**[HashMap](https://www.educative.io/edpresso/what-is-a-hashmap-in-java)** and **TreeMap** both store values in the form of key-value pairs. There are built-in classes, in Java, for both of them.

## Similarities
First, let's take a brief look at the similarities between them:

1. Manual synchronization​ is *required* for multi-threaded usage.
2. Both use an [*Iterator*](https://www.educative.io/edpresso/using-iterators-in-java) for traversal.
3. First incorporated in Java Development Kit 1.2.

## Differences
The following are the differences between the two classes:

<center>HashMap</center>
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* Allows *one* null as a key and *multiple* nulls as values
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* No iteration order
 
 
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* Insertion/deletion *requires* O(1) time
 
 
 
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* Implements the Map interface
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<center>TreeMap</center>
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* Nulls are *only* allowed as values
 
 
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* Iterates in the natural order or the order provided by the Comparator
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* Insertion/deletion *requires* O(log n) time, where **n** is the number of elements in the TreeMap
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* Implements NavigableMap interface
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HashMap vs. TreeMap in Java

Similarities

Differences

Hierarchy