TreeMap in java

Let’s learn TreeMap in java.

TreeMap in java

• Underlying data structure for TreeMap is Red-Black Tree.
• Insertion order is not preserved in TreeMap.
• In TreeMap all elements will be inserted according to some sorting order of Keys.
• Duplicate keys are not allowed. But duplicate values are allowed.
• For empty TreeMap, first entry with null key is allowed. But after inserting that entry if we insert any other null entry then we will get run time exception saying NullPointerException.

Let’s see TreeMap example.

import java.util.TreeMap;

class TreeMapDemo
{
    public static void main(String[] args)
    {
        TreeMap m = new TreeMap();
        m.put(101, "Ravi");
        m.put(103, "Yash");
        m.put(102, "Tejas");
        m.put(103, 106);
        System.out.println(m);
    }
}

Output:

{101=Ravi, 102=Tejas, 103=106}

In the above java program if we add heterogeneous keys then ClassCastException is thrown.

import java.util.TreeMap;

class TreeMapDemo
{
    public static void main(String[] args)
    {
        TreeMap m = new TreeMap();
        m.put(101, "Ravi");
        m.put(103, "Yash");
        m.put(102, "Tejas");
        m.put(103, 106);
        m.put("Abhi", "Tejas");
        System.out.println(m);
    }
}

Output:

Exception in thread “main” java.lang.ClassCastException: class java.lang.Integer cannot be cast to class java.lang.String

Now in the above non-empty TreeMap if we insert “null” key then NullPointerException is thrown. Because if we compare anything with “null” NullPointerException is thrown.

import java.util.TreeMap;

class TreeMapDemo
{
    public static void main(String[] args)
    {
        TreeMap m = new TreeMap();
        m.put(101, "Ravi");
        m.put(103, "Yash");
        m.put(102, "Tejas");
        m.put(103, 106);
        m.put(null, "Tejas");
        System.out.println(m);
    }
}

Output:

Exception in thread “main” java.lang.NullPointerException

Methods of treemap

Treemap constructors

TreeMap t = new TreeMap(); for default natural sorting order.

TreeMap t = new TreeMap(Comparator c); for customized sorting order.

TreeMap t = new TreeMap(SortedMap m);

TreeMap t = new TreeMap(Map m);

Reference – Oracle Docs