ArrayList in Java do not prevent the list from having duplicate values. But there are ways if you want to get unique values from the ArrayList and each way is explained with an example.
Method 1(Using Stream API’s distinct() Method): For Java 8, You can use Java 8 Stream API. To get distinct values, the distinct() method is an intermediate operation that also filters the stream to pass the next operation. Java Stream collect() is used to collect the stream elements to a collection (in this case a list).
Syntax:
Stream<T> distinct()
Return type: Stream is an interface and the function. It returns a stream consisting of distinct elements.
Java
// Java program to demonstrate// how to Get Unique Values from ArrayListimport java.util.*;import java.util.stream.Collectors;public class Main { public static void main(String[] args) { // Create ArrayList of Integers ArrayList<Integer> Numbers = new ArrayList<Integer>(); // add elements to ArrayList Numbers.add(1); Numbers.add(2); Numbers.add(1); Numbers.add(4); Numbers.add(2); List<Integer> UniqueNumbers = Numbers.stream().distinct().collect( Collectors.toList()); System.out.println("Unique Values of ArrayList"); // iterate through List for (int i = 0; i < UniqueNumbers.size(); ++i) { System.out.println(UniqueNumbers.get(i)); } }} |
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Method 2(Using HashSet): One can convert the ArrayList to HashSet which does not allow duplicate values. For conversion, use the constructor of HashSet class which accepts Collection as an argument.
Java
// Java program to demonstrate// how to Get Unique Values from ArrayListimport java.util.ArrayList;import java.util.HashSet;public class Main { public static void main(String[] args) { // Create ArrayList of Integer ArrayList<Integer> Numbers = new ArrayList<Integer>(); // add elements to ArrayList Numbers.add(1); Numbers.add(2); Numbers.add(1); Numbers.add(4); Numbers.add(2); /*Converting List to HashSet using constructor. */ HashSet<Integer> hashSetNumbers = new HashSet<Integer>(Numbers); System.out.println("Unique Values of ArrayList"); // iterate through HashSet for (Integer strNumber : hashSetNumbers) System.out.println(strNumber); }} |
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Maintaining order with no duplicate element’s insertion in the ArrayList.
LinkedHashSet is an ordered version of HashSet that maintains a doubly-linked List across all elements. Let us iterate through the elements in the order in which they were inserted. Following is the code that maintains insertion order and does not allow duplicate elements.
Java
// Maintaining order with no duplicate elements insertion in// the ArrayList.import java.util.*;public class Main { public static void main(String[] args) { // Using LinkedHashSet we can prevent // insertion of duplicate elements // as it implements the Set interface LinkedHashSet<Integer> UniqueList = new LinkedHashSet<Integer>(); // Adding elements to LinkedHashSet UniqueList.add(1); UniqueList.add(2); UniqueList.add(3); UniqueList.add(3); // duplicate 3 wont be inserted UniqueList.add(2); // duplicate 2 wont be inserted // Converting LinkedHashSet to List List<Integer> ls = new ArrayList<Integer>(UniqueList); // Printing list System.out.println(ls); }} |
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Method 4 : Using ArrayList ,add() and contains() method
Java
// Maintaining order with no duplicate elements insertion in// the ArrayList.import java.util.*;public class Main { public static void main(String[] args) { List<Integer> ls = new ArrayList<Integer>(); ls.add(1); ls.add(2); ls.add(1); ls.add(4); ls.add(5); List<Integer>ls1 = new ArrayList<Integer>(); for(int i=0;i<ls.size();i++) { System.out.println(ls.get(i)); int x; if(!ls1.contains(ls.get(i))) { x=ls.get(i); ls1.add(x); } } System.out.println(ls1); }} |
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