Are there any differences between different methods in each of the following groups of element retrieve operations in LinkedList?
Returning null + removing operations: poll(), pollFirst().
Returning null + not removing operations: peek(), peekFirst().
Throwing exception + removing operations: pop(), remove(), removeFirst().
Throwing exception + not removing operations: element(), getFirst().
Similar duplications exists in insertion methods.
If there is no such difference, I would expect it to be mentioned in the javadoc of the methods (something like the good old "This is exactly like calling ..."). Is it only a sloppy documentation, or am I missing anything?
There is no difference between them, and it is listed in the documentation too, but you have to do some recursive searching to get there.
LinkedList implements two interfaces - Queue and Deque. And Deque extends from Queue.
Now, Deque has defined the method - Deque#pollFirst() and inherited the method - Queue#poll().
So, LinkedList has basically these two methods defined for the two interfaces it implements.
And about the similarity between those two methods, it is listed in documentation of Deque as:
This interface extends the Queue interface. When a deque is used as a queue, FIFO (First-In-First-Out) behavior results. Elements are added at the end of the deque and removed from the beginning. The methods inherited from the Queue interface are precisely equivalent to Deque methods as indicated in the following table:
And there is a table listing the methods of Queue class and the equivalent Deque method. See Deque#poll(), Deque#peek() for e.g. They clearly list the equivalent method.