How To Convert A List Into Map Inward Coffee Viii - Illustration Tutorial

One of the mutual tasks inwards Java programming is to convert a listing to map too I lead hold written nearly this inwards the yesteryear too today we'll catch how Java 8 makes this occupation easier. Btw, travel it Java seven or Java 8, yous demand to proceed something inwards heed piece converting a listing to map because they are 2 unlike information structure. For example, the listing allows duplicate elements but keys inwards a map must travel unique, the value tin travel duplicated but a duplicate primal may crusade a occupation inwards Java 8. Similarly, unopen to other draw is order. H5N1 listing is an ordered collection but the map doesn't guarantee whatever lodge unless yous create upwards one's heed to utilization LinkedHashMap, which keeps insertion lodge or TreeMap which keeps mapping inwards the sorted lodge of keys. This is i of the of import details which many Java beginners forget too and hence pass hours to chase subtle bugs.

If your programme has whatever dependency on the lodge of elements inwards the listing they volition non piece of work equally expected if yous utilization a map. So, yous should travel mindful of these full general details piece converting a listing to map inwards Java. This is truthful irrespective of the Java version.

Btw, if yous are novel to Java world, I advise yous to kickoff become through a comprehensive course of didactics on Java similar The Complete Java Masterclass on Udemy. It non alone provides organized too construction learning but also yous volition larn to a greater extent than inwards less time.  It's also i of the most up-to-date course, late updated for Java eleven features.

Anyway, let's catch the occupation at hand. Assume yous lead hold a listing of courses too yous desire to create a map where keys should travel the championship of the course of didactics too value should travel the course of didactics object itself. How volition yous exercise that?

How to convert List<V> into Map<K, V> inwards Java 7

It's slowly inwards Java 7, all yous demand to exercise is become through the List, extract a primal from each object too add together both primal too value into the map, but how volition yous exercise that inwards Java 8 mode similar yesteryear using lambda expression too streams?


We'll catch that but let's kickoff write the JDK seven version to convert a List<V> into Map<K, V>:

private Map<String, Course> toMap(List<Course> listOfCourses) {     in conclusion Map<String, Course> courses = new HashMap<>();     for (final Course electrical flow : listOfCourses) {       hashMap.put(current.getTitle(), current);     }     return courses;   }

This code is real uncomplicated too slowly to read, let's directly catch the Java 8 version to observe out whether Java 8 actually makes your life slowly when it comes to writing twenty-four hours to twenty-four hours code:

Map<String, Course > result = listOfCourses                                  .stream()                                  .collect(                                  Collectors.toMap(Course::getTitle,                                                  Function.identity()));

Wow, it only took i draw to convert a listing of objects into a map, which had taken i piece of work inwards JDK 7. So, it looks Java 8 actually makes the developer's life easy.

Anyway, let's endeavour to empathise what's going on here. Well, yous lead hold a listOfCourses, which is a List too and hence yous called the stream() method which returns a Stream associated alongside that list. After that, yous lead hold called the collect() method which is used to accumulate elements from Stream.

Since nosotros are non doing whatever filtering at that topographic point is no telephone phone to filter(), the collect() method too hence uses a Collector which tin combine results inwards a map. All it needs is i method to extract the key, which is Course::getTitle too i method to extract value which is Function.identity() i.e. the object itself.

We lead hold also used a method reference to shorten the primal extractor. You tin farther see What's New inwards Java 8: Lambdas to larn to a greater extent than nearly how to convert lambda appear to a method reference.

Btw, at that topographic point is a lead deal of here. The ordering of elements has lost because Map doesn't guarantee whatever order. If yous desire to proceed Courses inwards the same lodge they appeared inwards List, nosotros demand to utilization a Map which provides ordering guarantees e.g. LinkedHashMap which keeps elements inwards the lodge they are inserted.

We also demand to nation this to Collector hence that it volition collect elements within a LinkedHashMap rather than a full general Map. Let's re-write the code to attain that:

 Map<String, Course > result = listOfCourses         .stream()         .collect(         Collectors.toMap(Course::getTitle,                           Function.identity(),                           LinkedHashMap::new));

This looks skilful now. The lodge of elements inwards both List too Map are same now, but at that topographic point is even hence i to a greater extent than affair yous demand to lead hold aid to proceed this code total proof too exceed the evidence of time.

If yous remember, List allows duplicates but if yous endeavour to insert a duplicate primal inwards the Map it overrides the values, that would lead hold been a nightmare inwards this case, but thankfully Java 8 protects you. Instead of silently overwriting a value inwards such condition, it throws an exception equally shown below when it encountered a duplicate chemical constituent inwards the source listing (see Modern Java inwards Action )

Though, yous tin resolve this fault yesteryear only telling Collector how to resolve collision e.g. what to exercise when it encounters a duplicate key. It tin exercise cypher too proceed the master copy mapping or it tin override too update the value. You tin instruct collector whatever yous desire yesteryear providing an extra parameter to the Collectors.toMap() method equally shown inwards the next code:

    Map<String, Course > result = listOfCourses         .stream()         .collect(         Collectors.toMap(Course::getTitle,                           Function.identity(),                          (e1, e2) -> e2,                           LinkedHashMap::new));

Here inwards illustration of a duplicate, nosotros are using the minute chemical constituent equally a primal to generated LinkedHashMap. You tin also lead the kickoff object or only concatenate kickoff too minute equally per your need.


That's all nearly how to convert a listing of objects List<V> to a map of keys too values e.g. Map<K, V>. It's super slowly inwards Java 8. All yous demand to know is how to utilization collect() and Collector. Though yous should travel mindful of the essential departure betwixt List too Map information construction e.g. i is ordered collection piece other is not. Transferring values from List to Map agency yous volition lose the lodge if yous don't save them e.g. yesteryear using a LinkedHashMap.

Other Java tutorials too Resources yous may similar to explore
The Complete Java MasterClass
How to form HashMap yesteryear keys values inwards Java 8?
How to take away an entry from HashMap inwards Java?
How to utilization map() too flatMap() methods inwards Java?
How to exercise map-reduce inwards Java 8?
10 Java 8 Stream Interview Questions
What's New inwards Java 8: Lambdas

Thanks for reading this article, if yous similar this article too hence delight part alongside your friends too colleagues. If yous lead hold whatever questions or feedback too hence delight driblet a note.

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