Java Cheat Sheet · through Java 21 LTS

01Setup & Runjavac · java

java --version
Check the installed JDK version.
javac Hello.java★
Compile source → Hello.class bytecode.
java Hello★
Run the class (no .class suffix).
java Hello.java11
Run a single source file directly — no javac.
jshell9
Interactive REPL to try snippets.
jar --create --file app.jar -C out .
Bundle classes into a runnable archive.
java -jar app.jar
Run an executable jar (needs Main-Class).

02Program Anatomythe skeleton

package com.acme.app;
First line — the namespace of this file.
import java.util.List;★
Pull in a type from another package.
import static java.lang.Math.max;
Import a static member; call max(a,b).
public class Main { ... }
One public class per file, named like the file.
public static void main(String[] a)★
The entry point the JVM calls.
void main() { ... }21ᴾ
Preview: instance main, no class needed.
0
Three comment forms.

03Primitives & Literals8 built-in value types

int i = 42; long L = 42L;★
32-bit vs 64-bit integers (note the L).
double d = 3.14; float f = 3.14f;★
64-bit vs 32-bit floating point.
boolean b = true; char c = 0;
true/false; a single UTF-16 code unit.
byte / short
8-bit and 16-bit integers.
int m = 1_000_000;7
Underscores group digits for readability.
0xFF 0b1010 071
Hex, binary, and (legacy) octal literals.
int n = Integer.MAX_VALUE + 1;
Silently wraps to a negative — no overflow error.

04Variables & varnames & inference

int count = 0;
Declare with an explicit type + assign.
final int MAX = 100;★
final = assign once; the constant idiom.
var list = new ArrayList<String>();★10
Type inferred from the initializer.
var x = switch(k){...};10
Works with any expression that has a type.
var y; var z = null;
Illegal — var needs an initializer with a real type.
fields / params / returns
var is local variables only.

05Operatorsarithmetic & logic

+ - * / %
% is remainder; 5/2==2 for ints.
++i i-- += -= *=
Increment / decrement / compound assign.
== != < > <= >=★
Comparisons — but == on objects checks identity.
&& || !
Logical, short-circuiting left→right.
cond ? a : b
Ternary — the only expression if.
& | ^ ~ << >> >>>
Bitwise; >>> = unsigned right shift.
o instanceof String s★16
Type test + bind s in one step.

06Stringsimmutable text

String s = 0;★
A literal; String objects never change.
a == b vs a.equals(b)★
Use .equals() for value equality, not ==.
0 + 1 + true
+ concatenates, coercing to text.
s.length() .charAt(i) .substring(1,4)
Core accessors (indices are 0-based).
s.strip() .isBlank() .repeat(3) .lines()11
Modern String helpers.
0.formatted(name, n)15
Instance form of String.format.
var t = 0;★15
Text block — literal newlines, no escaping.
new StringBuilder().append(x).toString()
Mutable builder — use it inside loops.

07Arraysfixed-size sequences

int[] a = new int[5];★
Zero-filled; length is fixed forever.
int[] a = {1, 2, 3};
Literal initializer.
a.length
A field, not a method (unlike String).
int[][] grid = new int[3][4];
Multidimensional = array of arrays.
Arrays.sort(a); Arrays.toString(a);
Utilities live in java.util.Arrays.
Arrays.asList(a) Arrays.copyOf(a,n)
Bridge to List / grow a copy.
for (int x : a) { ... }★
Enhanced for over any array/Iterable.

08Control Flowbranch & loop

if (c) {...} else if (d) {...} else {...}★
Standard branching.
while (c) {...} do {...} while (c);
Pre- vs post-tested loops.
for (int i=0; i<n; i++) {...}
Classic counting loop.
for (var e : coll) {...}★
For-each over arrays & collections.
break; continue;
Exit / skip the current iteration.
outer: for(...) { break outer; }
Labeled break jumps out of nested loops.

09Switchstatement → expression → pattern

switch(x){ case 1: ...; break; default: ...; }
Classic statement — beware fall-through.
switch(x){ case 1 -> a(); default -> b(); }★14
Arrow form: no fall-through, no break.
int r = switch(x){ case 1 -> 10; default -> 0; };14
Switch expression — returns a value.
case 1, 2, 3 -> ...; default -> { yield v; }14
Multi-label; yield returns from a block.
case Integer i -> i * 2;★21
Pattern matching — switch on type + bind.
case Integer i when i > 0 -> ...;21
Guarded pattern with when.
case null -> ...;21
Handle null as its own case.
exhaustiveness21
Patterns/enums need all cases or a default.

10Classes & Objectsthe building block

class Point { int x, y; }
Fields hold per-object state.
Point(int x, int y){ this.x = x; this.y = y; }★
Constructor initializes a new instance.
var p = new Point(1, 2);★
new allocates on the heap.
public / private / protected / (default)
Access levels — default is package-private.
static int count; static void help(){...}
Belongs to the class, not an instance.
Point(){ this(0,0); }
Overload + delegate with this(...).

11Inheritanceextend & override

class Dog extends Animal {...}★
Single inheritance of a superclass.
super(name); super.speak();
Call the parent constructor / method.
0 String speak(){...}★
Opt-in check that you really override.
abstract class Shape { abstract double area(); }
Abstract = can't instantiate; forces override.
final class / final method
final blocks further subclassing/overriding.
0 equals / hashCode / toString
The Object methods you usually redefine together.

12Interfacesa contract of behavior

interface Drawable { void draw(); }★
Pure abstract methods = a capability.
class C implements A, B {...}
Multiple interfaces (unlike class extends).
default void hi(){...}8
A method body inside an interface.
static Drawable of(){...}8
Static factory on the interface itself.
private helper(){...}9
Shared internals for default methods.
0 interface F { R apply(T t); }★8
Exactly one abstract method → usable as a lambda.

13Recordstransparent data carriers

record Point(int x, int y) {}★16
Immutable data class in one line.
p.x() p.y()16
Auto accessors named after components.
equals / hashCode / toString16
Generated for you from the components.
record R(int x){ R { if(x<0) throw...; } }16
Compact constructor validates/normalizes.
case Point(int x, int y) -> x + y;★21
Record pattern deconstructs in a switch.
case Line(Point(var x,var y), var e) -> ...;21
Patterns nest arbitrarily deep.

14Enums & Sealedclosed sets of types

enum Day { MON, TUE, WED }★
A fixed set of named constants.
Day.values() Day.valueOf(0) d.ordinal()
Built-in helpers on every enum.
enum P { A(1); final int n; P(int n){this.n=n;} }
Enums can hold fields, constructors, methods.
sealed interface Shape permits Circle, Square {}★17
Restrict who may implement/extend.
final / sealed / non-sealed17
Each permitted subtype picks one of these.
sealed + switch17
Compiler checks the switch covers every subtype.

15Genericstype-safe containers

class Box<T> { T get(){...} }★
Parameterize a class by a type.
Box<String> b = new Box<>();★7
Diamond <> infers the type.
<T> T first(List<T> xs){...}
Generic method with its own type var.
<T extends Number> ...
Upper bound — T is a Number or subtype.
List<? extends Number> List<? super Integer>
Wildcards: producer-extends, consumer-super (PECS).
new T[] x instanceof List<String>
Impossible — generics are erased at runtime.

16Collectionslists, sets, maps

List<String> xs = new ArrayList<>();★
Resizable, ordered, indexable.
Map<String,Integer> m = new HashMap<>();★
Key→value; keys need good equals/hashCode.
Set<Integer> s = new HashSet<>();
Unique elements, no order.
xs.add / .get(i) / .size() / .contains(x)
Everyday operations.
m.getOrDefault / .putIfAbsent / .computeIfAbsent8
Ergonomic Map methods.
List.of(1,2,3) Map.of(0,1)9
Compact immutable factories.
xs.getFirst() xs.getLast() xs.reversed()★21
Sequenced Collections — uniform ends.
LinkedHashMap (order) TreeMap (sorted)
Pick the map that matches your ordering need.

17Lambdas & Streamsfunctional pipelines

Runnable r = () -> System.out.println(0);★8
Lambda = an inline implementation.
list.forEach(System.out::println);8
Method reference :: — terse lambda.
Function / Predicate / Consumer / Supplier8
The core java.util.function shapes.
stream().filter(x -> x>0).map(f).toList()★8
Lazy pipeline; toList() collects.
.collect(Collectors.groupingBy(f))8
Group / join / count into a result.
.reduce(0, Integer::sum) .count() .anyMatch(p)8
Fold to a single value / terminal ops.
Optional<T> .map .filter .orElse(x) .ifPresent★8
Model "maybe absent" instead of null.
reuse a stream
A stream is one-shot — build a fresh one each time.

18Exceptionsfail loudly, recover well

try {...} catch (IOException e) {...} finally {...}★
Handle; finally always runs.
try (var f = open()) {...}★7
try-with-resources auto-closes (AutoCloseable).
catch (IOException | SQLException e) {...}7
Multi-catch shares one handler.
throw new IllegalArgumentException(0);
Raise an exception.
void read() throws IOException {...}
Checked exceptions must be declared.
checked vs unchecked
RuntimeException needn't be caught/declared; others must.
helpful NullPointerException messages14
The NPE now names the exact null variable.

19Concurrencythreads, old & new

new Thread(runnable).start();
Spin up an OS-backed platform thread.
var ex = Executors.newFixedThreadPool(4);5
Reuse a bounded pool of platform threads.
ex.submit(task); future.get();
Hand off work; block for the result.
Thread.ofVirtual().start(task);★21
A lightweight virtual thread (millions are fine).
Executors.newVirtualThreadPerTaskExecutor()★21
One virtual thread per task — for blocking I/O.
synchronized AtomicInteger ConcurrentHashMap
Guard shared mutable state.
CompletableFuture.supplyAsync(f).thenApply(g)8
Compose async stages.

java cheat sheet 21 LTS

Modern Java · 8 → 21

Four ideas worth a picture

values vs references

checked vs unchecked

sealed + exhaustive switch

virtual vs platform threads

Worth memorizing