Java Cheat Sheet

Free Java cheat sheet: the most-used Java syntax and methods at a glance — searchable and beginner-friendly.

Basics

Concept	Syntax	Example
Main method	`public static void main(String[] args)`	`Entry point of every Java program`
Print	`System.out.println()`	`System.out.println("Hello");`
Variable	`type name = value;`	`int age = 25;`
Array	`type[] name = new type[size]`	`int[] nums = {1, 2, 3};`

Learn it in a lesson: Introduction to Java · Variables and Data Types · Arrays

OOP

Concept	Syntax	Example
Class	`class Name { }`	`class Dog { String name; }`
Constructor	`ClassName(params) { }`	`Dog(String n) { name = n; }`
Inheritance	`class B extends A { }`	`class Puppy extends Dog { }`
Interface	`interface Name { method(); }`	`interface Runnable { void run(); }`
Abstract class	`abstract class Name { }`	`abstract class Shape { abstract double area(); }`

Learn it in a lesson: Object-Oriented Programming · Inheritance · Interfaces and Abstract Classes

Collections

Concept	Syntax	Example
List	`List<T> l = new ArrayList<>()`	`List<String> names = new ArrayList<>(); names.add("Ada");`
Map	`Map<K,V> m = new HashMap<>()`	`Map<String,Integer> ages = new HashMap<>(); ages.put("Ada", 36);`
Set	`Set<T> s = new HashSet<>()`	`new HashSet<>(list) // removes duplicates`
For-each	`for (T item : collection)`	`for (String s : list) { ... }`
Default on miss get() returns null for a missing key, which is where NullPointerExceptions come from.	`map.getOrDefault(k, d)`	`counts.getOrDefault(word, 0) + 1`
Count into a map The tidiest word-count line in Java.	`map.merge(k, 1, Integer::sum)`	`counts.merge(word, 1, Integer::sum);`
Immutable literal Java 9+. Throws if you try to add to it — that is the point.	`List.of / Map.of`	`List.of("a", "b")`

Learn it in a lesson: Collections Framework · ArrayList · HashMap · Immutability

Exception Handling

Concept	Syntax	Example
Try-catch	`try { } catch (Exception e) { }`	`try { int x = 1/0; } catch (ArithmeticException e) { ... }`
Throw	`throw new Exception(msg)`	`throw new IllegalArgumentException("Invalid");`
Finally	`try { } finally { }`	`finally { connection.close(); }`
Custom exception	`class MyEx extends Exception { }`	`class NotFoundEx extends RuntimeException { }`

Learn it in a lesson: Exception Handling

Strings

Concept	Syntax	Example
Length	`str.length()`	`"hello".length() // 5`
Substring	`str.substring(start, end)`	`"hello".substring(0, 3) // "hel"`
Split	`str.split(regex)`	`"a,b,c".split(",") // ["a","b","c"]`
Format	`String.format("%s is %d", a, b)`	`String.format("Hi %s", name);`

Learn it in a lesson: Strings

Streams

Concept	Syntax	Example
Filter and collect toList() is Java 16+; before that, .collect(Collectors.toList()).	`.stream().filter().toList()`	`names.stream().filter(n -> n.length() > 3).toList();`
Map	`.map(fn)`	`names.stream().map(String::toUpperCase).toList();`
Sum	`.mapToInt().sum()`	`orders.stream().mapToInt(Order::getQty).sum();`
Sort	`.sorted(Comparator.comparing(fn))`	`people.stream().sorted(Comparator.comparing(Person::age)).toList();`
Group	`Collectors.groupingBy`	`people.stream().collect(Collectors.groupingBy(Person::city));`
Join to a string	`Collectors.joining(", ")`	`names.stream().collect(Collectors.joining(", "));`

Learn it in a lesson: Streams API Advanced Operations & Pipelines · Comparable & Comparator · Collectors & Grouping

Null & Optional

Concept	Syntax	Example
Optional Says "might not be there" in the signature instead of documenting it and hoping.	`Optional<T>`	`Optional<User> findById(long id);`
Unwrap safely Never call .get() without checking — it throws the same way null did.	`.orElse(x) / .orElseThrow()`	`user.orElseThrow(() -> new NotFoundException(id));`
Chain	`.map().filter()`	`user.map(User::email).filter(e -> e.contains("@"));`
Guard a parameter Fails at the boundary with a useful message rather than deep inside later.	`Objects.requireNonNull(x, "name")`	`this.name = Objects.requireNonNull(name, "name");`

Learn it in a lesson: Working with Optionals the Right Way