java.util.TreeMap visualized: interactive view from a new perspective

java.util.TreeMap stores Key Value pairs like java.util.HashMap, and enables the retrieval of a Value given a Key.  TreeMap differs from HashMap in that the Keys are ordered by their natural order.  Internally the TreeMap maintains a Tree data structure, used to order and locate keys.

This tutorial, complete with an easy to understand and interactive UML visualization, explains how TreeMap works.

This is the another instalment of our java.util.Map series.  Earlier parts include:

• Hashmap Introduction Tutorial
• Hashmap and the Importance of HashCode
• Java 8, Map::computeIfAbsent Tutorial

Class Diagram

Take a look at the Class Diagram above.  You may note the following interesting points in this diagram:

• java.util.TreeMap implements java.util.SortedMap: All sorted maps, order keys by their natural ordering. E.g. natural ordering of strings would result in alphabetic ordering
• PersonKey implements java.lang.Comparable: this enables PersonKey instances to be sorted.  More specifically PersonKey implements the java.lang.Comparable::compareTo method.  The method returns 0, if the PersonKey is equal to the object passed in.  Alternatively compareTo should return a positive integer if PersonKey is greater than the object passed in, or a negative integer should be returned.

Step through TreeMap in action

Step through the visualisation from the above image, and read the yellow comments and the associated code.

When the code is executed, the following is output.  Note how the results have been ordered.  Observe the PersonKey::compareTo method and note how the last name is primarily used to order, and, secondarily, if the last names are equal, the first name is then used to order.

Last Name  First Name  Rating
---------------------------------
Jackson    Bill        2
Smith      Andrew      1
Smith      Bill        5
Smith      Jack        4


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