Java · Theory

Java Collection Interface

Theory

222/270

Java Collection Interface

Collection Interface 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 Collection Interface: 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 Collection Interface 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 Java Collection Interface. Start from main, go line by line, and stop at each print. That output is the proof for Collection Interface.

Exam tip

Explain Java Collection Interface as: definition → why → one code idea.

Example

// Java
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 Java Collection Interface.

Short notes

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

// Java 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"); } }

Java Collection Interface

Example

Short notes

Questions

Java Collection Interface

Example

Short notes

Questions