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

One of the mutual tasks inward Java programming is to convert a listing to map in addition to I receive got written virtually this inward the past times in addition to today we'll regard how Java 8 makes this chore easier. Btw, live on it Java seven or Java 8, you lot demand to continue something inward hear spell converting a listing to map because they are ii unlike information structure. For example, the listing allows duplicate elements but keys inward a map must live on unique, the value tin live on duplicated but a duplicate primal may crusade a occupation inward Java 8. Similarly, only about other draw of piece of work concern is order. H5N1 listing is an ordered collection but the map doesn't guarantee whatsoever lodge unless you lot create upward one's hear to role LinkedHashMap, which keeps insertion lodge or TreeMap which keeps mapping inward the sorted lodge of keys. This is i of the of import details which many Java beginners forget in addition to thus pass hours to chase subtle bugs.

If your programme has whatsoever dependency on the lodge of elements inward the listing they volition non move equally expected if you lot role a map. So, you lot should live on mindful of these full general details spell converting a listing to map inward Java. This is truthful irrespective of the Java version.

Btw, if you lot are novel to Java world, I propose you lot to root become through a comprehensive course of report on Java similar The Complete Java Masterclass on Udemy. It non solely provides organized in addition to construction learning but also you lot volition larn to a greater extent than inward less time.  It's also i of the most up-to-date course, of late updated for Java xi features.

Anyway, let's regard the chore at hand. Assume you lot receive got a listing of courses in addition to you lot desire to create a map where keys should live on the championship of the course of report in addition to value should live on the course of report object itself. How volition you lot exercise that?

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

It's slow inward Java 7, all you lot demand to exercise is become through the List, extract a primal from each object in addition to add together both primal in addition to value into the map, but how volition you lot exercise that inward Java 8 fashion similar past times using lambda expression in addition to streams?


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

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

This code is real unproblematic in addition to slow to read, let's instantly regard the Java 8 version to detect out whether Java 8 actually makes your life slow when it comes to writing twenty-four lx minutes catamenia to twenty-four lx minutes catamenia 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 business office inward JDK 7. So, it looks Java 8 actually makes the developer's life easy.

Anyway, let's endeavor to sympathise what's going on here. Well, you lot receive got a listOfCourses, which is a List in addition to thus you lot called the stream() method which returns a Stream associated amongst that list. After that, you lot receive got called the collect() method which is used to accumulate elements from Stream.

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

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

Btw, at that topographic point is a guide handgrip of here. The ordering of elements has lost because Map doesn't guarantee whatsoever order. If you lot desire to continue Courses inward the same lodge they appeared inward List, nosotros demand to role a Map which provides ordering guarantees e.g. LinkedHashMap which keeps elements inward the lodge they are inserted.

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

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

This looks expert now. The lodge of elements inward both List in addition to Map are same now, but at that topographic point is yet i to a greater extent than affair you lot demand to receive got tending to continue this code total proof in addition to overstep the seek of time.

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

Though, you lot tin resolve this mistake past times only telling Collector how to resolve collision e.g. what to exercise when it encounters a duplicate key. It tin exercise zero in addition to continue the master mapping or it tin override in addition to update the value. You tin instruct collector whatever you lot desire past times providing an extra parameter to the Collectors.toMap() method equally shown inward the next code:

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

Here inward instance of a duplicate, nosotros are using the 2nd chemical constituent equally a primal to generated LinkedHashMap. You tin also select the root object or only concatenate root in addition to 2nd equally per your need.


That's all virtually how to convert a listing of objects List<V> to a map of keys in addition to values e.g. Map<K, V>. It's super slow inward Java 8. All you lot demand to know is how to role collect() and Collector. Though you lot should live on mindful of the essential departure betwixt List in addition to Map information construction e.g. i is ordered collection spell other is not. Transferring values from List to Map agency you lot volition lose the lodge if you lot don't save them e.g. past times using a LinkedHashMap.

Other Java tutorials in addition to Resources you lot may similar to explore
The Complete Java MasterClass
How to form HashMap past times keys values inward Java 8?
How to take an entry from HashMap inward Java?
How to role map() in addition to flatMap() methods inward Java?
How to exercise map-reduce inward Java 8?
10 Java 8 Stream Interview Questions
What's New inward Java 8: Lambdas

Thanks for reading this article, if you lot similar this article thus delight portion amongst your friends in addition to colleagues. If you lot receive got whatsoever questions or feedback thus delight drib a note.

P. S. - If you lot are looking for only about gratuitous courses to larn novel features introduced inward Java 8 in addition to Java nine thus this listing of free Java 8 in addition to Java nine Courses can live on real useful.

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