Java · Theory

Java HashSet

Theory

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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.

Exam tip

Pick Set type by: speed vs order vs sorted.

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.

Short notes

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

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

Example

Short notes

Questions

Java HashSet

Example

Short notes

Questions