Java HashSet — Java Technical Interview Questions & Answers

Java HashSet

Java HashSet

  • HashSet belongs to the Collections story. List = ordered, duplicates OK (ArrayList). Set = unique (HashSet). Map = key → value (HashMap). Pick by need, not by what you revised last.
  • For HashSet: ArrayList is the daily List: get(i) is fast, add at end is fast. HashMap lookup by key is average O(1). HashSet is unique values, no duplicates.
  • Use HashSet with generics: List<String>, Map<String, Integer>. Iterate with for-each or iterator. Don’t use a raw array when the size must grow.
  • Let's take this on the board with one tiny Main class — no extra files. Java HashSet — duplicate A is ignored; size stays 2. Start from main, go line by line, and stop at each print. That output is the proof for HashSet.

Technical Interview Tip: Pick Set type by: speed vs order vs sorted.

Code Examples for Java HashSet

Example

import java.util.HashSet;
public class Main {
  public static void main(String[] args) {
    HashSet<String> set = new HashSet<>();
    set.add("A");
    set.add("B");
    set.add("A"); // ignored
    System.out.println(set.size()); // 2
    System.out.println(set.contains("B"));
  }
}

Java HashSet — duplicate A is ignored; size stays 2.

How to Explain Java HashSet to an Interviewer

What is HashSet?

HashSet — List = order + duplicates OK. Set = unique. Map = key → value. HashSet — pick it only when that structure matches the problem. HashSet — generics List<String>. ArrayList for daily lists. HashMap for lookup.

Why do we use HashSet in Java?

Makes the program clearer or safer HashSet — List = order + duplicates OK. Set = unique. Map = key → value.

Write one small example of HashSet.

Show syntax Say what prints

What mistake do beginners make with HashSet?

Only the name, no example HashSet — using arrays for everything that must grow, or mixing List/Set/Map.

Where is HashSet used in a real program?

Day-to-day Java code