What is the fundamental difference between HashMap and TreeMap?

Main differences between HashMap and TreeMap in Java:

• Data Structure:

  • HashMap is based on a hash table.
  • TreeMap is based on a red-black tree (a balanced search tree).

• Order of Elements:

  • HashMap does not guarantee the order of elements.
  • TreeMap stores elements in sorted order by key.

• Operation Complexity:

  • HashMap: insert, delete, and search operations are on average O(1).
  • TreeMap: insert, delete, and search operations are O(log n).

• Key Requirements:

  • In HashMap, keys must correctly implement hashCode() and equals() methods.
  • In TreeMap, keys must be comparable (implement Comparable) or a Comparator must be provided.

• Usage:

  • HashMap is suitable when access speed is important and order does not matter.
  • TreeMap is suitable when a sorted set of data by key is needed.

Example:

Map<String, Integer> hashMap = new HashMap<>();
Map<String, Integer> treeMap = new TreeMap<>();

hashMap.put("banana", 2);
hashMap.put("apple", 5);
hashMap.put("orange", 3);

// Order is not guaranteed
System.out.println(hashMap.keySet()); // e.g., [orange, banana, apple]

// In TreeMap, keys are sorted
System.out.println(treeMap.keySet()); // [apple, banana, orange]