🔴 Advanced · Lesson 24

Collections (ArrayList, HashMap)

What are Collections?

The Collections framework provides ready-made data structures like lists and maps that grow dynamically — unlike fixed-size arrays.

ArrayList

import java.util.ArrayList;
ArrayList<String> names = new ArrayList<>();
names.add("Aman");
names.add("Riya");
System.out.println(names.get(0));   // Aman
System.out.println(names.size());   // 2

HashMap

import java.util.HashMap;
HashMap<String,Integer> marks = new HashMap<>();
marks.put("Aman", 88);
marks.put("Riya", 95);
System.out.println(marks.get("Riya"));   // 95

Summary

ArrayList = dynamic list; add/get/size.
HashMap = key-value pairs; put/get.

Collections क्या हैं?

Collections framework lists और maps जैसी ready-made data structures देता है जो dynamically बढ़ती हैं — fixed-size arrays के उलट।

ArrayList

import java.util.ArrayList;
ArrayList<String> names = new ArrayList<>();
names.add("Aman");
names.add("Riya");
System.out.println(names.get(0));   // Aman
System.out.println(names.size());   // 2

HashMap

import java.util.HashMap;
HashMap<String,Integer> marks = new HashMap<>();
marks.put("Aman", 88);
marks.put("Riya", 95);
System.out.println(marks.get("Riya"));   // 95

सारांश

ArrayList = dynamic list; add/get/size।
HashMap = key-value pairs; put/get।

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