HashMap vs TreeMap vs LinkedHashMap: Key Differences Explained

HashMap vs TreeMap vs LinkedHashMap

Introduction

Java's Map interface, a crucial part of the Collections Framework, is where key-value pairs are stored. HashMap, TreeMap, and LinkedHashMap are three of the most extensively used Map interface implementations. Despite the fact that all three classes have the basic function of storing key-value pairs, there are significant differences in their internal implementations, performance characteristics, and use cases.

Overview of HashMap, TreeMap, and LinkedHashMap

HashMap

HashMap in Java is like the legacy Hashtable class, but it is not synchronized. In addition to allowing us to store null elements, there should only be one null key. Since Java 5, it has been expressed as HashMap, where V is the value and K is the key. It implements the Map interface and inherits the AbstractMap class.

The Map interface, which is implemented by the Java HashMap class, enables us to store key-value pairs in which the keys must differ. The element of the relevant key will be replaced if you attempt to insert the duplicate key. Data deletion and modification are made easier by the key index. The HashMap class is part of the java.util package.

Ordering: HashMap does not maintain any order of its available elements. In a hashmap, it does not preserve any order of insertion, and there is no guarantee of any order iteration.

Performance: After the evaluations of basic operations such as put() and get(), HashMap always provides constant-time performance of (O(1)). It is also assumed that the hash function distributes the keys evenly across the buckets.

Null Keys and Values: HashMap allows multiple numbers of null values but, favours only one null key.

Thread safety: For operations like thread-safe, generally we can use ConcurrentHashMap or Collections.synchronizedMap().

TreeMap

TreeMap class is meant to be the implementation of the Map interface where the backing implementation pertains mainly to Red-Black trees. It provides functional key-value storage in a sorted way, based on either the custom comparator passed at instantiation or natural ordering of the keys.

Ordering: TreeMap always maintains its keys in the sorted order of how they are compared. That can be the natural order of the keys if they implement the Comparable interface, or TreeMap can hold a custom Comparator in its constructor. By the way, if sorting in that order doesn't matter, then you're not getting all the benefits from the TreeMap data structure, but it will still work.

Performance: For basic operations get() and put(), TreeMap provides performance that is logarithmic to the size of the map because it is always balanced. Thus, performance is O(log n).

Null Keys and Values: TreeMap did not allow null keys until now, but it allows null values. This is acceptable as null can be compared against anything.

Thread Safety: This was already known before that TreeMap was not thread-safe; we would need to execute thread-safe operations using Collections.synchronizedSortedMap().

LinkedHashMap

It defines the iteration order. `LinkedHashMap` is an implementation of the Map interface. It maintains a doubly linked list traversing all of its entries.

Ordering: LinkedHashMap preserves the insertion ordering by default, but it also can preserve access ordering (when entries are accessed last, index-for index). This requires setting the accessOrder to true in the constructor.

Performance: LinkedHashMap delivers O(1) performance for get() and put() operations just like HashMap because it maintains a doubly linked list however, it has a slightly higher overhead because it must maintain the linked list.

Null Keys and Values: You can have as many null values as you want with a LinkedHashMap in addition to a single null key.

Thread Safety: LinkedHashMap is not meant for use as a thread-safe class; you may want to consider using it in composition with Collections.synchronizedMap(), though.

Key Differences Between HashMap, TreeMap, and LinkedHashMap

In this section, we will discuss the key differences between HashMap, TreeMap, and LinkedHashMap that cover points such as Internal Data Structure, Performance, Use Cases, Ordering of Elements, Null Keys and Null Values, Memory Overhead, and Thread Safety. Let’s discuss it.

Internal Data Structures

·        HashMap: Hash table and the hash map involve that key and value mapping and What bucket will be used to store the pair of key and value is decided based on the hash code received once the hashing of the key is done.

·        TreeMap: It stores key-value pairs using Red-Black Trees, which is a kind of self-balancing binary search tree. Maintaining keys in sorted order allows range queries and traversals of the keys.

·        LinkedHashMap: It is identical to that of HashMap. The difference is it maintains the insertion order. Keys along with values are maintained in a double-linked list.

 

Performance

·        HashMap: If the keys are evenly distributed and the hash function is good, a hash map offers constant-time performance (O (1)) for simple operations like get() and put(). Nevertheless, the performance may deteriorate to O(n) in the worst situation (for example, when there are numerous hash collisions).

·        TreeMap: Because of its balanced tree structure, it offers logarithmic-time performance (O (log n)) for simple operations like get() and put(). Because of this, TreeMap executes more quickly for range queries and ordered traversal but more slowly than HashMap for simple operations.

·        LinkedHashMap: Similar to HashMap, LinkedHashMap offers constant-time performance (O (1)) for simple operations like get() and put(). But because it has to keep the linked list up to date, the overhead is a little higher.

Use Cases

·        HashMap: Perfect for situations requiring quick lookups, insertions, and deletions and where element order is irrelevant. It is extensively utilized in general-purpose map implementations, indexing, and caching.

·        TreeMap: Perfect for tasks requiring sorted keys, like range queries or alphabetical dictionary maintenance. It is also helpful for tasks like locating the first or last key—or keys—within a given range.

·        LinkedHashMap: Perfect for situations where elements must be accessed or inserted in a specific order. When access order is crucial in LRU (Least Recently Used) cache implementations, it is frequently utilized.

Ordering of Elements

·        HashMap: The hash map does not maintain elemental order. Refreshing the map does not guarantee that the iteration sequence will remain the same.

·        TreeMap: Keeps the keys in their original order or according to a custom comparator. Consequently, TreeMap can be used in situations where sorted keys are necessary.

·        LinkedHashMap: By default, it keeps the entries' insertion order. If set up properly, it can also preserve access order. LinkedHashMap can therefore be used in situations where maintaining the order of insertion or access is necessary.

Null Keys and Values

·        HashMap: Supports multiple null values and a single null key.

·        TreeMap: It allows null values but not null keys because null cannot be compared.

·        LinkedHashMap: LinkedHashMap permits one null key and multiple null values, just like HashMap.

Memory Overhead

·        HashMap: Since the hash map only needs to store the hash table and the key-value pairs, it has the least amount of memory overhead of the three.

·        TreeMap: Because TreeMap needs an additional tree structure (Red-Black tree) to keep the keys sorted, its memory overhead is higher.

·        LinkedHashMap: Because LinkedHashMap uses an extra linked list to preserve the insertion or access order, it has a marginally higher memory overhead than HashMap.

Thread Safety

·        Consequently, TreeMap, LinkedHashMap, and HashMap not being thread-safe by default, the collections could be ConcurrentHashMap or synchronizedMap(). To initiate hash maps, linked hash maps, and collections in a synchronized manner, the programmers could complete the task by invoking any of the two methods. If thread safety is necessary for TreeMap, the programmers could use synchronizedSortedMap().

Example Use Cases

HashMap

·        Caching: Caching systems typically use HashMap since quick lookups are important, and element ordering is not essential.

·        Indexing: HashMap is used in indexing structures since fast access to data is required, e.g., in search engine inverted indexes.

TreeMap

·        Sorted Data: When you need to keep a dictionary or other sorted collection of key-value pairs in alphabetical order, you use a TreeMap.

·        Range Queries: A TreeMap can be used to provide the range query more effectively, finding all key values within a given range.

LinkedHashMap

·        LRU Cache: LinkedHashMap serves as the common choice for developers building LRU (Least Recently Used) caches when maintaining access order is required.

·        Maintaining Insertion Order: The LinkedHashMap data structure helps maintain the sequence of events when it is necessary to track their chronological order.

Summary of Key Differences

Feature	HashMap	TreeMap	LinkedHashMap
Internal Structure	Hash Table.	Red-Black Tree.	Hash Table + Linked List.
Performance	O(1) for operations like get/put.	O(log n) for operations like get/put.	O(1) for operations like get/put.
Use Cases	Caching, Indexing.	Sorted Data and range Queries.	LRU Cache, Insertion Order.
Ordering of Elements	No Order.	Sorted Order.	Insertion/Access Order.
Null Keys/Values	Allows one null key.	No null keys.	Allows one null key.
Thread Safety	Not thread-safe	Not thread-safe	Not thread-safe

 

Conclussion

In conclusion, while HashMap, TreeMap, and LinkedHashMap all implement the Map interface in Java, they differ in terms of ordering, performance, and underlying data structures. Choosing the right one depends on the specific needs for ordering and efficiency in your application.

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