Java · Theory

Collections in Java

Theory

194/270

Collections in Java

The Collections Framework gives ready-made structures for groups of objects. List: ordered, duplicates allowed — ArrayList. Set: unique elements — HashSet. Map: key → value — HashMap (Map is separate but part of the same story).

Prefer generics: List<String> so types stay safe. Pick by need: fast index get → ArrayList; uniqueness → HashSet; key lookup → HashMap. Don’t use a raw array for everything that must grow or shrink.

Draw List vs Set vs Map in ten seconds with one property each.

Let's take this on the board with one tiny Main class — no extra files. Collections in Java — output: 3. Three real names: Asha, Ravi, Neha. size() is 3. Start from main, go line by line, and stop at each print. That output is the proof for Collections.

Diagram

List  → order + duplicates   (ArrayList)
  Set   → unique only          (HashSet)
  Map   → key → value          (HashMap)

Exam tip

List / Set / Map — one line each.

Example

import java.util.ArrayList;

public class Main {
  public static void main(String[] args) {
    ArrayList<String> names = new ArrayList<>();
    names.add("Asha");
    names.add("Ravi");
    names.add("Neha");
    System.out.println(names.size()); // 3
  }
}

Collections in Java — output: 3. Three real names: Asha, Ravi, Neha. size() is 3.

Short notes

DefCollections Framework = ready-made structures for groups of objects.
RuleList = ordered + duplicates OK. Set = unique. Map = key → value.
RememberUse generics: List<String>.
UseArrayList for index, HashSet for unique, HashMap for lookup.

Java · Theory

Collections in Java

← All stacks

Theory

194/270

Collections in Java

Draw List vs Set vs Map in ten seconds with one property each.

Diagram

List  → order + duplicates   (ArrayList)
  Set   → unique only          (HashSet)
  Map   → key → value          (HashMap)

Exam tip

List / Set / Map — one line each.

Example

import java.util.ArrayList;

public class Main {
  public static void main(String[] args) {
    ArrayList<String> names = new ArrayList<>();
    names.add("Asha");
    names.add("Ravi");
    names.add("Neha");
    System.out.println(names.size()); // 3
  }
}

Collections in Java — output: 3. Three real names: Asha, Ravi, Neha. size() is 3.

Short notes

DefCollections Framework = ready-made structures for groups of objects.
RuleList = ordered + duplicates OK. Set = unique. Map = key → value.
RememberUse generics: List<String>.
UseArrayList for index, HashSet for unique, HashMap for lookup.

Collections in Java

Example

Short notes

Questions

Collections in Java

Example

Short notes

Questions