What does deterministic mean?

Question

I am reading the Java Hashmap documentation but I don't understand this sentence.

Note that the iteration order for HashMap is non-deterministic. If you want deterministic iteration, use LinkedHashMap.

What does deterministic mean?

Answer 1

The simplest definition:

Given the same inputs, you always get the same outputs.

Above, it's saying that iterating through the exact same HashMap may give different results at different times, even when you haven't changed anything. Usually that doesn't matter, but if it does, you should use a LinkedHashMap.

Answer 2

In an order which can be "determined" in advance.

Because of the way hashing works, the elements in the map are "scrambled" into arbitrary locations. The scrambling positions cannot easily be determined in advance -- they aren't determinable -- you don't know the resulting order.