Java Hashtable — Java Technical Interview Questions & Answers

Java Hashtable

Java Hashtable

  • Hashtable 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 Hashtable: 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 Hashtable 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 Hashtable — treeMap prints keys in sorted order. Start from main, go line by line, and stop at each print. That output is the proof for Hashtable.

Technical Interview Tip: Say which Map keeps sorted keys (TreeMap).

Code Examples for Java Hashtable

Example

import java.util.TreeMap;
public class Main {
  public static void main(String[] args) {
    TreeMap<String, Integer> m = new TreeMap<>();
    m.put("b", 2);
    m.put("a", 1);
    System.out.println(m); // {a=1, b=2}
  }
}

Java Hashtable — treeMap prints keys in sorted order.

How to Explain Java Hashtable to an Interviewer

What is Hashtable?

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

Why do we use Hashtable in Java?

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

Write one small example of Hashtable.

Show syntax Say what prints

What mistake do beginners make with Hashtable?

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

Where is Hashtable used in a real program?

Day-to-day Java code