup in an array
• Check if a key exists: Just a lookup in the array
• Set an element based on key O(1)*: another
lookup, and we won’t worry about the
situation in which we need to grow the
hashmap
Note that this only works if the hashCode
function is good
After today you will…
• Be able to explain how HashMap and ArrayList
work
• Be able to derive/reason about the runtimes
of ArrayList and HashMap
Derive, don’t
memorize