How to Convert an ArrayList or an Array into a TreeSet in Java?

In Java, ArrayList allows random access because it works like an array. It means you can access any element by its index in O(1) at any time. A TreeSet is a sorted set implementation of the Set interface based on a TreeMap. It provides log(n) time complexity for basic operations like add, remove, and contains due to its tree implementation.

In this article, we will learn how to convert an ArrayList or an Array into a TreeSet in Java.

Syntax to convert an ArrayList into a TreeSet:

ArrayList<Type> arrayList =  (initialize and add elements to the ArrayList)
TreeSet<Type> treeSet = new TreeSet<>(arrayList);

Program to convert an ArrayList to a TreeSet in Java

Implementation:

Below is the implementation to convert an ArrayList to a TreeSet.

Java

// Java Program to convert an ArrayList to a TreeSet in Java
import java.util.ArrayList;
import java.util.TreeSet;

public class ArrayListToTreeSetExample {
    public static void main(String[] args) {
       // Create an ArrayList
        ArrayList<String> courseList = new ArrayList<>();
        courseList.add("C++");
        courseList.add("Java");
        courseList.add("Python");
        courseList.add("Java"); // Duplicate entry

        // Create a TreeSet
        TreeSet<String> courseSet = new TreeSet<>();

        // Add elements from ArrayList to TreeSet
        courseSet.addAll(courseList);

        // Print the elements
        System.out.println("Elements of CourseList: " + courseList);
        System.out.println("Elements of CourseSet: " + courseSet);
    }
}

Output

Elements of CourseList: [C++, Java, Python, Java]
Elements of CourseSet: [C++, Java, Python]

Explanation of the Program:

• In the above program, an ArrayList of Strings[courseList] is created and some elements[courseName] are added including a duplicate.
• A TreeSet[courseSet] of Strings is created to hold the elements from ArrayList.
• The addAll() method is used to add all elements of ArrayList to TreeSet in one shot.
• TreeSet automatically sorts the elements and removes duplicates due to its set implementation.
• The elements of ArrayList and TreeSet are printed to show the conversion and sorting/deduplication.
  

Syntax to convert an Array into a TreeSet:

T[] array = {array elements}; 
// Convert array to TreeSet 
TreeSet<T> treeSet = new TreeSet<>(Arrays.asList(array));

Program to convert an Array to a TreeSet in Java

Implementation:

Below is the implementation to convert an Array to a TreeSet.

Java

// Java Program to convert an Array to a TreeSet
import java.util.Arrays;
import java.util.TreeSet;

public class ArrayToTreeSetExample {
    public static void main(String[] args) {
        // Create an array
        String[] course = {"Java", "Python", "Python", "C++"};

        // Create a TreeSet
        TreeSet<String> courseSet = new TreeSet<>();

        // Add elements from array to TreeSet
        courseSet.addAll(Arrays.asList(course));

        // Print the elements
        System.out.println("Elements of Course Array: " + Arrays.toString(course));
        System.out.println("Elements of Course Set: " + courseSet);
    }
}

Output

Elements of Course Array: [Java, Python, Python, C++]
Elements of Course Set: [C++, Java, Python]

Explanation of the Program:

• A String array is created with some course names including a duplicate.
• A TreeSet of Strings is made to hold the elements from the array.
• Arrays.asList() converts the array to a List for adding to the TreeSet.
• addAll() adds all elements of the list to the TreeSet in one call.
• TreeSet automatically sorts and removes duplicates when printing elements.