How do you maintain order in a set?

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How to Maintain Insertion Order While Getting Unique Values from ArrayList in Java?

ArrayList is a part of collection framework and is present in java.util package. It provides us with dynamic arrays in Java. Though, it may be slower than standard arrays but can be helpful in programs where lots of manipulation in the array is needed. This class is found in java.util package. If we want to maintain the insertion order of the elements, we are supposed to use LinkedHashSet. LinkedHashSet maintains the order in which the elements are inserted.

Example 1:

Java

import java.util.*; class GFG { public static void main(String[] args) { ArrayList arrayList = new ArrayList<>(); arrayList.add( 100 ); arrayList.add( 200 ); arrayList.add( 100 ); arrayList.add( 500 ); arrayList.add( 200 ); arrayList.add( 300 ); arrayList.add( 200 ); arrayList.add( 600 ); LinkedHashSet set = new LinkedHashSet<>(arrayList); System.out.println( "Unique values in inserted order" ); System.out.println(set); } }

Output Unique values in inserted order [100, 200, 500, 300, 600]

Example 2:

In this example, instead of using a wrapper class, we will use a user-defined class and will maintain insertion order while getting unique values from ArrayList. Since we are using a user-defined class so we are supposed to override the hashCode method and equals method so that our LinkedHashSet will be able to identify similar elements otherwise our LinkedHashSet will consider every element as a unique element.

Java

import java.util.*; class friendsDetail { private String name; private String nickName; public friendsDetail(String name, String nickName) { this .name = name; this .nickName = nickName; } public String getName() { return name; } public void setName(String name) { this .name = name; } public String getnickName() { return nickName; } public void setNickName( int id) { this .nickName = nickName; } @Override public boolean equals(Object o) { if ( this == o) return true ; if (!(o instanceof friendsDetail)) return false ; friendsDetail that = (friendsDetail)o; return Objects.equals(getName(), that.getName()) && Objects.equals(nickName, that.nickName); } @Override public int hashCode() { return Objects.hash(getName(), nickName); } public String toString() { return "(" + this .getName() + ":" + this .getnickName() + ")" ; } } class GFG { public static void main(String[] args) { ArrayList originalArrayList = new ArrayList<>(); System.out.println( "Our ArrayList " ); originalArrayList.add( new friendsDetail( "Raushan" , "Chamgader" )); originalArrayList.add( new friendsDetail( "Yashdeep" , "Dopa" )); originalArrayList.add( new friendsDetail( "Shishya" , "Gorilla" )); originalArrayList.add( new friendsDetail( "Sonika" , "Chipkali" )); originalArrayList.add( new friendsDetail( "Himanshu" , "Lalten" )); originalArrayList.add( new friendsDetail( "Sarthak" , "Nagin" )); originalArrayList.add( new friendsDetail( "Tsering" , "Battak" )); originalArrayList.add( new friendsDetail( "Abhishek" , "Liquid" )); originalArrayList.add( new friendsDetail( "Shishya" , "Gorilla" )); originalArrayList.add( new friendsDetail( "Suraj" , "Bhindi" )); originalArrayList.add( new friendsDetail( "Sonika" , "Chipkali" )); originalArrayList.add( new friendsDetail( "Himanshu" , "Lalten" )); originalArrayList.add( new friendsDetail( "Sarthak" , "Nagin" )); for (friendsDetail friend : originalArrayList) { System.out.println(friend); } LinkedHashSet linkedHashSet = new LinkedHashSet<>(originalArrayList); System.out.println( "

Unique elements in inserted order

" ); for (friendsDetail friend : linkedHashSet) { System.out.println(friend); } } }

Output Our ArrayList (Raushan:Chamgader) (Yashdeep:Dopa) (Shishya:Gorilla) (Sonika:Chipkali) (Himanshu:Lalten) (Sarthak:Nagin) (Tsering:Battak) (Abhishek:Liquid) (Shishya:Gorilla) (Suraj:Bhindi) (Sonika:Chipkali) (Himanshu:Lalten) (Sarthak:Nagin) Unique elements in inserted order (Raushan:Chamgader) (Yashdeep:Dopa) (Shishya:Gorilla) (Sonika:Chipkali) (Himanshu:Lalten) (Sarthak:Nagin) (Tsering:Battak) (Abhishek:Liquid) (Suraj:Bhindi)

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import java.util.Collections; import java.util.Enumeration; import java.util.HashSet; import java.util.Iterator; import java.util.Set; /** * Copyright (c), AnkitMittal JavaMadeSoEasy.com */ public class HashSetTest { public static void main(String args[]) { Set hashSet = new HashSet(); hashSet.add( "ankit" ); hashSet.add( "javaMadeSoEasy" ); System. out .println( "-------use iterator-------" ); // fail-fast Iterator iterator = hashSet.iterator(); while (iterator.hasNext()) { hashSet.add( "newElement1" ); //unComment to avoid ConcurrentModificationException System. out .println(iterator.next()); } System. out .println( "-------use Enumeration-------" ); // fail-fast Enumeration listEnum = Collections. enumeration (hashSet); while (listEnum.hasMoreElements()) { hashSet.add( "newElement2" ); //unComment to avoid ConcurrentModificationException System. out .println(listEnum.nextElement()); } System. out .println( "-------use enhanced for loop-------" ); // enhanced for loop is fail-fast for (String string : hashSet) { hashSet.add( "newElement3" ); //unComment to avoid ConcurrentModificationException System. out .println(string); } } }

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