When Should I Use `HashMap` vs. `Map` in Java?

Differences Between HashMap and Map in Java

When creating a map collection in Java, we typically use the HashMap class. However, we can also declare it as a Map interface. This raises the question: what's the difference between these two approaches?

The key distinction lies in the type of access we have to the object. Using HashMap creates an object with a HashMap interface, while Map provides the same object with a Map interface. The underlying object remains the same in both cases.

The advantage of using the Map interface is the ability to change the underlying implementation without breaking the contract with the code that uses it. If the map is declared as a HashMap, any change in the implementation requires updating the contract.

To illustrate, consider the example of a class Foo that contains internal maps shared with subclasses. If Foo is initialized with HashMaps and a subclass method processes both things and moreThings using the declared HashMap type, changing Foo to use TreeMap would break the code.

However, if Foo had declared things and moreThings as Map instead of HashMap, both Foo and the subclass would remain unaffected even if the implementation is changed to TreeMap.

Therefore, unless there's a specific reason to use the specific implementation, it's considered best practice to code to the most general interface (Map in Java). This approach enhances flexibility and ensures resilience against implementation changes.

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