What is TreeMap in Java with example?

The underlying data-structure isÂ Red-black-Tree.
Insertion order is not preserved and it is based on some sorting order of keys.
Duplicate key is not allowed but values can be duplicated.
If we are depending on default natural sorting order then keys should be heterogeneous and comparable. Otherwise, we will get Runtime exception saying ClassCastException.
If we are defining our own sorting by comparator then keys need not be homogeneous and comparable. We can take heterogeneous, non-comparable objects also.
Whether we are depending on default natural sorting order or customized sorting order there are no-restrictions for values. We can take heterogeneous, non-comparable objects also.

Null Acceptance

For non-empty TreeMap if we are trying to insert an entry with a null key then we will get RuntimeException saying NullPointerException.
For empty TreeMap as the first entry with a Null key is allowed but after inserting that entry if we are trying to insert any other entry then we will get RuntimeException saying NullPointerException.
Note:-Â the above acceptance rule applicable until 1.6 only.Â 1.7 version onward null is not allowed for Key.
But for values we can use Null any number of times there is no restriction whether it is 1.6v or 1.7v.

Constructors

1	TreeMap m=new TreeHashMap()	Creates an empty TreeMap object for default natural sorting
2	TreeMap m=new TreeMap(Comparator c)	Creates an empty TreeMap object for customized sorting order
3	TreeMap m=new TreeMap(SortedMap s)	Creates an empty TreeMap object with sorting order as given
4	TreeMap m=new TreeMap(Map m)	create a TreeMap with equivalent given Map object.

//Demo program for cutomized sorting
package com.java4us;

import java.util.Comparator;
import java.util.TreeMap;

class Test {
    public static void main(String[] args) throws InterruptedException {
        TreeMap m = new TreeMap(new myComparator());
        m.put(101, "Welcome");
        m.put(102, "To");
        m.put(109, "Java4US");
        m.put(106, "Dot");
        m.put(105, "Com");
        System.out.println(m);  //{109=Java4US, 106=Dot, 105=Com, 102=To, 101=Welcome}
    }
}

class myComparator implements Comparator {

    public int compare(Object obj1, Object obj2) {
        Integer i1 = (Integer) obj1;
        Integer i2 = (Integer) obj2;
        if (i1 > i2) {
            return -1;
        } else if (i1 < i2) {
            return +1;
        } else
            return 0;
    }
}