Chapter 60: HashSet in Java

Chapter 59 introduced Set as a bag without duplicates. HashSet is the usual Hash implementation: fast average add, remove, and contains when keys spread nicely across buckets.

LinkedHashSet (Chapter 61) remembers insertion order. TreeSet (Chapter 62) keeps sorted order.

1. Topic title

HashSet: hash the element, find a bucket, let equals settle collisions

2. What it means

Internally a HashSet wraps a HashMap-style structure: values are dummy placeholders; the set element is the key. When you add("java"), the string’s hashCode() suggests a bucket; equals confirms whether "java" is already there.

Average time for add/contains/remove is constant with respect to size—until hashes clump, then buckets grow long chains or trees (implementation detail you rarely tune at lesson sixty).

3. Why it is used

Tag clouds, permission names, visitor ids today—anywhere you need “have I seen this?” without caring which slot it lives in.

4. Mental image

Picture mail slots labeled by a quick math fingerprint of the envelope. equals is the clerk who opens the slot and checks the real address if two fingerprints collided.

5. Java code example

import java.util.HashSet;
import java.util.Set;
 
public class HashSetDemo {
    public static void main(String[] args) {
        Set<String> langs = new HashSet<>();
        langs.add("Go");
        langs.add("Java");
        langs.add("Go");
        System.out.println(langs.size());
        System.out.println(langs.contains("Rust"));
 
        langs.remove("Go");
        System.out.println(langs);
 
        langs.add(null);
        langs.add(null);
        System.out.println("null count path size=" + langs.size());
    }
}

HashSet allows one null element (as of typical JDK behavior—still treat null in sets as a smell in production models).

6. Explanation of code

Duplicate "Go" does not grow the set. remove returns true when something was removed. Second add(null) is ignored—uniqueness still applies.

7. Common mistakes

Relying on println order for tests—iteration order can change between runs or JVM versions.

Storing objects whose equals/hashCode change after insert—you can “lose” elements behind wrong buckets.

Using HashSet when you need sorted output—reach for TreeSet or sort a List copy.

8. Best practices

If you need predictable iteration, Chapter 61’s LinkedHashSet is the lighter move before jumping to TreeSet.

For domain objects in a set, define equals/hashCode together around stable business keys (id fields), not volatile display strings unless that is truly your identity rule.

9. Small practice task

Feed List.of("a","b","a","c","b") into new HashSet<>(...) and print size plus contains("c"). Then iterate and notice order is not alphabetical.

Beginner tip

HashSet is not magic dedupe dust—it dedupes by equals. Two different Person objects with the same id but default Object.equals still count as two entries until you fix equality.