In Java collections, when should you prefer using a HashMap over a TreeMap, and when is a TreeMap the better choice?

Introduction / Context:
Java provides several Map implementations, including HashMap and TreeMap, each with different performance characteristics and ordering behaviour. Choosing between them is a common design decision when building data structures for applications. Interviewers often ask when you would use HashMap versus TreeMap to check whether you understand the trade off between constant time operations and maintaining a sorted order of keys.

Given Data / Assumptions:

• We are using the java.util.HashMap and java.util.TreeMap classes.
• We care about both performance and whether the keys are kept in sorted order.
• Both maps store key value pairs and do not allow duplicate keys.
• Thread safety is not provided by either implementation by default.

Concept / Approach:
HashMap is backed by a hash table. It offers average time complexity close to O(1) for get and put operations, assuming a good hash function and low collision rate. However, it does not maintain any order of keys; iteration order is effectively arbitrary and may change when the map is modified. TreeMap, by contrast, is typically implemented as a Red Black tree and maintains keys in sorted order based on their natural ordering or a provided Comparator. Operations like get and put run in O(log n) time. Therefore, you choose HashMap when you want fast access and do not need sorted keys, and TreeMap when you specifically require sorted traversal, range queries, or ordered operations.

Step-by-Step Solution:
Step 1: Consider the use case where you have frequent lookups by key and do not care about the order in which keys are iterated. Here, HashMap provides excellent performance with average constant time operations. Step 2: Consider a second use case where you need to iterate over keys in sorted order, perform range queries (such as all keys between A and M), or retrieve the smallest or largest key frequently. TreeMap supports these operations efficiently because it maintains a sorted key structure. Step 3: Recognise that HashMap does not guarantee any particular iteration order, while TreeMap explicitly guarantees sorted order. Step 4: Note that neither HashMap nor TreeMap is thread safe by default; you need extra wrappers or concurrent variants for thread safety. Step 5: Match this understanding with the option that contrasts fast, unordered lookups (HashMap) with sorted key order (TreeMap).

Verification / Alternative check:
You can verify this by coding a simple example that inserts keys into a HashMap and a TreeMap and then iterates over the entries. The HashMap will produce keys in an order that appears random or dependent on hash codes, while the TreeMap will list keys sorted according to their natural ordering or the Comparator you provided. Additionally, measuring performance with a large number of keys will show that TreeMap operations grow roughly with log n, while HashMap operations remain near constant time under good conditions. These observations confirm the appropriate choice criteria described above.

Why Other Options Are Wrong:
Option Use HashMap only for primitive keys and TreeMap only for String keys: Both maps work with object keys; primitives must be boxed, and there is no such limitation. Option Use HashMap when you need thread safety and TreeMap when you need non thread safe access: Neither implementation is thread safe by default; thread safety requires additional mechanisms. Option Use HashMap for small collections and TreeMap for large collections, regardless of ordering needs: Collection size alone is not the main decision factor; ordering and operation complexity are more important.

Common Pitfalls:
A common mistake is to choose TreeMap without needing sorted order, which may introduce unnecessary overhead compared to HashMap. Another pitfall is relying on the iteration order of HashMap, which is not stable between runs or modifications unless you use LinkedHashMap instead. Developers may also confuse thread safety with the type of map; for concurrency, classes like ConcurrentHashMap are more appropriate. For exam answers, clearly state that HashMap is for fast, unordered access and TreeMap is for sorted key access.

Final Answer:
You should Use HashMap when you need fast key based lookup without any ordering, and use TreeMap when you need keys sorted in natural or custom order.