Java Cheat Sheet 2026 (Syntax, Collections, Classes, Streams)

Java Cheat Sheet

Quick reference from FreeCodingCourses.com

Variables and types

int x = 5;: primitive int. Note the semicolon.
double d = 3.14; boolean ok = true;: other primitives.
String s = "hi";: String is a class, capital S.
var list = new ArrayList<String>();: var infers the type (Java 10+).
final int MAX = 100;: final means it cannot be reassigned.

Strings

s.length() s.charAt(0): length and a character.
s.equals(other): compare content. Never use == for strings.
s.toUpperCase() s.trim(): case change and trim.
s.split(",") String.join(",", list): split and join.
String.format("Hi %s", name): build a formatted string.

Collections

List<String> xs = new ArrayList<>();: a growable list.
xs.add("a"); xs.get(0); xs.size();: add, read, count.
Map<String,Integer> m = new HashMap<>();: key/value map.
m.put("a", 1); m.get("a");: set and read.
m.getOrDefault("b", 0): read with a fallback.

Classes

public class User { private String name; }: a class with a field.
public User(String name) { this.name = name; }: a constructor.
public String getName() { return name; }: a getter method.
User u = new User("Ada");: create an instance.

Control flow

if (x > 0) { ... } else { ... }: parentheses and braces.
for (int i = 0; i < 5; i++) { ... }: classic counted loop.
for (String x : xs) { ... }: enhanced for: loop over items.
while (cond) { ... }: loop while true.
switch (x) { case 1 -> ...; default -> ...; }: arrow switch (Java 14+).

Streams

xs.stream().filter(x -> x > 0): keep matching items.
.map(x -> x * 2): transform each item.
.collect(Collectors.toList()): gather back into a list.
xs.forEach(System.out::println): run something per item.

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Java questions, answered

Is Java the same as JavaScript?

No, and the names are a historical accident. Java is a compiled, statically typed language often used for large back-end systems and Android. JavaScript runs in the browser and on Node.js and is dynamically typed. Beyond a C-style syntax, they are unrelated.

Why do I compare strings with equals and not ==?

In Java, == checks whether two variables point to the exact same object in memory. Two different String objects with the same text are not the same object, so == can be false even when the text matches. .equals() compares the actual characters, which is what you almost always want.

What are streams for?

Streams let you process a collection in a readable pipeline: filter, map, then collect, instead of writing manual loops. They shine when you are transforming or filtering lists. Under the hood they still loop; they just read more clearly for data work.

Java cheat sheet