What is the default value for HashMap in Java?

Java Hashmap Question?

• For my homework, we are supposed to implement a Contact List(i.e. a phone book) using a hashmap, with the keys being the names of the people and the values being their phone number(s). I have implemented that much correctly. However, we are then supposed to include a method that displays the keys/values of the hashmap in alphabetical order. My original thought was to simply copy the values of the hashmap over to a treemap which would sort them by default, but my professor said there was a simpler way and I cannot think of one. Does anyone have an idea for a simple method to display the keys/values of the hashmap in alphabetical order?

• Answer:

  Well, I guess if you cannot leverage the TreeMap to back your contact list then here is one possible way to do it. Writing this off the top of my head so no guarantee as to compiling etc. Create a new TreeSet Object with the keys from your HashMap. Iterate through the new TreeSet and get the values from the HashMap for the key. Print out the key and the values. Sample: //Assume that you stored your hashMap as HashMap<String, List<String>> public void printListing() { Set<String> mySet = new TreeSet<String>(myHashMap.keySet()); for(String myKey : mySet) { print(myKey); } } private void print(String aKey) { //Using the key, get the list of phone numbers and print them out. List<String> myPhones = hashMap.get(aKey); StringBuffer myBuf = new StringBuffer(aKey); if(myPhones != null) { for(String myNumber : myPhones) { myBuf.append(myNumber); } } System.out.println(myBuf.toString()); }