Difference between TreeMap and TreeSet in Java? Answer

Difference between TreeSet and TreeMap in Java
The main difference between TreeMap and TreeSet is that TreeMap is an implementation of Map interface while TreeSet is an implementation of the Set interface. There are some similarities between both TreeMap and TreeSet and few differences as well. In this Java tutorial, we will first see similarities between TreeMap and TreeSet, and then you will learn some differences between TreeMap and TreeSet in Java. 

The key point to remember about TreeMap and TreeSet is that they use compareTo() or compare() method to compare object, So if uses put a String object in TreeSet of Integers, add() method will throw ClassCastException at runtime prior to Java 5.

From Java 5 you can use Generics to avoid this happening by declaring TreeMap and TreeSet with parametrized version. If you want to master the Java Collection framework by heart, you can see the Java Generics and Collection book by Maurice Naftalin, one of the best works on the Java Collections framework.

Similarities between TreeMap and TreeSet in Java

Here is a list of similarities between TreeMap and TreeSet in Java:

1. Both TreeMap and TreeSet are sorted data structures, which means they keep their element in predefined Sorted order. Sorting order can be natural sorting order defined by Comparable interface or custom sorting Order defined by Comparator interface. 

Both TreeMap and TreeSet has overloaded constructor which accepts a Comparator, if provided all elements inside TreeSet or TreeMap will be compared and Sorted using this Comparator.

2. Both TreeSet and TreeMap implements base interfaces e.g. TreeSet implements Collection and Set interface so that they can be passed to a method where a Collection is expected and TreeMap implements java.util.Map interface, which means you can pass it when a Map is expected.

3. TreeSet is practically implemented using TreeMap instance, similar to HashSet which is internally backed by HashMap instance. See my post Internal Implementation of HashSet to learn more. 

4. Both TreeMap and TreeSet are non-synchronized Collections, hence can not be shared between multiple threads. You can make both TreeSet and TreeMap synchronized by wrapping them into the Synchronized collection by calling Collections.synchroinzedMap() method.

5. Iterator returned by TreeMap and TreeSet are fail-fast, which means they will throw ConcurrentModificationException when TreeMap or TreeSet is modified structurally once Iterator is created. this fail-fast behavior is not guaranteed but works in the best effort.

6. Both TreeMap and TreeSet are slower than there Hash counterpart like HashSet and HashMap and instead of providing constant-time performance for add, remove, and get operation they provide performance in O(log(n)) order.

TreeSet vs TreeMap in Java

Now let's see some differences between TreeSet vs TreeMap in Java:

1. Major difference between TreeSet and TreeMap is that TreeSet implements Set interface while TreeMap implements Map interface in Java.

2.Second difference between TreeMap and TreeSet is the way they store objects. TreeSet stores only one object while TreeMap uses two objects called key and Value. Objects in TreeSet are sorted while keys in TreeMap remain in sorted order.

3. Third difference between TreeSet and TreeMap is that, former implements NavigableSet while later implements NavigableMap in Java.

4. Fourth difference is that duplicate objects are not allowed in TreeSet but duplicates values are allowed in TreeMap.

That's all on the difference between TreeSet and TreeMap in Java. Both provide sorting but their usage is different. TreeMap is used to keep mappings between key and values in sorted order while TreeSet is used to keep just one element in sorted order. 

TreeSet also doesn't allow duplicates but TreeMap does allow duplicate values. If you find any other significant difference between TreeMap and TreeSet then please post as a comment.

Further Learning
Difference between TreeSet and HashSet in Java
Difference between HashMap and ArrayList in Java

And now one question for you? What is difference between TreeMap and LinkedHashMap in Java?