Java · Theory

Java EnumMap

Theory

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Java EnumMap

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

Exam tip

Explain Java EnumMap 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 EnumMap.

Short notes

DefEnumMap — List = order + duplicates OK. Set = unique. Map = key → value.
RuleEnumMap — pick it only when that structure matches the problem.
RememberEnumMap — generics List<String>. ArrayList for daily lists. HashMap for lookup.
TrapEnumMap — 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 EnumMap

Example

Short notes

Questions

Java EnumMap

Example

Short notes

Questions