How Hash Maps Work — API Cheat Sheet

This is a reference page, not a graded exercise. If there's one data structure worth having cold, it's this one — it's the answer to "can I do better than O(n^2)" more often than anything else.

Java — HashMap<K, V>

Want to...	Method	Example
Create	new HashMap<>()	Map<String, Integer> m = new HashMap<>();
Insert / overwrite	.put(k, v)	m.put("a", 1);
Read	.get(k)	returns null if missing — NPE risk if you unbox into a primitive
Read with a default	.getOrDefault(k, default)	m.getOrDefault("a", 0)
Check a key exists	.containsKey(k)	m.containsKey("a")
Remove	.remove(k)
Iterate keys	.keySet()	for (String k : m.keySet())
Iterate values	.values()
Iterate pairs	.entrySet()	for (var e : m.entrySet()) { e.getKey(); e.getValue(); }
Size	.size()
Increment-or-insert	.merge(k, 1, Integer::sum)	classic frequency-counter one-liner
Insert only if absent	.computeIfAbsent(k, key -> new ArrayList<>())	great for building a Map<K, List<V>>

C# — Dictionary<K, V>

Want to...	Method	Example
Create	new Dictionary<K, V>()	var m = new Dictionary<string, int>();
Insert / overwrite	indexer [] or .Add(k, v)	m["a"] = 1; — [] overwrites, .Add throws if the key exists
Read	indexer []	m["a"] — throws KeyNotFoundException if missing
Read safely	.TryGetValue(k, out v)	the idiomatic way to read without risking an exception
Read with a default	.GetValueOrDefault(k, default)
Check a key exists	.ContainsKey(k)
Remove	.Remove(k)
Iterate keys	.Keys	property
Iterate values	.Values	property
Iterate pairs	foreach (var kv in m)	kv.Key, kv.Value
Size	.Count	property, no parens

Python — dict

Want to...	Method	Example
Create	{} or dict()	m = {}
Insert / overwrite	[]	m["a"] = 1
Read	[]	m["a"] — raises KeyError if missing
Read with a default	.get(k, default)	m.get("a", 0)
Check a key exists	in	"a" in m
Remove	.pop(k) or del m[k]	.pop returns the value
Iterate keys	for k in m	dicts iterate keys by default
Iterate values	.values()
Iterate pairs	.items()	for k, v in m.items()
Size	len(m)	builtin, not a method
Frequency counting	collections.Counter(iterable)	does the whole "count occurrences" pattern in one line
Default-on-missing	collections.defaultdict(int) / defaultdict(list)	avoids manual .get(k, 0) bookkeeping

JavaScript / TypeScript — Map<K, V>

Want to...	Method	Example
Create	new Map()	const m = new Map<string, number>();
Insert / overwrite	.set(k, v)	m.set("a", 1);
Read	.get(k)	returns undefined if missing
Check a key exists	.has(k)	m.has("a")
Remove	.delete(k)
Iterate keys	.keys()
Iterate values	.values()
Iterate pairs	.entries() or for (const [k, v] of m)	Map preserves insertion order
Size	.size	property, no parens, and not .length
Note	A plain {} object also works as a string-keyed map (obj["a"] = 1), but Map is safer: any value can be a key, no accidental collisions with inherited Object.prototype properties, and .size just works.

The gotcha that costs the most time: reading a missing key throws/crashes in some languages (C# [], Python []) and silently returns null/undefined in others (Java .get, JS .get). Know which behavior your language gives you before you write m.get(k) + 1 and get a null-pointer or NaN you don't understand.