Key existence check in HashMap

Question

Is checking for key existence in HashMap always necessary?

I have a HashMap with say a 1000 entries and I am looking at improving the efficiency. If the HashMap is b

Answer 1

You can also use the computeIfAbsent() method in the HashMap class.

In the following example, map stores a list of transactions (integers) that are applied to the key (the name of the bank account). To add 2 transactions of 100 and 200 to checking_account you can write:

HashMap> map = new HashMap<>();
map.computeIfAbsent("checking_account", key -> new ArrayList<>())
   .add(100)
   .add(200);

This way you don't have to check to see if the key checking_account exists or not.

• If it does not exist, one will be created and returned by the lambda expression.
• If it exists, then the value for the key will be returned by computeIfAbsent().

Really elegant!