Types and Usage of Sets

Types and Usage of Sets

Generally, sets are a type of collection which stores unique values. Uniqueness is determined by the equals() and hashCode() methods.

Sorting is determined by the type of set.

HashSet - Random Sorting

Version ≥ Java SE 7
Set<String> set = new HashSet<> ();
set.add("Banana");
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set.add("Banana");
set.add("Apple");
set.add("Strawberry");
 
// Set Elements: ["Strawberry", "Banana", "Apple"]

LinkedHashSet - Insertion Order

Version ≥ Java SE 7
Set<String> set = new LinkedHashSet<> ();
set.add("Banana");
set.add("Banana");
set.add("Apple");
set.add("Strawberry");
 
// Set Elements: ["Banana", "Apple", "Strawberry"]

TreeSet - By compareTo() or Comparator

Version ≥ Java SE 7
Set<String> set = new TreeSet<> ();
set.add("Banana");
set.add("Banana");
set.add("Apple");
set.add("Strawberry");
 
// Set Elements: ["Apple", "Banana", "Strawberry"]
Version ≥ Java SE 7
Set<String> set = new TreeSet<> ((string1, string2) -> string2.compareTo(string1));
set.add("Banana");
set.add("Banana");
set.add("Apple");
set.add("Strawberry");
 
// Set Elements: ["Strawberry", "Banana", "Apple"]

Create a list from an existing Set

Using a new List

List<String> list = new ArrayList<String>(listOfElements);

Using List.addAll() method

 Set<String> set = new HashSet<String>();
 set.add("foo");
 set.add("boo");
 
 List<String> list = new ArrayList<String>();
 list.addAll(set);

Using Java 8 Steam API

List<String> list = set.stream().collect(Collectors.toList());

Eliminating duplicates using Set

Suppose you have a collection elements, and you want to create another collection containing the same elements but with all duplicates eliminated: 

Collection<Type> noDuplicates = new HashSet<Type>(elements);

Example:

List<String> items = new ArrayList<>(
    Arrays.asList("Alice", "Bob", "Charlie", "Alice", "Eve", "Charlie", "Bob"));
Set<String> uniqueItems = new HashSet<>(items);
System.out.println("uniqueItems = " + uniqueItems);

Output:

uniqueItems = [Alice, Bob, Charlie, Eve]

Declaring a HashSet with values

You can create a new class that inherits from HashSet:

Set<String> h = new HashSet<String>() {{
   add("a");
   add("b");
}};

One line solution:

Set<String> h = new HashSet<String>(Arrays.asList("a", "b"));

Using guava:

Sets.newHashSet("a", "b", "c")

Using Streams:

Set<String> set3 = Stream.of("a", "b", "c").collect(toSet());

List vs Set

What are differences between List and Set collection at the top level and How to choose when to use List in java and when to use Set in Java

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
 
public class DifferentSetAndListExample {
    public static void main(String[] args) {
        System.out.println("List example .....");
        List<String> myList = new ArrayList<>();
        myList.add("A");
        myList.add("B");
        myList.add("C");
        myList.add("D");
        myList.add("A");
 
        for (String item : myList) {
            System.out.println(item);
        }
 
        System.out.println("Set example .....");
        Set<String> mySet = new HashSet<>();
        mySet.add("X");
        mySet.add("Y");
        mySet.add("Z");
        mySet.add("W");
        mySet.add("X");
        mySet.add("Y");
        mySet.add("K");
 
        for (String item : mySet) {
            System.out.println(item);
        }
    }
}
 

Output:

List example .....
A
B
C
D
A
Set example .....
K
X
Y
Z
W

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