What is the difference between HashMap and LinkedHashMap data structures in Java?

HashMap and LinkedHashMap are implementations of the Map interface in Java, but they have a key difference in the order of element storage:

• HashMap does not guarantee the order of elements. Key-value pairs are stored without a specific order, which may change when adding or removing elements.

• LinkedHashMap maintains the insertion order of elements. This is achieved by using a linked list that preserves the order of key insertion. LinkedHashMap can also be configured for access order, which is useful, for example, for implementing caches.

Example:

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

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

System.out.println(hashMap.keySet());       // Order may be arbitrary
System.out.println(linkedHashMap.keySet()); // Order: [one, two, three]

Thus, if the order of elements is important, it is recommended to use LinkedHashMap.