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Working of HashMap

Theory

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Working of HashMap

Working of HashMap 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 Working of HashMap: 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 Working of HashMap 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. Working of HashMap — output: 90. HashMap stores Asha→90, Ravi→85. get("Asha") looks up the key. Start from main, go line by line, and stop at each print. That output is the proof for Working of HashMap.

Diagram

put("Asha", 90)
  get("Asha") → 90
  get("Neha") → null

Exam tip

Explain bucket + equals for collisions in one line.

Example

import java.util.HashMap;

// HashMap demo
public class Main {
  public static void main(String[] args) {
    HashMap<String, Integer> marks = new HashMap<>();
    marks.put("Asha", 90);
    marks.put("Ravi", 85);
    System.out.println(marks.get("Asha"));
  }
}

Working of HashMap — output: 90. HashMap stores Asha→90, Ravi→85. get("Asha") looks up the key.

Short notes

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

import java.util.HashMap; // HashMap demo public class Main { public static void main(String[] args) { HashMap<String, Integer> marks = new HashMap<>(); marks.put("Asha", 90); marks.put("Ravi", 85); System.out.println(marks.get("Asha")); } }

Working of HashMap

Example

Short notes

Questions

Working of HashMap

Example

Short notes

Questions