The principle of Java TreeMap

TreeMap is a Red-Black tree based NavigableMap implementation. The map is sorted according to the natural ordering of its keys, or by a Comparator provided at map creation time, depending on which constructor is used.
This implementation provides guaranteed log(n) time cost for the containsKey, get, put and remove operations. Algorithms are adaptations of those in Cormen, Leiserson, and Rivest’s Introduction to Algorithms.

The defination of TreeMap is below:

public class TreeMap<K,V>
    extends AbstractMap<K,V>
    implements NavigableMap<K,V>, Cloneable, java.io.Serializable

This class entends the AbstractMap class and in the same time implements NavigableMap, Cloneable, Serializable interface.

It has the following significant attributes:

//Comparator, We could use this interface to control the order of internal node of TreeMap
private final Comparator<? super K> comparator;
//Entry of TreeMap, the internal class of TreeMap
private transient Entry<K,V> root = null;
//The capacity of container
private transient int size = 0;
//The modification times ofd TreeMap
private transient int modCount = 0;
//The Entry RED color of Red-Black Tree
private static final boolean RED = false;
//The Entry Black color of Red-Black Tree
private static final boolean BLACK = true;

As for Entry class, it has the following attributes

K key;
V value;
Entry<K,V> left = null;
Entry<K,V> right = null;
Entry<K,V> parent;
boolean color = BLACK;

TreeMap class is similar to HashMap or LinkedHashMap class. But the main difference between them is that HashMap/LinkedHashMap is unordered collections while TreeMap is sorted in an ascending order by its key.
When we want to keep the order of keys in the Map, we need to utilize TreeMap class.

Here is a sample on how to use it:

/* Declare a TreeMap */
TreeMap<Integer, String> tmap = new TreeMap<Integer, String>();

/* Add element */
tmap.add(1, "Data1");
tmap.put(23, "Data2");
tmap.put(70, "Data3");
tmap.put(4, "Data4");
tmap.put(2, "Data5");

/* Display content */
for (Entry<Integer, String> entry : tmap.entrySet()) {
	System.out.println(entry.getKey() + " : " + entry.getValue());
}

And the output is below:

1 : Data1
2 : Data5
4 : Data4
23 : Data2
70 : Data3

As we can see, the random ordered inserted data is displayed in the ascending order of keys.