Main differences between Internal and External Iterator are...

1 - An Internal Iterator controls the iteration itself. In an External Iterator collection controls the iteration.

2 - Internal Iterator can iterate elements in individually as well as in

3 - Bulk (like forEach). External iterator iterates element one by one.

4 - Internal Iterator does not have to iterate elements only sequentially. External Iterator always iterates sequentially.

5 - Internal Iterator supports declarative programming style that goes well with functional programming. External Iterator follows imperative style OOPS programming.

6 - Some people consider Internal Iterator code more readable than that of External Iterator.