When working with Java, it is common to use different data structures to store and manage elements efficiently. Two commonly used data structure classes are HashMap and LinkedHashMap. While both classes are used to store key-value pairs, they have some key differences in terms of ordering and performance. In this article, we will explore the differences between HashMap and LinkedHashMap in Java.

HashMap

HashMap is an implementation of the Map interface that provides constant-time performance for basic operations, such as inserting and retrieving elements. The key feature of HashMap is that it does not maintain any specific order of the elements. It uses a technique called hashing to store and retrieve elements based on the hash code of their keys.

Here are some key features of HashMap:

• Fast retrieval: HashMap’s performance is optimal for retrieving elements based on their keys.
• No ordering: The elements in a HashMap are not stored in any particular order.
• Null keys and values: HashMap allows null as both key and value.
• Thread-safe: HashMap is not synchronized and thus not thread-safe. If multiple threads access a HashMap concurrently, it must be synchronized externally.

LinkedHashMap

LinkedHashMap, on the other hand, is a subclass of HashMap that maintains the insertion order of the elements. It adds a doubly-linked list to the underlying implementation, which helps in maintaining the order of elements.

Here are some key features of LinkedHashMap:

• Maintains insertion order: LinkedHashMap preserves the order in which elements are added to it.
• Iteration order: The iteration order of a LinkedHashMap is the same as the insertion order.
• Slower performance: LinkedHashMap has slightly slower performance compared to HashMap due to the additional operations to maintain the insertion order.
• Null keys and values: Like HashMap, LinkedHashMap allows null as both key and value.
• Thread-safe: LinkedHashMap is not synchronized and needs to be externally synchronized for concurrent access.

When to use HashMap or LinkedHashMap?

Now that we have understood their differences, the choice between HashMap and LinkedHashMap depends on your specific use case. Here are some guidelines:

• If you don’t care about the order of elements and need the best performance for inserting, retrieving, and deleting elements, HashMap is a good choice.
• If you need to iterate or access elements in the order they were added, LinkedHashMap is more suitable.
• If you need a thread-safe version, you can use ConcurrentHashMap instead of HashMap or LinkedHashMap.

In conclusion, HashMap and LinkedHashMap are both useful data structures in Java, but with different characteristics. Understanding their differences will help you choose the right one for your specific requirement.