Java Collections — Free Java Tutorial

TL;DR: Learn Java Collections in Java with a free, beginner-friendly tutorial, examples and practice for Indian students on Syllab.in.

Java Collections in Java

Java Collections Framework provides pre-built data structures. The most used: ArrayList (dynamic array — like Python list), HashMap (key-value pairs — like Python dict), LinkedList, HashSet, TreeMap.

ArrayList<Type>: A resizable list. Unlike arrays, it grows automatically. Key methods: add(), get(), remove(), size(), contains(), indexOf(), clear(), sort(). Use when you need an ordered, indexed list that can grow.

HashMap<K, V>: Stores key-value pairs with O(1) lookup by key. Key methods: put(), get(), remove(), containsKey(), containsValue(), keySet(), values(), entrySet(). Use when you need fast lookups by key (like a phone book or dictionary).

// ArrayList
ArrayList<String> list = new ArrayList<>();
list.add("item");
list.get(0);
list.remove(0);
list.size();

// HashMap
HashMap<String, Integer> map = new HashMap<>();
map.put("Arjun", 85);
map.get("Arjun");       // 85
map.containsKey("key");

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