When should you prefer HashMap and when should you avoid it?

In Java, a HashMap is a part of the Java Collections Framework and is used to store data in key-value pairs. When considering whether to use HashMap, keep the following in mind:

When to Prefer HashMap

• Fast Lookups: If you need a data structure that allows for fast retrieval of data, HashMap is ideal due to its average O(1) time complexity for get and put operations.
• Unordered Collection: If the order of entries does not matter, HashMap is a good choice since it does not maintain any order of its keys or values.
• Performance: HashMap can outperform other map implementations like TreeMap or LinkedHashMap in scenarios where keys and their associations are primarily accessed.

When to Avoid HashMap

• Thread Safety: If you are working in a multi-threaded environment where concurrent access is possible, consider using ConcurrentHashMap instead, as HashMap is not synchronized.
• Ordered Collection: If you need to maintain the order of insertion, then LinkedHashMap or TreeMap would be more suitable options.
• Null Keys and Values: HashMap allows one null key and multiple null values; if null references are not acceptable in your dataset, you should choose another Map implementation.