TreeMaps – Java Collections Framework

If you want to save a sequence of key/value pairs and if you want them ordered by the key, what would be your ideal option? Don’t have an idea? Don’t worry. Let’s talk about that today.

Hello coders, I am back with a new article in my Java Collections Framework article series. Today I am going to discuss TreeMap class in Java. TreeMap class extends the SortedMap Interface and that extends the Map Interface.

HashMaps vs TreeMaps

TreeMaps are considerably similar to HashMaps. There are two main differences between these two classes.

1. HashMaps are implemented as a Hash Table. TreeMaps are implemented based on Red-Black Tree Structure

If you are not aware of Hash Tables and Red-Black Tree Structure, please refer to the linked articles to get a better idea. I will be talking about Hash Tables later in this article series as well.

2. HashMap does not maintain order. TreeMaps are ordered by the key.

Here is a small code snippet that explains the difference between HashMaps and TreeMaps.

        //Creating a TreeMap
        TreeMap<Integer, String> myTreeMap = new TreeMap<>();

        myTreeMap.put(34, "Smith");
        myTreeMap.put(62, "Virat");
        myTreeMap.put(1, "Marnus");
        myTreeMap.put(8, "Stokes");
        myTreeMap.put(2, "Williamson");

        System.out.println("TreeMap : " + myTreeMap);

        //Creating a HashMap
        HashMap<Integer, String> myHashMap = new HashMap<>();

        myHashMap.put(39, "Smith");
        myHashMap.put(61, "Virat");
        myHashMap.put(9, "Marnus");
        myHashMap.put(13, "Stokes");
        myHashMap.put(4, "Williamson");

        System.out.println("HashMap : " + myHashMap);

TreeMap : {1=Marnus, 2=Williamson, 8=Stokes, 34=Smith, 62=Virat}
HashMap : {4=Williamson, 39=Smith, 9=Marnus, 61=Virat, 13=Stokes}

Similarities

1. Both are unsynchronized and should be synchronized explicitly to be used in a thread-safe environment

2. Only contains unique elements. (Key should be unique)

3. Keys can’t be null. Values can be null.

        //Creating a TreeMap
        TreeMap<Integer, String> myTreeMap = new TreeMap<>();

        myTreeMap.put(34, "Smith");
        myTreeMap.put(62, "Virat");
        myTreeMap.put(null, "Bro"); // Causes NullPointerException
        myTreeMap.put(4, null); //This is fine

        System.out.println("TreeMap : " + myTreeMap);

4. Methods are almost the same.

I am not going to list down the methods here. You can take a look at all the available methods here in the Java Documentation

Now you know where to go to when you want to store key/value pairs ordered by the key.

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