ArrayList: In Java, ArrayList can have duplicates as well as maintains insertion order.
HashSet: HashSet is the implementation class of Set. It does not allow duplicates and uses Hashtable internally.
There are four ways to convert ArrayList to HashSet :
- Using constructor.
- Using add() method by iterating over each element and adding it into the HashSet.
- Using addAll() method that adds all the elements in one go into the HashSet.
- Using stream
Method 1: Using constructor
In this example, we will create an ArrayList object and pass it into the constructor of HashSet. It is the simplest method to convert into HashSet.
Java
// Java program to convert ArrayList// to HashSet using constructorimport java.util.HashSet;import java.util.ArrayList;import java.io.*;class GFG { public static void main(String[] args) { // adding elements to ArrayList object ArrayList<String> gfg = new ArrayList<>(); gfg.add("data structure"); gfg.add("competitive programming"); gfg.add("Interviews"); gfg.add("FANG"); gfg.add("FANG"); // pass ArrayList object into the HashSet object // constructor HashSet<String> hashSet = new HashSet<>(gfg); // printing every element in Set for (String value : hashSet) { System.out.println(value); } }} |
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Method 2: Using add() method:
In this, we will iterate over Arraylist and add every element in HashSet.
Java
// Java program to convert ArrayList// to HashSet using add() methodimport java.io.*;import java.util.ArrayList;import java.util.HashSet;class GFG { public static void main(String[] args) { ArrayList<String> gfg = new ArrayList<>(); // adding element to ArrayList gfg.add("data structure"); gfg.add("competitive programming"); gfg.add("Interviews"); gfg.add("FANG"); gfg.add("FANG"); // adding each Arraylist element inside the HashSet HashSet<String> hashSet = new HashSet<>(); // printing each element gfg.forEach(value -> { hashSet.add(value); }); for (String value : hashSet) { System.out.println(value); } }} |
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Method 3: Using addAll() method
In this example, we will simply add the complete ArrayList object using addALL() method in the HashSet.
Java
// Java program to convert ArrayList// to HashSet using addAll() methodimport java.io.*;import java.util.ArrayList;import java.util.HashSet;class GFG { public static void main(String[] args) { ArrayList<String> gfg = new ArrayList<>(); // adding element into ArrayList object gfg.add("data structure"); gfg.add("competitive programming"); gfg.add("Interviews"); gfg.add("FANG"); gfg.add("FANG"); // adding ArrayList to hashset using addAll() method HashSet<String> hashSet = new HashSet<>(); hashSet.addAll(gfg); // printing each element of hashset for (String value : hashSet) { System.out.println(value); } }} |
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Method 4: Using Streams.
In this approach we will use streams to iterate over each ArrayList element and then add each item into a set using the collect method. Streams are part of java 8
Java
// Java program to convert ArrayList// to HashSet using streamsimport java.io.*;import java.util.ArrayList;import java.util.Set;import java.util.HashSet;import java.util.stream.*;class GFG { public static void main(String[] args) { ArrayList<String> gfg = new ArrayList<>(); gfg.add("data structure"); gfg.add("competitive programming"); gfg.add("Interviews"); gfg.add("FANG"); gfg.add("FANG"); // using stream to convert the ArrayList into set // then typecast into HashSet<String> HashSet<String> hashSet = (HashSet<String>)gfg.stream().collect( Collectors.toSet()); // printing values of the hashSet for (String value : hashSet) { System.out.println(value); } }} |
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