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Java Java Data Structures Exploring the Java Collection Framework Maps

Jonathan Grieve
Jonathan Grieve
Treehouse Moderator 91,250 Points

Map Collections - Review

  • Every key has at the most ONE value. Maps match keys to values.

  • problems solved include counts of values and categorising items into groups.

  • With maps you define types for the keys and the values like so. ( Map <String, String> nameOfMap)


Map<String, String> name = new HashMap<String, String>();
  • You can add to maps with the put() method which takes a key and a value as its parameters.
   name.put("YANA", "You Are Not Alone");

This returns null first time when using the put method on a map because originally Maps are created with no initial value. Each put method returns the previous value of a particular key.

  • Keys can be set to null

other methods

  • The Map interface includes methods for basic operations (such as put, get, remove, containsKey, containsValue, size, and empty), bulk operations (such as putAll and clear), and collection views (such as keySet, entrySet, and values).

+.remove("key") - Remove key value pairs in a Map by the key. returns value when removing +.contains() - check a map has a particular key.

  • .get() - Check a key exists

Using KeySet() - return all the keys in a set


Set<String> variable = variable.keySet();

------> EntrySet() used to loop through keys and values. Map.Entry is an interclass, only used within the map, only available to the map.

for ( Map.Entry entry: variable.entrySet() ) {
    System.out.printf("%s stands for %s %n, entry.getKey(), entry.getValue() );

Other implementations

In the documentation K stands for the type and V, the type of value. There are interfaces for HashMap, Map with HashMap being the default. There's also LinkedHashMap inteface

  • Interface Map<K, V>
  • Class HashMap<K,V>
  • Class LinkedHashMap<K,V> - maps with values in the order they were added
  • Class TreeMap - predictable sorting order

e.g. For each treet, loop through each HashTag.

Map<String, Integer> hashTagCounts = new HashMap<String, Integer>();

for (Treet treet : treets) {
  for(String hashTag : treet.getHashTags() ) {
     Integer count = hashTagCounts.get(hashTag);
     if(count==null) {
     hashTagCounts.put(hashTag, count);

e.g. A map that is keyed on "author"

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);


System.out.printf("Treets by author: %s %n", treetsByAuthor ); System.out.printf("Treets by nickrp: %s %n", treetsByAuthor.get("nickrp") );

  • The use of the Collections Maps, Lists and Sets are very common in Java.
  • Always use the interface for the type declaration. You can change the implementation later if you need to.
  • Queue is another interface.

Collections Recap

List - List<String> variable= new ArrayList<String>(); Sets - Set<String> variable = new Set<String>(); Maps - Map<String, String> variable = new HashMap<String, String>();

Nelson Fleig
Nelson Fleig
23,608 Points

Thank you for the recap Jonathan!