Solution for https://twitter.com/tagir_valeev/status/1402089474805354499

StringBuilderInHashMap.java

import java.util.*;
import java.util.stream.*;

public class StringBuilderInHashMap {
        public static void main(String[] args) {
        List<StringBuilder> list = Stream.generate(() -> {
                while (true) {
                    StringBuilder sb = new StringBuilder("a");
                    int hc = sb.hashCode();
                    if (((hc ^ (hc >>> 16)) & 0x3F) == 0) {
                        return sb;
                    }
                }
            }).limit(40)
            .sorted(Comparator.comparingInt(Object::hashCode))
            .toList();
        StringBuilder sb = Stream.generate(StringBuilder::new)
            .dropWhile(b -> Collections.binarySearch(list, b,
                Comparator.comparingInt(Object::hashCode)) < 0)
            .findFirst().get();
        Set<StringBuilder> set = new HashSet<>(list);
        set.add(sb);
        System.out.println(set.contains(sb)); // prints "true"
        sb.append("oops");
        System.out.println(set.contains(sb)); // prints "false"
    }
}