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Java Java Data Structures Exploring the Java Collection Framework Maps

raul cortes
raul cortes
5,745 Points

why second parameter is this....

In this code

/if(authoredTreets == null){ authoredTreets= new ArrayList<Treet>(); treetsByAuthor.put(treet.getAuthor(),authoredTreets); }/ why are the teacher putting an empty variable into the map variable, because in that moment he is initializing authoredTreets. Its hard to me to understand that step.

Thank you a lot.

2 Answers

Alexander Nikiforov
.a{fill-rule:evenodd;}techdegree seal-36
Alexander Nikiforov
Java Web Development Techdegree Graduate 22,174 Points

Tried my best, take a look

Map<String, List<Treet>> treetsByAuthor = new HashMap<>;

for (Treet treet : treets) {

       String currentAuthor = treet.getAuthor();

      // first we try to get treets by author from the Map above

      LIst<Treet> authoredTreets = treetsByAuthor.get(currentAuthor);

      // two scenarios are possible

      // 1. authoredTreets will be null, because in the Map there are no 
      // treets mapped to "currentAuthor"
      // That is why we do the following "if"
      if (authoredTreets == null) {
             // Create empty array with treets to put in our Map.
             authoredTreets = new ArrayList<Treet>();

            // Put that empty array for key "currentAuthor"
            //  { "currentAuthor" : new ArrayList<> }
           treetsByAuthor.put(currentAuthor, authoredTreets);

          // Logical step would be to write here
          //  authoredTreets.add(treet);
          // but because for both scenarios we do the same, we can add that later
         // see below

      // 2. authoredTreets already have treets with currentAuthor
      // In this case we just add treets in a map for current author
      // And this code also works if we just created empty array for
     // current author

    // note: we don't need to write "put" second time, because authored treets is
    // pointing to the array that is in a Map, that is why by adding treet to
    // "authoredTreets" we add it to Map as well


Or you know I found even better in community:


raul cortes
raul cortes
5,745 Points

THank you a lot, I understood the Idea, I'll read the link too :)

hugs from Chile