How do LinkedList and HashMap differ? How does LinkedHashMap differ from HashMap?

LinkedList vs HashMap:

• LinkedList is a data structure implementing a doubly linked list. It stores elements in insertion order and allows efficient addition and removal of elements at the beginning and end of the list.
• HashMap is a data structure for storing key-value pairs with fast access by key (constant time on average). The order of elements is not guaranteed.

LinkedHashMap vs HashMap:

• LinkedHashMap extends HashMap, maintaining the insertion order of elements (or access order if access-order mode is enabled).
• In HashMap, the order of elements is arbitrary and can change during operations.

Example of using LinkedHashMap to preserve insertion order:

Map<String, Integer> map = new LinkedHashMap<>();
map.put("one", 1);
map.put("two", 2);
map.put("three", 3);

for (String key : map.keySet()) {
    System.out.println(key);
}
// Output: one, two, three

In HashMap, the order of keys can be any.