Main differences between a HashMap and a TreeMap in Java are:

1. Order: A HashMap does not maintain any order of its keys.

In a HashMap there is no guarantee that the element

inserted first will be retrieved first.

2. In a TreeMap elements are stored according to natural

ordering of elements. A TreeMap uses compareTo()

method to store elements in a natural order.

3. Internal Implementation: A HashMap uses Hashing

internally. A TreeMap internally uses Red-Black tree

implementation.

4. Parent Interfaces: A HashMap implements Map interface.

TreeMap implements NavigableMap interface.

5. Null values: A HashMap can store one null key and

multiple null values. A TreeMap can not contain null key

but it may contain multiple null values.

6. Performance: A HashMap gives constant time performance

for operations like get() and put(). A TreeMap gives order

of log(n) time performance for get() and put() methods.

7. Comparison: A HashMap uses equals() method to compare

keys. A TreeMap uses compareTo() method for

maintaining natural ordering.

8. Features: A TreeMap has more features than a HashMap. It

has methods like pollFirstEntry() , pollLastEntry() ,

tailMap() , firstKey() , lastKey() etc. that are not provided

by a HashMap

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