Sets

A set is a collection with no duplicates. It answers "is this item in the collection?" very quickly, which makes sets ideal for tracking visited pages, unique tags, or which users have voted. Java offers three main implementations with different ordering guarantees.

HashSet

JAVA

import java.util.HashSet;
import java.util.Set;

public class Main {
    public static void main(String[] args) {
        Set<String> tags = new HashSet<>();
        System.out.println(tags.add("java"));
        System.out.println(tags.add("sql"));
        System.out.println(tags.add("java"));
        System.out.println(tags.size());
        System.out.println(tags.contains("sql"));
        tags.remove("sql");
        System.out.println(tags);
    }
}

Output

true
true
false
2
true
[java]

add returns false when the element was already present. HashSet makes no promise about iteration order — don't rely on the order it prints in.

Removing duplicates

JAVA

import java.util.ArrayList;
import java.util.LinkedHashSet;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        List<String> visits = List.of("home", "shop", "home", "cart", "shop", "checkout");
        List<String> unique = new ArrayList<>(new LinkedHashSet<>(visits));
        System.out.println(unique);
    }
}

Output

[home, shop, cart, checkout]

LinkedHashSet remembers insertion order, so this removes duplicates while keeping the first occurrence of each.

TreeSet: sorted

JAVA

import java.util.TreeSet;

public class Main {
    public static void main(String[] args) {
        TreeSet<Integer> scores = new TreeSet<>(java.util.List.of(72, 95, 61, 88, 95));
        System.out.println(scores);
        System.out.println(scores.first() + " " + scores.last());
        System.out.println(scores.ceiling(80));
        System.out.println(scores.headSet(80));
        System.out.println(scores.descendingSet());
    }
}

Output

[61, 72, 88, 95]
61 95
88
[61, 72]
[95, 88, 72, 61]

TreeSet keeps elements sorted and offers range queries: the smallest element at least 80 (ceiling), everything below 80 (headSet), and so on.

Choosing an implementation

Class	Order	Speed of add/contains
`HashSet`	None	Fastest (constant time)
`LinkedHashSet`	Insertion order	Nearly as fast
`TreeSet`	Sorted	Slower (logarithmic)
`Set.of(…)`	None (unmodifiable)	Fast

Hash sets rely on correct equals and hashCode (lesson 35); tree sets rely on compareTo or a Comparator.

Set operations

JAVA

import java.util.Set;
import java.util.TreeSet;

public class Main {
    public static void main(String[] args) {
        Set<String> anaLikes = Set.of("jazz", "film", "hiking", "chess");
        Set<String> benLikes = Set.of("chess", "football", "jazz");

        Set<String> both = new TreeSet<>(anaLikes);
        both.retainAll(benLikes);

        Set<String> either = new TreeSet<>(anaLikes);
        either.addAll(benLikes);

        Set<String> onlyAna = new TreeSet<>(anaLikes);
        onlyAna.removeAll(benLikes);

        System.out.println("Both: " + both);
        System.out.println("Either: " + either);
        System.out.println("Only Ana: " + onlyAna);
    }
}

Output

Both: [chess, jazz]
Either: [chess, film, football, hiking, jazz]
Only Ana: [film, hiking]

retainAll (intersection), addAll (union) and removeAll (difference) modify the set they're called on, so copy first.

Try it yourself

Find the first repeated word in a sentence.

import java.util.HashSet;
import java.util.Set;

public class Main {
    public static void main(String[] args) {
        String sentence = "the quick fox saw the lazy dog and the cat";
        Set<String> seen = new HashSet<>();
        for (String word : sentence.split(" ")) {
            if (!seen.add(word)) {
                System.out.println("First repeat: " + word);
                break;
            }
        }
        System.out.println(seen.size() + " distinct words before stopping");
    }
}

Edit the code, copy it into Main.java and run java Main.java (Java 25+). How to set up Java

Output

First repeat: the
4 distinct words before stopping

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