HashMap, TreeMap

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HashMap, TreeMap

HashMap and TreeMap are two important classes in Java used to store key-value pairs, but they have different internal implementations and behaviors.

HashMap

Uses a hash table to store data.

Keys are unordered – insertion order is not maintained.

Offers constant time performance (O(1)) for put(), get(), and remove() operations in the average case.

Allows one null key and multiple null values.

Not synchronized (not thread-safe by default).

Example:

java

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HashMap<Integer, String> map = new HashMap<>();

map.put(1, "Apple");

map.put(2, "Banana");

TreeMap

Uses a Red-Black Tree (a self-balancing binary search tree).

Keys are stored in sorted (ascending) order.

Operations like put(), get(), and remove() take O(log n) time.

Does not allow null keys, but allows multiple null values.

Useful when natural ordering or custom sorting of keys is needed.

Example:

java

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TreeMap<Integer, String> map = new TreeMap<>();

map.put(3, "Mango");

map.put(1, "Apple");

Differences:

Ordering: HashMap is unordered; TreeMap is ordered.

Performance: HashMap is faster; TreeMap is slower due to sorting.

Null Key: HashMap allows one null key; TreeMap doesn't.

Use HashMap when you need fast access and ordering doesn't matter.

Use TreeMap when you need sorted data based on keys.

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