What is the difference between `HashMap`, `TreeMap`, and `LinkedHashMap` in Java?

Q: What is the difference between `HashMap`, `TreeMap`, and `LinkedHashMap` in Java?

Learn the answer to "What is the difference between `HashMap`, `TreeMap`, and `LinkedHashMap` in Java?" with detailed explanations, code examples, and best practices on DeployU.

The Scenario

You are a backend engineer at a social media company. You are writing a new service that needs to store a collection of key-value pairs. You are not sure whether to use a HashMap, a TreeMap, or a LinkedHashMap.

The Challenge

Explain the difference between a HashMap, a TreeMap, and a LinkedHashMap in Java. What are the pros and cons of each approach, and which one would you choose for this use case?

Wrong Approach

A junior engineer might think that they are interchangeable. They might not be aware of the difference in ordering or the performance implications of choosing one over the other.

Addresses symptoms, not root cause

Right Approach

A senior engineer would be able to provide a detailed explanation of the differences between a `HashMap`, a `TreeMap`, and a `LinkedHashMap`. They would also be able to explain the trade-offs between each approach and would have a clear recommendation for which one to use in this use case.

Step 1: Understand the Key Differences

Feature	`HashMap`	`TreeMap`	`LinkedHashMap`
Ordering	Unordered.	Ordered by key.	Ordered by insertion order.
Performance	O(1) for `get` and `put`.	O(log n) for `get` and `put`.	O(1) for `get` and `put`.
Implementation	Backed by a hash table.	Backed by a red-black tree.	Backed by a hash table and a linked list.
Use Cases	When you need fast access to key-value pairs and do not care about the order.	When you need to iterate over the keys in a sorted order.	When you need to iterate over the keys in the order in which they were inserted.

Step 2: Choose the Right Tool for the Job

For our use case, it depends on whether we need to be able to iterate over the key-value pairs in a specific order.

If we do not need to iterate over the key-value pairs in any specific order, we should use a HashMap.
If we need to iterate over the key-value pairs in sorted order, we should use a TreeMap.
If we need to iterate over the key-value pairs in the order in which they were inserted, we should use a LinkedHashMap.

Systematic, production-ready debugging

Practice Question

You want to create a cache that evicts the least recently used items. Which of the following would be the most appropriate?