In Java 8, how do I transform a Map to another Map using a lambda?

Protection from Telekinesis: Defensive Copy

When your map contains custom objects, create defensive copies to protect the original map from unexpected mutations:

Map<K, CustomObject> copiedMap = originalMap.entrySet()
    .stream()
    .collect(Collectors.toMap(
        Map.Entry::getKey, // Stickin' to the original key
        entry -> new CustomObject(entry.getValue()) // You shall not mutate! - Gandalf probably
));


Here, calling a copy constructor guarantees every value in copiedMap is a separate instance, untouched by those pesky mutations.

Power-level Over 9000: Efficiency with Streams

Working with larger datasets? Using Stream API teamed with Collectors.toMap() is like becoming Super Saiyan in terms of performance. But watch out for those transformations:

• Don't get too fancy with transformations inside the stream, unless you wanna go Kaioken and suffer the drawbacks (looking at you Goku!)
• Supercharge your dataset processing with Parallel streams.
• Method references are not only easier on the eyes, but can also boost performance (ah, clean and efficient - just like Vegeta's fighting style!).

Mutations, Not Radiation: In-place Updates

Consider Map.replaceAll() to mutate the values in place:

originalMap.replaceAll((key, value) -> yourUpdate(value));


This avoids the cost of a new map and may save the day when suffocating under memory issues. It does however, perform a Scorpio-style Mortal Kombat finisher on the original Map.

Enhance! Guava's Special Technique

When transformation gets complex, call in Guava for backup:

Map<K, V> lazyMap = Maps.transformValues(originalMap, value -> new Value(value));


Behold, Maps.transformValues, Guava's special move that offers a lazy view of the transformed map without actually creating a new one.

Best Practices Guide for Map-transforming Ninja

Crystal Clear Code with Method References

Ditch lambdas and use method references that give the code the perfect Zen balance:

.collect(Collectors.toMap(
    Entry::getKey, 
    CustomObject::new // Spawning new objects like Agent Smith!
));


Copy-cat or Clone? Choose wisely

If your values are mutable objects, don't end up with a Spiderman-Clone-Saga situation. If you're thinking of a shallow copy with clone(), ensure the objects support it and make sure a clone is really what you need.

No Rehashing While Working at Hash's Diner

Methods like HashMap.clone() may give you the Hash House blues by causing internal rehashing which can hit performance. Stick to stream transformations where you can.

Assertions: Your Guard Against Identity Theft

Are those new instances real or fake? You can assert your defensive copies are genuine:

assertNotSame("Defensive copy, you had one job!", originalMap.get(key), newMap.get(key));
