How to Convert List to Map?

java

prompt-engineering

collections

hashmap

byNikita Barsukov·Sep 21, 2024

Effortlessly migrate from a List to a Map using the JavaScript Hulk, I mean Java, specifically Streams and Collectors.toMap(). Yes, it's that powerful! You need to specify the key and value for your Map. For a List of objects, use a fitting attribute as the key and our superhero combatant, the object itself, as the value:

List<YourObject> list = // ...;
Map<PropertyType, YourObject> map = list.stream()
    .collect(Collectors.toMap(YourObject::getProperty, Function.identity())); // "I am Ironman" of functions

YourObject::getProperty is the key extractor, and Function.identity() is literally Tony Stark of functions saying "I am Ironman", means the object is the value. For those tricky duplicate keys, no worries, we've got an Infinity Gauntlet for them:

Map<PropertyType, YourObject> map = list.stream()
    .collect(Collectors.toMap(YourObject::getProperty, Function.identity(), (first, second) -> first)); // "I am inevitable"

Handling duplicates like a pro

Ever had twins at home? Well, non-unique keys might feel the same in your List. To handle these mischievous little devils, we need a merging function. Say, Santa has two lists of gifts for the same kid, merge them:

Map<PropertyType, Integer> map = list.stream()
    .collect(Collectors.toMap(YourObject::getKey, YourObject::getValue, Integer::sum)); // "Thank you, Santa!"

This is where Santa unifies his lists -- Integer::sum works as a universal gift distributor for duplicate keys.

Mind the Map

It's all fun and games until someone gets lost. Don't let your Map be a haunted maze. Choose wisely between HashMap, TreeMap or LinkedHashMap based on your needs — every map is unique, just like you.

Map<PropertyType, YourObject> linkedHashMap = list.stream()
    .collect(Collectors.toMap(YourObject::getProperty, Function.identity(),
           (existing, replacement) -> existing, LinkedHashMap::new)); // "Just like grandma's recipe"

Speed is key

Slow and steady wins the race, unless you're coding. Fret not, performance can be improved by initializing a HashMap with the size from the List — like knowing how much pasta to cook based on the number of guests.

Alternatives exist

Did you know Google Guava Collections is like the "Avengers Assemble" moment for collections? They also provide Maps.uniqueIndex() for simple List to Map conversions:

ImmutableMap<PropertyType, YourObject> map = Maps.uniqueIndex(list, YourObject::getProperty); // "Guava to the rescue"

It's like Captain America's secret weapon in your toolbox.

Embrace complexity

Feel like a mad scientist with complex objects? Fret not! Custom lambda expressions can be your "Avengers Assemble" moment.

Map<String, Integer> personMap = persons.stream()
    .collect(Collectors.toMap(
        p -> p.getName() + " " + p.getSurname(), // The Avengers' roll call
        Person::getAge, // Their age, not power level
        (age1, age2) -> age1 // Let's not mess with time here
    ));

To Map or not to Map

At times, one must choose between the agility of a Panther (List) and the wisdom of an Owl (Map). Maps work out better for frequent reads, but remember, with great power comes great memory overhead.