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Java

Need some help understanding why some parts of my coding challenge answer are what they are?!

So, with this challenge I managed to get most of the skeleton down correctly (at first I was thrown by creating a method which returns a map, whilst also declaring the hashMap inside of the method!).

Here's the answer that worked: public Map<String, Integer> getCategoryCounts() { Map<String, Integer> categoryCounts = new HashMap<String, Integer>();

  for (BlogPost post : mPosts) {
    String category = post.getCategory();
    Integer count = categoryCounts.get(category);
    if (count == null) {
     count = 0; 
    }
    count ++;
    categoryCounts.put(category, count);
  }

return categoryCounts; }

A few of the finer details were lost on me however: 1)String category = post.getCategory(); Can this be declared outside of the for-loop? Also why is the method of getCategory() referring to post, rather than BlogPost? Especially as it is the class BlogPost that contains this getter for category?

2)for (BlogPost post : mPosts) Just to get this straight in my head: this is saying that for every post of type BlogPost (object) within the list of posts (mPosts)......we should etc etc? Am I saying that the right way round?

3) Integer count = categoryCounts.get(category); Like during the class tutorial, this line plus the accompanying if block is basically defining what the count is. If no count exists for a particular category, then one is begun at 0 for it. Regardless we then increment the count for that category. Finally the categoryCounts is ammended accordingly. Correct?

Finally the part .get(category) is referring to the map, so is this essentially searching for the particular category (key from the map)?

Ah, I somehow posted this at the top of the Java Forum! It's from Java Data Structures >>> Exploring the JCF >>> Maps The task is: In Blog.java add a new method called getCategoryCounts. It should return a Map of category-to-count calculated by looping over all the posts.

https://teamtreehouse.com/library/java-data-structures/exploring-the-java-collection-framework/maps-2

Code formatted

public Map<String, Integer> getCategoryCounts() {
    Map<String, Integer> categoryCounts = new HashMap<String, Integer>();

    for (BlogPost post : mPosts) {
        String category = post.getCategory();
        Integer count = categoryCounts.get(category);
        if (count == null) {
            count = 0; 
        }
    count ++;
    categoryCounts.put(category, count);
    }
  return categoryCounts;
}

1 Answer

Brendan Whiting
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Brendan Whiting
Front End Web Development Techdegree Graduate 84,704 Points

Sorry for the late response:

1) No, you can't declare String category = post.getCategory() outside of the for loop, because post is a local variable that will only exist in the loop, and each time around the loop has a different value. When you're saying for (BlogPost post : mPosts) {} it's saying loop through mPosts and give me a local variable post for each time through the loop.

getCategory() is a method on the variable post which is an instance of a BlogPost class. If you had a static method, you would call it directly on the class (BlogPost.someStaticMethod()), but in this case, we're calling the method on this object which is an instance of the class, so we call the method on this variable (post.getCategory()).

2) and 3) Yes, sounds right to me.