Java · Theory

Collections class

Theory

213/270

Collections class

Collections class 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 Collections class: 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 Collections class 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. Output: Asha scored 72 then Pass. Real Main. Swap marks/name to show one idea from Collections class. Start from main, go line by line, and stop at each print. That output is the proof for Collections class.

Exam tip

Explain Collections class as: definition → why → one code idea.

Example

// Collections
public class Main {
  public static void main(String[] args) {
    int marks = 72;
    String name = "Asha";
    System.out.println(name + " scored " + marks);
    if (marks >= 40) System.out.println("Pass");
  }
}

Output: Asha scored 72 then Pass. Real Main. Swap marks/name to show one idea from Collections class.

Short notes

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

// Collections public class Main { public static void main(String[] args) { int marks = 72; String name = "Asha"; System.out.println(name + " scored " + marks); if (marks >= 40) System.out.println("Pass"); } }

Collections class

Example

Short notes

Questions

Collections class

Example

Short notes

Questions