JavaTreeMap

Java TreeMap

ATreeMap is a collection that stores key/value pairsin sorted order by key.

It is part of thejava.util package and implements theMap interface.

Create a TreeMap

Create aTreeMap that storesString keys andString values:

import java.util.TreeMap; // Import the TreeMap classTreeMap<String, String> capitalCities = new TreeMap<>();

Now you can use methods likeput(),get(), andremove() to manage sorted key/value pairs.

Add Items

Use theput() method to add key/value pairs:

import java.util.TreeMap;public class Main {  public static void main(String[] args) {    TreeMap<String, String> capitalCities = new TreeMap<>();    capitalCities.put("England", "London");    capitalCities.put("India", "New Dehli");    capitalCities.put("Austria", "Wien");    capitalCities.put("Norway", "Oslo");    capitalCities.put("Norway", "Oslo"); // Duplicate    capitalCities.put("USA", "Washington DC");    System.out.println(capitalCities);  }}

Access an Item

Useget() with the key to access its value:

capitalCities.get("England");

Remove Items

Useremove() to delete a key/value pair by key:

capitalCities.remove("England");

Useclear() to remove all items:

capitalCities.clear();

TreeMap Size

Usesize() to count the number of key/value pairs:

capitalCities.size();

Note: The size only counts unique keys. If a key is added more than once, only the latest value is kept.

Loop Through a TreeMap

Loop through the items of aTreeMap with a for-each loop.

Note: Use thekeySet() method if you only want the keys, and use thevalues() method if you only want the values:

// Print keysfor (String i : capitalCities.keySet()) {  System.out.println(i);}

// Print valuesfor (String i : capitalCities.values()) {  System.out.println(i);}

// Print keys and valuesfor (String i : capitalCities.keySet()) {  System.out.println("key: " + i + " value: " + capitalCities.get(i));}

TreeMap vs HashMap

The var Keyword

From Java 10, you can use thevar keyword to declare aTreeMap variable without writing the type twice. The compiler figures out the type from the value you assign.

This makes code shorter,but many developers still use the full type for clarity. Sincevar is valid Java, you may see it in other code, so it's good to know that it exists:

// Without varTreeMap<String, String> capitalCities = new TreeMap<String, String>();// With varvar capitalCities = new TreeMap<String, String>();

The Map Interface

Note: Sometimes you will see bothMap andTreeMap in Java code, like this:

import java.util.Map;import java.util.TreeMap;Map<String, String> capitalCities = new TreeMap<>();

This means the variable (capitalCities) is declared as aMap (the interface), but it stores aTreeMap object (the actual map). SinceTreeMap implements theMap interface, this is possible.

It works the same way, but some developers prefer this style because it gives them more flexibility to change the type later.