Java Cheat Sheet · through Java 25 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.
javac -d out src/*.java
Compile a package tree into an output dir.
java -cp out:libs/* Main
Set the classpath (; on Windows).
java --enable-preview --source 25 X.java25
Required to run any preview-feature code.
jshell9
Interactive REPL to try snippets.
javap -c Hello
Disassemble bytecode to see what javac produced.
jar --create --file app.jar -C out .
Bundle classes into an 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 java.util.*;
Wildcard import — a whole package (not recursive).
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[] args)★
The entry point the JVM calls.
args.length args[0]
Command-line arguments arrive here.
System.out.println(0);★
Print a line to standard output.
void main() { ... }25
Final in 25: instance main in a compact source file — no class, no static, no args.
import module java.base;25
Module import (JEP 511) — every exported package at once.
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.
fields default: 0 · 0.0 · false · null
Fields (not locals) start at these zero-values.
1_000_000 1e9 0xFF 0b10107
Digit separators, scientific, hex, binary.
int t = (int) 3.9; 0
Cast narrows & truncates; widening is automatic.
long big = 10_000_000_000L;
Past ~2.1 billion you need long.
int n = Integer.MAX_VALUE + 1;
Silently wraps to a negative — no overflow error.
1 + 1 0
char is a number — + does arithmetic, not text.

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.
final var id = nextId();10
final and var combine.
var x = switch(k){...};10
Works with any expression that has a type.
for (var i = 0; i < n; i++)10
var in a loop header is fine.
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.
0 + x + 1
+ concatenates when either side is a String.
== != < > <= >=★
Comparisons — but == on objects checks identity.
&& || !★
Logical, short-circuiting left→right.
cond ? a : b
Ternary — the only expression if.
& | ^ ~ << >> >>>
Bitwise; >>> = unsigned right shift.
Math.floorMod(-3, 5) 08
True modulo; % can be negative.
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 ==.
s.length() s.charAt(i) s.isEmpty()
Core accessors (indices are 0-based).
s.substring(1,4) s.indexOf(0) s.contains(1)
Slice & search.
s.startsWith(p) .endsWith(q) .replace(a,b)
Prefix/suffix tests & replacement.
s.toUpperCase() .toLowerCase() .trim()
Case & whitespace.
s.split(0) String.join(1, list)★8
Split to an array / join an Iterable.
s.strip() .isBlank() .repeat(3) .lines()11
Modern helpers (Unicode-aware strip).
0.formatted(name, n)15
Instance form of String.format.
var t = 0;★15
Text block — literal newlines, no escaping.
switch (cmd) { case 0 -> ...; }7
You can switch on String values.
new StringBuilder().append(x).toString()
Mutable builder — use it inside loops.
s.equalsIgnoreCase(t) s.compareTo(t)
Case-insensitive compare / lexicographic order.

07Numbers & Mathparse · box · compute

int n = Integer.parseInt(0);★
Parse text → number (throws on bad input).
double d = Double.parseDouble(0);
Same for floating point.
String s = Integer.toString(42); 0 + 42
Number → text.
Integer boxed = 42; int back = boxed;5
Auto-boxing between int and Integer.
Integer.MAX_VALUE Long.MIN_VALUE
Wrapper constants for the limits.
Math.abs .max .min .pow .sqrt .round★
The everyday math toolbox.
Math.random() new Random().nextInt(6)
Random doubles [0,1) / bounded ints.
ThreadLocalRandom.current().nextInt(1, 7)7
Preferred RNG in concurrent code.
new BigDecimal(0).add(...)★
Exact decimals — use for money, never double.
BigInteger.valueOf(2).pow(100)
Arbitrary-precision integers.

08Arraysfixed-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).
a[0] = 9; int x = a[i];
Index access; out-of-range throws.
int[][] grid = new int[3][4];
Multidimensional = array of arrays.
Arrays.sort(a); Arrays.toString(a);★
Utilities live in java.util.Arrays.
Arrays.fill(a, 7) Arrays.equals(a, b)
Bulk set / value compare.
Arrays.copyOf(a, n) a.clone()
Grow a copy / shallow clone.
Arrays.stream(a).sum()8
Bridge an array into the Stream world.
for (int x : a) { ... }★
Enhanced for over any array/Iterable.
int sum(int... nums)5
Varargs — call sum(1,2,3); it's an array inside.

09Control 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.
if (x == null) return;
Guard clause — bail early to cut nesting.
assert x > 0 : 0;
Off by default — enable with java -ea.

10Switchstatement → 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, default -> ...;21
Handle null; combine with the default branch.
exhaustiveness21
Patterns/enums need all cases or a default.

11Classes & 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.
double dist(){ return Math.hypot(x, y); }
Instance method — works on this object's fields.
int getX(){ return x; } void setX(int v){...}
Getter/setter idiom around private fields.
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(...).
static class Node{} class Inner{}
Nested (static) vs inner (holds outer this).

12Inheritanceextend & 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.
Animal a = new Dog(); a.speak();★
Upcast + dynamic dispatch = polymorphism.
if (a instanceof Dog d) d.fetch();16
Safe downcast with a pattern.
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.

13Interfacesa contract of behavior

interface Drawable { void draw(); }★
Pure abstract methods = a capability.
class C implements A, B {...}★
Multiple interfaces (unlike class extends).
interface Shape extends Drawable, Sized {}
Interfaces can extend several others.
int MAX = 100;
Interface fields are implicitly public static final.
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.

14Recordstransparent 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.
record P(...){ double d(){...} static P zero(){...} }16
Add methods & static factories freely.
record Pair<A,B>(A a, B b) {}16
Records can be generic and implement interfaces.
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.
records are immutable & final16
No setters, no subclassing — that's the point.

15Enums & 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.
switch (day) { case MON -> ...; }
In a switch, use bare constant names.
enum Op { ADD { int f(){...} }; abstract int f(); }
Per-constant method bodies.
EnumSet.of(A, B) new EnumMap<>(Day.class)5
Fast, compact enum-keyed set/map.
sealed interface Shape permits Circle, Square {}★17
Restrict who may implement/extend.
sealed class Base permits A, B {}17
Sealing works on classes too.
final / sealed / non-sealed17
Each permitted subtype picks one of these.

16Genericstype-safe containers

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

17Collectionslists, 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.
Deque<Integer> dq = new ArrayDeque<>();6
Stack (push/pop) & queue (offer/poll) in one.
xs.add / .get(i) / .size() / .contains(x)
Everyday operations.
xs.removeIf(x -> x.isBlank());8
Filter in place with a predicate.
m.getOrDefault / .putIfAbsent / .computeIfAbsent★8
Ergonomic Map methods.
for (var e : m.entrySet()) e.getKey()
Iterate entries; also keySet()/values().
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.
Collections.max / .min / .frequency
Handy static helpers.

18Comparing & Sortingnatural & custom order

class P implements Comparable<P> { ... }★
Give a type a natural order.
public int compareTo(P o){ return age - o.age; }
Negative / 0 / positive = before / equal / after.
xs.sort(Comparator.naturalOrder());★8
Sort using Comparable order.
Comparator.comparing(P::name)★8
Build a comparator from a key extractor.
.thenComparing(P::age)8
Break ties with a second key.
.reversed() Comparator.reverseOrder()8
Flip the direction.
Comparator.comparingInt(String::length)8
Primitive-specialized — avoids boxing.
Comparator.nullsFirst(naturalOrder())8
Decide where nulls sort.
Collections.sort(list) list.sort(cmp)8
In-place sorts; streams have .sorted(cmp).

19Lambdas & Method Refsfunctions as values

Runnable r = () -> System.out.println(0);★8
Lambda = an inline implementation.
(a, b) -> a + b x -> x * 28
Params infer types; one expr needs no braces/return.
Function / Predicate / Consumer / Supplier★8
The core java.util.function shapes.
BiFunction / UnaryOperator / BinaryOperator8
Two-arg & same-type variants.
Math::sqrt 08
Method reference to a static method.
str::length 08
Bound to a specific instance.
String::toUpperCase 0★8
Unbound — the first arg becomes the receiver.
ArrayList::new 08
Constructor reference = a factory.

20Streamslazy data pipelines

list.stream() Arrays.stream(a) Stream.of(1,2)8
Get a stream from a source.
.filter(x -> x > 0) .map(String::trim)★8
Lazy intermediate ops — nothing runs yet.
.sorted(comparing(P::age)) .distinct()8
Order / dedupe.
.limit(10) .skip(5) .peek(sysout)8
Slice / debug mid-pipeline.
.flatMap(List::stream)8
Flatten a stream of collections into one.
.toList() .collect(Collectors.toSet())★16
Terminal — materialize the result.
.collect(groupingBy(f, counting()))★8
Group into a Map with a downstream.
.collect(joining(0)) toMap(k, v)8
Join to a String / build a map.
.reduce(0, Integer::sum) .count() .anyMatch(p)8
Fold to one value / other terminals.
IntStream.range(0,n).sum() .average()8
Primitive streams avoid boxing.
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.

21Exceptionsfail 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.
class MyException extends RuntimeException {}
Define your own (unchecked here).
throw new MyException(0, cause);
Wrap & chain the original via the cause.
e.getMessage() e.getCause() e.printStackTrace()
Inspect what went wrong.
Objects.requireNonNull(x, 0);7
Fail fast on null arguments.
checked vs unchecked★
RuntimeException needn't be caught/declared; others must.
helpful NullPointerException messages14
The NPE now names the exact null variable.

22Dates & Timejava.time

LocalDate d = LocalDate.now();★8
A date with no time or zone.
LocalDate.of(2023, 9, 19) d.plusDays(10)★8
Construct / shift (returns a new object).
LocalTime LocalDateTime ZonedDateTime8
Time, date+time, and zoned variants.
Instant.now()8
A machine timestamp (UTC epoch).
Duration.ofHours(3) Period.ofDays(10)8
Time-based vs date-based amounts.
Duration.between(a, b) Period.between(x, y)8
Difference between two temporals.
d.getYear() .getMonthValue() .getDayOfWeek()8
Read individual fields.
d.isBefore(x) d.isAfter(x)8
Compare chronologically.
DateTimeFormatter.ofPattern(0)★8
Parse & format with a pattern.
java.util.Date / Calendar8
Legacy & mutable — prefer java.time.

23Console & Files I/Oin, out, disk

System.out.println(x) .print(x)★
Line / no-newline output.
System.out.printf(0, k, v)★5
Formatted output (%n = newline).
System.err.println(msg)
Write to standard error.
var sc = new Scanner(System.in);5
Read typed input: nextInt(), nextLine().
Path p = Path.of(0);★11
Build a filesystem path.
Files.writeString(p, text)★11
Write a whole String to a file.
String s = Files.readString(p);★11
Read a whole file into a String.
Files.readAllLines(p) Files.lines(p)8
List of lines / lazy Stream of lines.
try (var br = Files.newBufferedReader(p))7
Stream a large file line by line.
Files.exists(p) .createDirectories(p) .delete(p)7
Everyday filesystem checks & ops.

24Concurrencythreads, old & new

new Thread(runnable).start();
Spin up an OS-backed platform thread.
Thread.sleep(1000); t.join();
Pause / wait for a thread to finish.
var ex = Executors.newFixedThreadPool(4);5
Reuse a bounded pool of platform threads.
Future<Integer> f = ex.submit(callable); f.get();5
Callable returns a value; get() blocks.
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 volatile AtomicInteger
Guard shared state / ensure visibility.
var lock = new ReentrantLock(); lock.lock();5
Explicit lock — always unlock in finally.
ConcurrentHashMap CopyOnWriteArrayList5
Thread-safe collections.
CompletableFuture.supplyAsync(f).thenApply(g)★8
Compose async stages.
list.parallelStream().filter(p).count()8
Data parallelism for CPU-bound work.
new StructuredTaskScope.ShutdownOnFailure()21ᴾ
Preview: treat related tasks as one unit.

java cheat sheet 25 LTS

Modern Java · 8 → 25

Six ideas worth a picture

values vs references

checked vs unchecked

sealed + exhaustive switch

virtual vs platform threads

the collections family

how a stream runs

Worth memorizing