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

19 Jan, 2021
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
// Java program to Maintain Insertion Order While// Getting Unique Values from ArrayList  import java.util.*;  class GFG {    public static void main(String[] args)    {          // Creating an arrayList        ArrayList<Integer> arrayList = new ArrayList<>();          // adding elements to 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);          // creating an LinkedHashSet and        // adding arrayList elements to linkedHashSet        LinkedHashSet<Integer> set            = new LinkedHashSet<>(arrayList);          System.out.println(            "Unique values in inserted order");        System.out.println(set);    }}
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
// Java program to Maintain Insertion Order While// Getting Unique Values from ArrayList  import java.util.*;class friendsDetail {      // class field    private String name;    private String nickName;      // parameterised constructor    public friendsDetail(String name, String nickName)    {        this.name = name;        this.nickName = nickName;    }      // getter for name    public String getName() { return name; }      // setter for name    public void setName(String name) { this.name = name; }      // getter for nickname    public String getnickName() { return nickName; }      // setter for 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);    }      // overriding toString method    public String toString()    {        // return super.toString();        return "(" + this.getName() + ":"            + this.getnickName() + ")";    }}class GFG {    public static void main(String[] args)    {          ArrayList<friendsDetail> originalArrayList            = new ArrayList<>();          System.out.println("Our ArrayList\n");          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"));          // Displaying output using enhanced for loop        for (friendsDetail friend : originalArrayList) {            System.out.println(friend);        }          LinkedHashSet<friendsDetail> linkedHashSet            = new LinkedHashSet<>(originalArrayList);          System.out.println(            "\nUnique elements in inserted order\n");          // Displaying output using enhanced for loop        for (friendsDetail friend : linkedHashSet) {            System.out.println(friend);        }    }}
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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