Why getWordsPrefixedWith(String prefix) is a Private method?

We make methods private when we don't want for end users of our application, or API to access some methods.

In this case we make getHashTags() and getMentions() public, because in the end these methods will be used in our Application, or in any other class.

The purpose of the getWordsPrefixedWith is ONLY to serve in THIS class, i.e. only here, nothing else, that is why we make it private.

One can make it protected for Unit Testing purposes, but that is whole different story, and we don't want to make it public

 // this one is public, with re-use of helpful method
  public List<String> getHashTags() {
    return getWordsPrefixedWith("#");
  }

// this one is public with re-use of helpful method
  public List<String> getMentions() {
    return getWordsPrefixedWith("@");
  }

  private List<String> getWordsPrefixedWith(String prefix) {
    List<String> results = new ArrayList<String>();
    for (String word : getWords()) {
      if (word.startsWith(prefix)) {
        results.add(word);
      }
    }
    return results;
  }

Rule of thumb is simple: if you write "helpful" method to be used in class only, then you better make it private

Here is the nice video about access-modifiers by Craig

https://teamtreehouse.com/library/access-level-modifiers