HashMap Cheat Sheet: A Quick Reference for Java Developers

1. Definition:

• HashMap is a part of the Java Collections Framework.
• It implements the Map interface and stores data in key-value pairs.

2. Key Features:

• It allows null values and a single null key.
• It does not guarantee the order of elements.

3. Key and Value:

• Each key in a HashMap must be unique.
• Values can be duplicated.

4. Performance:

• Provides constant-time performance for basic operations like get and put, on average.
• The actual performance depends on factors like the capacity and hash function quality.

5. Methods:

• put(key, value): Inserts a key-value pair into the HashMap.
• get(key): Retrieves the value associated with the specified key.
• remove(key): Removes the key and its associated value.
• containsKey(key): Checks if the HashMap contains a specific key.
• containsValue(value): Checks if the HashMap contains a specific value.
• size(): Returns the number of key-value pairs in the HashMap.
• keySet(): Returns a Set of all keys in the HashMap.

6. Iterating Over HashMap:

• You can iterate over the keys, values, or entries (key-value pairs) using iterators or the enhanced for loop.

7. Load Factor and Capacity:

• The load factor determines when the HashMap should resize.
• The default load factor is 0.75, and the default initial capacity is 16.

8. Collisions:

• Hash collisions occur when multiple keys hash to the same index.
• HashMap uses a linked list (or a red-black tree in Java 8 and later) to handle collisions.

9. Synchronization:

• HashMap is not synchronized, meaning it is not thread-safe.
• For thread-safe operations, consider using ConcurrentHashMap or synchronizing the HashMap externally.

10. Recommended Use:

• HashMap is commonly used when you need a fast, efficient way to look up values based on keys.
• For thread-safe and concurrent applications, consider alternatives like ConcurrentHashMap or synchronization.