Finding Duplicates in Array using Treeset Example in Java

Introduction

Finding Duplicates in Array Using TreeSet is a classic Java console program that demonstrates the concept with complete source code and sample output. The Collections Framework provides ArrayList, HashMap, HashSet and related data structures.

This tutorial walks through the program line by line, explains how the logic works, and highlights best practices you can apply in your own code.

Definition

TreeSet uses a red-black tree implemented by a java.util.TreeMap. The red-black tree makes sure that there are no duplicates. Additionally, it allows TreeSet to implement java.util.SortedSet. Thus TreeSet itself sorts the values in it.

Syntax

TreeSet<variable-type> variableName = new TreeSet<variable-type>();

Example Program

import java.util.Arrays;
import java.util.TreeSet;

public class DuplicatesInArrayUsingTreeSet {

    public static void main(String[] args) {
        String[] array = new String[]{"a", "d", "z", "x", "t", "b", "a", "z"};
        System.out.println("Input Array is : " + (Arrays.toString(array)));
        TreeSet<String> treeSet = new TreeSet<String>();
        for (String str : array) {
            if (!treeSet.add(str)) {
                System.out.println("Duplicate Entry is: " + str);
            }
        }
        System.out.println("TreeSet is : " + treeSet);
    }
}

Sample Output

Input Array is : [a, d, z, x, t, b, a, z]
Duplicate Entry is: a
Duplicate Entry is: z
TreeSet is : [a, b, d, t, x, z]

When to use

Use this finding duplicates in array using treeset example when learning or revising core Java syntax.

How it works

1. Execution begins in the main method — the JVM calls this method when you run the class.

2. import java.util.Arrays; imports a class used later in the program.

3. import java.util.TreeSet; imports a class used later in the program.

4. String[] array = new String[]{"a", "d", "z", "x", "t", "b", "a", "z"}; updates a variable used in the calculation or output.

5. A println / print call writes text to the console — part of the sample output below.

6. TreeSet<String> treeSet = new TreeSet<String>(); updates a variable used in the calculation or output.

7. The if statement runs the nested code only when the condition is true.

8. A println / print call writes text to the console — part of the sample output below.

Best Practices

• Use meaningful variable and class names that describe their purpose.
• Compile and run the program locally — modify values to see how output changes.
• Read compiler errors carefully; they usually point to the exact line to fix.

Common Mistakes

• Copying code without understanding each line — practice by changing one statement at a time.
• Mismatching the public class name and the .java filename.
• Forgetting semicolons at the end of statements.