Java · Theory

Java Stack

Theory

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Java Stack

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

Exam tip

Explain Java Stack 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 Stack.

Short notes

DefStack — List = order + duplicates OK. Set = unique. Map = key → value.
RuleStack — pick it only when that structure matches the problem.
RememberStack — generics List<String>. ArrayList for daily lists. HashMap for lookup.
TrapStack — 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 Stack

Example

Short notes

Questions

Java Stack

Example

Short notes

Questions