Java · Theory

Queue & PriorityQueue

Theory

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Queue & PriorityQueue

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

Exam tip

Explain Queue PriorityQueue as: definition → why → one code idea.

Example

// Queue
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 Queue & PriorityQueue.

Short notes

DefQueue & PriorityQueue — List = order + duplicates OK. Set = unique. Map = key → value.
RuleQueue & PriorityQueue — pick it only when that structure matches the problem.
RememberQueue & PriorityQueue — generics List<String>. ArrayList for daily lists. HashMap for lookup.
TrapQueue & PriorityQueue — using arrays for everything that must grow, or mixing List/Set/Map.

// Queue 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"); } }

Queue & PriorityQueue

Example

Short notes

Questions

Queue & PriorityQueue

Example

Short notes

Questions