import java.util.*;

public class Main {
    public static void main(String[] args) {
        List<Integer> input = List.of(42, 7, 19, 7, 3, 42);
        System.out.println("HashSet       " + new HashSet<>(input));
        System.out.println("LinkedHashSet " + new LinkedHashSet<>(input));
        System.out.println("TreeSet       " + new TreeSet<>(input));

        Set<String> seen = new HashSet<>();
        System.out.println(seen.add("x") + " " + seen.add("x"));   // second add is rejected
    }
}

HashSet       [19, 3, 7, 42]
LinkedHashSet [42, 7, 19, 3]
TreeSet       [3, 7, 19, 42]
true false

import java.util.*;

public class Main {
    public static void main(String[] args) {
        TreeSet<Integer> ts = new TreeSet<>(List.of(10, 20, 30, 40, 50));
        System.out.println(ts.first() + " " + ts.last());
        System.out.println(ts.floor(25) + " " + ts.ceiling(25));   // <= 25, >= 25
        System.out.println(ts.lower(20) + " " + ts.higher(20));    // < 20, > 20
        System.out.println(ts.floor(5));                           // none -> null
        System.out.println(ts.headSet(30) + " " + ts.tailSet(30)); // [..30) and [30..]
        System.out.println(ts.subSet(20, true, 40, true));
        System.out.println(ts.descendingSet());
        System.out.println(ts.pollFirst() + " -> " + ts);
    }
}

10 50
20 30
10 30
null
[10, 20] [30, 40, 50]
[20, 30, 40]
[50, 40, 30, 20, 10]
10 -> [20, 30, 40, 50]

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Set<Point> raw = new HashSet<>();
        raw.add(new Point(1, 2));
        raw.add(new Point(1, 2));
        System.out.println("class without equals/hashCode: " + raw.size());

        Set<PointRec> rec = new HashSet<>();
        rec.add(new PointRec(1, 2));
        rec.add(new PointRec(1, 2));
        System.out.println("record: " + rec.size());
    }
}

class Point {                        // uses Object's identity equals/hashCode
    final int x, y;
    Point(int x, int y) { this.x = x; this.y = y; }
}

record PointRec(int x, int y) {}     // records generate equals/hashCode from the fields

class without equals/hashCode: 2
record: 1

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Set<String> byLength = new TreeSet<>(Comparator.comparingInt(String::length));
        byLength.addAll(List.of("dog", "cat", "horse"));
        System.out.println(byLength);       // "cat" looked like a duplicate of "dog"
    }
}

[dog, horse]