How Strings Work — API Cheat Sheet

This is a reference page, not a graded exercise — there's nothing to run. The problem most people hit in an interview isn't the algorithm, it's freezing on "wait, what's the method called again?" with no autocomplete to bail them out. Read this once, then come back to it before your next mock interview.

Every language here treats strings as immutable — s.toUpperCase(), s.Replace(...), s.replace(...) etc. all return a new string, they never modify s in place. If you find yourself calling a string method and not using its return value, that's almost always a bug.

Java — String

Want to...	Method	Example
Get length	.length()	s.length()
Get char at index	.charAt(i)	s.charAt(0)
Slice out a substring	.substring(start) / .substring(start, end)	s.substring(1, 3)
Find a substring	.indexOf(sub) / .lastIndexOf(sub)	s.indexOf("lo") (-1 if not found)
Check if it contains	.contains(sub)	s.contains("lo")
Compare for equality	.equals(other) / .equalsIgnoreCase(other)	s.equals(t) — never use == on strings in Java
Split on a delimiter	.split(regex)	s.split(",") → String[]
Trim whitespace	.trim() / .strip()	s.strip()
Change case	.toUpperCase() / .toLowerCase()	s.toLowerCase()
Replace	.replace(a, b)	s.replace("a", "b")
To a char array	.toCharArray()	char[] cs = s.toCharArray();
Join a list of strings	String.join(sep, list)	String.join(", ", names)
Build a string from a number	String.valueOf(x)	String.valueOf(42)
Parse a number	Integer.parseInt(s)	throws if not numeric

C# — string

Want to...	Method	Example
Get length	.Length (property, no parens)	s.Length
Get char at index	indexer	s[0]
Slice out a substring	.Substring(start) / .Substring(start, length)	s.Substring(1, 2) — second arg is length, not end index
Find a substring	.IndexOf(sub) / .LastIndexOf(sub)	s.IndexOf("lo")
Check if it contains	.Contains(sub)	s.Contains("lo")
Compare for equality	== (overloaded for value equality) or .Equals(other)	s == t is safe in C#, unlike Java
Split on a delimiter	.Split(sep)	s.Split(',') → string[]
Trim whitespace	.Trim()	s.Trim()
Change case	.ToUpper() / .ToLower()	s.ToLower()
Replace	.Replace(a, b)	s.Replace("a", "b")
To a char array	.ToCharArray()	char[] cs = s.ToCharArray();
Join a list of strings	string.Join(sep, list)	string.Join(", ", names)
Check empty/null	string.IsNullOrEmpty(s)	guards against both at once
Parse a number	int.Parse(s) / int.TryParse(s, out x)	TryParse avoids the exception

Python — str

Want to...	Method	Example
Get length	len(s) (builtin, not a method)	len(s)
Get char at index	indexing	s[0]
Slice out a substring	slicing	s[1:3] (end-exclusive), s[::-1] reverses
Find a substring	.find(sub) (-1 if missing) / .index(sub) (raises)	s.find("lo")
Check if it contains	in	"lo" in s
Compare for equality	==	s == t
Split on a delimiter	.split(sep)	s.split(",") → list[str]
Trim whitespace	.strip()	s.strip()
Change case	.upper() / .lower()	s.lower()
Replace	.replace(a, b)	s.replace("a", "b")
To a list of chars	list(s)	strings are already iterable
Join a list of strings	sep.join(list)	", ".join(names) — join is a string method, not a list method
Check type of content	.isdigit() / .isalpha() / .isalnum()	s.isdigit()
Parse a number	int(s)	raises ValueError if not numeric

JavaScript / TypeScript — string

Want to...	Method	Example
Get length	.length (property, no parens)	s.length
Get char at index	indexing or .charAt(i)	s[0] or s.charAt(0)
Slice out a substring	.slice(start, end)	s.slice(1, 3) (end-exclusive; supports negative indices)
Find a substring	.indexOf(sub) / .lastIndexOf(sub)	s.indexOf("lo")
Check if it contains	.includes(sub)	s.includes("lo")
Compare for equality	===	s === t — always triple-equals
Split on a delimiter	.split(sep)	s.split(",") → string[]
Trim whitespace	.trim()	s.trim()
Change case	.toUpperCase() / .toLowerCase()	s.toLowerCase()
Replace	.replace(a, b) (first match) / .replaceAll(a, b)	s.replaceAll("a", "b")
To an array of chars	[...s] or s.split("")	[...s] handles unicode better
Join an array of strings	arr.join(sep)	names.join(", ") — join is an array method here, opposite of Python
Build from a number	String(x) or `${x}`	template literals are idiomatic
Parse a number	Number(s) / parseInt(s, 10)	Number("") is 0, watch for that

The one gotcha that trips up almost everyone across languages: every "mutating-looking" call is not mutating — s.toUpperCase() doesn't change s, it returns a new string you have to capture. If your code compiles but the output looks unchanged, this is the first thing to check.