Java program to remove duplicate elements in an Array

In this article, you will learn Java program to remove duplicate elements in an Array. Here, you will see total 4 approaches to do this program using Java. To understand first three approach, you should have basic understanding of integer array and looping concept. And for the last approach you should have the java collection framework knowledge.

Approach #1. Remove duplicate element in Array using temporary Array

package com.javacodepoint.array;

public class RemoveDuplicateInArrayDemo {

	// Method to remove duplicate elements
	public static int removeDuplicate(int arr[]) {

		int n = arr.length;
		// There is no duplicate element in array if length is 0 or 1
		if (n == 0 || n == 1) {
			return n;
		}
		// Temporary array to hold unique elements
		int[] temp = new int[n];
		int j = 0;
		for (int i = 0; i < n - 1; i++) {
			if (arr[i] != arr[i + 1]) {
				temp[j++] = arr[i];
			}
		}
		temp[j++] = arr[n - 1];

		// Update the original array
		for (int i = 0; i < j; i++) {
			arr[i] = temp[i];
		}
		return j;
	}

	// Main method
	public static void main(String[] args) {

		// Declare and Initialize an integer array
		int arr[] = { 1, 1, 4, 5, 5, 8, 10, 10, 20, 20 };

		int length = removeDuplicate(arr);

		// After removing the duplicate elements, printing unique elements
		for (int i = 0; i < length; i++) {
			System.out.print(arr[i] + " ");
		}

	}

}

OUTPUT:

1 4 5 8 10 20

Approach #2. Remove duplicate element in Array using separate index

package com.javacodepoint.array;

public class RemoveDuplicateInArrayDemo2 {

	// Method to remove duplicate elements
	public static int removeDuplicate(int arr[]) {

		// array length
		int n = arr.length;

		// There is no duplicate element in array if length is 0 or 1
		if (n == 0 || n == 1) {
			return n;
		}
		// Separate index
		int index = 0;
		for (int i = 0; i < n - 1; i++) {
			if (arr[i] != arr[i + 1]) {
				arr[index++] = arr[i];
			}
		}
		arr[index++] = arr[n - 1];

		return index;
	}

	// Main method
	public static void main(String[] args) {

		// Declare and Initialize an integer array
		int arr[] = { 5, 5, 10, 15, 16, 16, 20, 20, 25 };

		int length = removeDuplicate(arr);

		// After removing the duplicate elements, printing unique elements
		for (int i = 0; i < length; i++) {
			System.out.print(arr[i] + " ");
		}

	}

}

OUTPUT:

5 10 15 16 20 25

NOTE: In the first two approachs, the array should be in sorted order otherwise it will not work.

Approach #3. Remove duplicate elements in unsorted Array

package com.javacodepoint.array;

import java.util.Arrays;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

public class RemoveDuplicateInArrayDemo3 {

	// Method to remove duplicate elements
	public static int removeDuplicate(int arr[]) {

		// array length
		int n = arr.length;

		// There is no duplicate element in array if length is 0 or 1
		if (n == 0 || n == 1) {
			return n;
		}
		// Separate index
		int index = 0;
		for (int i = 0; i < n - 1; i++) {
			if (arr[i] != arr[i + 1]) {
				arr[index++] = arr[i];
			}
		}
		arr[index++] = arr[n - 1];

		return index;
	}

	// Main method
	public static void main(String[] args) {

		// Declare and Initialize an integer array
		int arr[] = { 15, 5, 0, 15, 5, 16, 20, 20, 10 };

		// Sort the array
		Arrays.sort(arr);

		int length = removeDuplicate(arr);

		// After removing the duplicate elements, printing unique elements
		for (int i = 0; i < length; i++) {
			System.out.print(arr[i] + " ");
		}

	}

}

OUTPUT:

0 5 10 15 16 20

Approach #4. Remove duplicate elements in Array using Collection framework

In this approach, we use the java.util.HashSet class from java collection framework to write the below program. The HashSet class doesn’t allow you to add duplicate element into it.

package com.javacodepoint.array;

import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

public class RemoveDuplicateInArrayDemo4 {

	// Method to remove duplicate elements
	public static int removeDuplicate(int arr[]) {

		// array length
		int n = arr.length;

		// There is no duplicate element in array if length is 0 or 1
		if (n == 0 || n == 1) {
			return n;
		}

		// To hold only unique elements
		Set<Integer> set = new HashSet<Integer>();

		for (int i = 0; i < n; i++) {
			set.add(arr[i]);
		}

		// Update the original array
		Iterator<Integer> it = set.iterator();
		int index = 0;
		while (it.hasNext()) {
			arr[index++] = it.next();
		}
		return index;
	}

	// Main method
	public static void main(String[] args) {

		// Declare and Initialize an integer array
		int arr[] = { 5, 5, 10, 15, 16, 16, 20, 20, 25 };

		int length = removeDuplicate(arr);

		// After removing the duplicate elements, printing unique elements
		for (int i = 0; i < length; i++) {
			System.out.print(arr[i] + " ");
		}

	}

}

OUTPUT:

16 20 5 25 10 15

See also: Java Program to Calculate Average Using Array.

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