Hashmap questions

[PhQ]

Why are hashmaps useful?
In what kind of situations should I use them?
And how do I use hashmaps correctly?

[Fubarable]

There's already much written on this that can easily be gleaned from the internet which makes me ask:

What are your answers based on what you've read and what are your specific points of confusion?
What's the underlying reason for these questions?

[PhQ]

What's the underlying reason for these questions?

I was just wondering, someone said in their blog that they told someone how to use hashmaps.

[Fubarable]

You use them if you want a collection where you want to access an object (the value) based another object (the key). For instance if you want to get a test score (the value) that is tied to a Student (the key). HashMaps are usually very quick and efficient at retrieving things.

To use them correctly you'd read the API and check out tutorials. Otherwise that's a very vague and broad question.