Trouble understanding how nested for loop and counting work. Appreciate help.

Looking at the creation of the Map interface and the nested for loops: Line 33, the second for statement, isn't treet.getHashTags() an ArrayList of words with hash tag prefixes. Each treet calls the getHashTags() method which returns the List of words with hash tags contained in the treet?

Don't understand how next line works, 34. Does hashTagCounts.get(hashTag) pull out each word with a hashtag in front? How does variable count take on a null value? Is Integer automatically switching to an object wrapper type?

Why check if count == null if getHashTags() above is a list of words with the hashtag prefix?

public class Example {

public static void main(String[] args) { Treet[] treets = Treets.load(); System.out.printf("There are %d treets. %n", treets.length); Set<String> allHashTags = new HashSet<String>(); Set<String> allMentions = new TreeSet<String>(); for (Treet treet : treets) { allHashTags.addAll(treet.getHashTags()); allMentions.addAll(treet.getMentions()); } System.out.printf("Hash tags: %s %n", allHashTags); System.out.printf("Mentions: %s %n", allMentions);

//Make key be hashtag, value be count so we can see what most popular hashtags were
Map<String, Integer> hashTagCounts = new HashMap<String, Integer>();// key type is String, value is Integer, Note values must be of type object
//Uppercase Integer will toggle to type int automatically - autoboxing
for  (Treet treet : treets) { //for each treet in treets
  for (String hashTag : treet.getHashTags()) {//loop through each hashtag, .getHashTags,of a specific treet
    Integer count = hashTagCounts.get(hashTag);//see if it exists
    if (count == null) {
      count = 0;
    }
    count++;
    hashTagCounts.put(hashTag, count);
  }
}
System.out.printf("Hash tag counts: %s %n", hashTagCounts);

Map<String, List<Treet>> treetsByAuthor = new HashMap<String, List<Treet>>();
for (Treet treet : treets) {
  List<Treet> authoredTreets = treetsByAuthor.get(treet.getAuthor());
  if (authoredTreets == null) {
    authoredTreets = new ArrayList<Treet>();
    treetsByAuthor.put(treet.getAuthor(), authoredTreets);
  }
  authoredTreets.add(treet);
}
System.out.printf("Treets by author: %s %n ", treetsByAuthor);
System.out.printf("Treets by nickrp: %s %n", treetsByAuthor.get("nickrp"));

// Treet originalTreet = treets[0];//pullout first one, there in sorting order // System.out.println("Hashtags:"); //for (String hashtag : originalTreet.getHashTags()) { // System.out.println(hashtag);

}
} package com.teamtreehouse; import java.io.Serializable; import java.util.ArrayList; import java.util.Arrays; import java.util.Date; import java.util.List;

public class Treet implements Comparable, Serializable { private static final long serialVersionUID = 7146681148113043748L; private String mAuthor; private String mDescription; private Date mCreationDate;

public Treet(String author, String description, Date creationDate) { mAuthor = author; mDescription = description; mCreationDate = creationDate; } @Override public String toString() { return String.format("Treet: \"%s\" by %s on %s", mDescription, mAuthor, mCreationDate); } @Override public int compareTo(Object obj) { Treet other = (Treet) obj; if (equals(other)) { return 0; } int dateCmp = mCreationDate.compareTo(other.mCreationDate); if (dateCmp == 0) { return mDescription.compareTo(other.mDescription); } return dateCmp; } public String getAuthor() { return mAuthor; } public String getDescription() { return mDescription; } public Date getCreationDate() { return mCreationDate; } public List<String> getWords() { String[] words = mDescription.toLowerCase().split("[^\w#@']+"); return Arrays.asList(words); } public List<String> getHashTags() {//return a list of strings calling it gethashtags return getWordsPrefixedWith("#"); } public List<String> getMentions() {//make a list of mentions, return a list of strings return getWordsPrefixedWith("@"); } private List<String> getWordsPrefixedWith(String prefix) {//always want return list interface,return a list of strings List<String> results = new ArrayList<String>(); //make a new list called results, its a new arraylist for (String word : getWords()) {// loop through array, check each word in the treet,to see if it has hashtag if (word.startsWith(prefix)) {//startsWith is special method of strings results.add(word); } } return results; } }