Java · Theory

Java Hashtable

Theory

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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.

Exam tip

Say which Map keeps sorted keys (TreeMap).

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.

Short notes

DefHashtable — List = order + duplicates OK. Set = unique. Map = key → value.
RuleHashtable — pick it only when that structure matches the problem.
RememberHashtable — generics List<String>. ArrayList for daily lists. HashMap for lookup.
TrapHashtable — using arrays for everything that must grow, or mixing List/Set/Map.

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

Example

Short notes

Questions

Java Hashtable

Example

Short notes

Questions