eclipse java

How does Eclipse IDE quick fix display suggestions?

I’m interested in how the eclipse ide can display suggestions to you and display the results so quickly. I work with a huge codebase with 1000’s and 1000’s of classes and I thought there would be some lag in suggesting misspelt class names for example.

How does it search for possible suggestions?
Why is it so quick?
Does it index all possibly situations to it can be quickly accessed?

Just something thats interests me because I use eclipse everyday 🙂