Java Cheat Sheet

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A searchable, printable Java reference — types, strings, collections, streams, records, pattern matching, concurrency and file I/O. Free.

Basics & types

var list = new ArrayList<String>();

Local variable type inference

int x = 42; double d = 3.14;

Primitive declarations

long big = 9_000_000_000L;

Digit separators + long literal

char c = 'A'; boolean ok = true;

Char and boolean primitives

final int MAX = 100;

Constant (cannot be reassigned)

int i = (int) 3.99;

Explicit narrowing cast (→ 3)

double d = x;

Implicit widening conversion

int n = Integer.parseInt("42");

String → int

String s = String.valueOf(42);

Number → string

Integer boxed = 42; int y = boxed;

Auto-boxing and unboxing

Strings & text blocks

s.length(); s.charAt(0);

Length and character access

s.substring(1, 3)

Slice (end index exclusive)

s.contains("x"); s.startsWith("a");

Search checks

s.replace("a", "b")

Replace all occurrences

s.strip()

Trim whitespace (Unicode-aware)

s.split(",")

Split into a String[]

String.join(", ", parts)

Join with a separator

"ab".repeat(3)

Repeat a string ("ababab")

s.isBlank(); s.isEmpty();

Whitespace-only / zero-length

"%s: %d".formatted(key, count)

Inline printf-style formatting

String html = """\n <p>Hi</p>\n """;

Multi-line text block

new StringBuilder().append(a).append(b)

Efficient string building in loops

Collections

List<String> l = List.of("a", "b");

Immutable list factory

Map<String, Integer> m = Map.of("a", 1);

Immutable map factory

Set<Integer> s = Set.of(1, 2, 3);

Immutable set factory

new ArrayList<>(List.of("a", "b"))

Mutable copy of a list

List.copyOf(list)

Immutable defensive copy

map.getOrDefault(key, 0)

Read with a fallback value

map.computeIfAbsent(k, x -> new ArrayList<>())

Lazily create grouped values

map.merge(key, 1, Integer::sum)

Increment a counter map

list.removeIf(s -> s.isBlank())

Bulk remove matching items

list.sort(Comparator.naturalOrder())

Sort a list in place

for (var e : map.entrySet())

Iterate map entries

list.forEach(System.out::println)

Iterate with a method reference

Streams & lambdas

list.stream().map(String::toUpperCase).toList()

Transform every element

.filter(n -> n > 0)

Keep matching elements

.sorted(Comparator.comparing(User::name))

Sort by a key extractor

.distinct().limit(10).skip(5)

Dedupe and paginate a stream

.collect(Collectors.groupingBy(User::city))

Group into Map<K, List<V>>

.collect(Collectors.joining(", "))

Join strings with a separator

.collect(Collectors.toMap(User::id, u -> u))

Build a map from a stream

.flatMap(List::stream)

Flatten nested collections

.mapToInt(Order::total).sum()

Numeric stream aggregation

.reduce(0, Integer::sum)

Fold to a single value

.anyMatch(s -> s.isBlank())

anyMatch / allMatch / noneMatch

IntStream.range(0, 10)

Stream of ints 0..9

Records & sealed classes

record Point(int x, int y) {}

Immutable data carrier

var p = new Point(1, 2); p.x();

Construct + generated accessors

equals(), hashCode(), toString()

Generated for free on records

record Range(int lo, int hi) { Range { if (lo > hi) throw new IllegalArgumentException(); } }

Compact constructor validation

record Point(...) implements Shape {}

Records can implement interfaces

static Point origin() { return new Point(0, 0); }

Static factory inside a record

sealed interface Shape permits Circle, Square {}

Restrict allowed subtypes

final class Circle implements Shape {}

Permitted type must be final...

non-sealed class Square implements Shape {}

...or sealed / non-sealed

Pattern matching & switch

if (obj instanceof String s) use(s);

instanceof pattern binds a var

int n = switch (day) { case MON -> 1; default -> 0; };

Switch as an expression

case MON, TUE -> "early week";

Multiple labels per arm

case X -> { ...; yield value; }

Block arm returns via yield

case String s when s.length() > 2 ->

Guarded pattern (when clause)

case Point(int x, int y) -> x + y;

Record pattern deconstruction

case null, default -> "none";

Handle null inside the switch

switch (shape) { case Circle c -> ... }

Exhaustive over sealed types

OOP essentials

interface Greeter { default String hi() { return "hi"; } }

Interface with default method

class Impl implements A, B {}

Implement multiple interfaces

class Admin extends User {}

Single-class inheritance

@Override public String toString()

Mark overridden methods

super.method(); super(args);

Call parent method / constructor

class Box<T> { T value; }

Generic class

<T extends Comparable<T>> T max(...)

Bounded type parameter

List<? extends Number> nums

Covariant wildcard

enum Status { ACTIVE, DONE }

Enum type (can hold methods)

abstract class Base { abstract void run(); }

Abstract class + method

public static User of(String n) { ... }

Static factory method idiom

Optional

Optional.of(v); Optional.empty();

Create (of throws on null)

Optional.ofNullable(maybeNull)

Wrap a possibly-null value

opt.isPresent(); opt.isEmpty();

Presence checks

opt.orElse(defaultValue)

Fallback value

opt.orElseGet(() -> compute())

Lazy fallback (only if empty)

opt.orElseThrow()

Get or throw NoSuchElement

opt.map(User::name).filter(n -> !n.isBlank())

Chain transformations safely

opt.ifPresent(v -> use(v))

Run only when present

opt.ifPresentOrElse(use, this::warn)

Branch on both cases

opt.get()

Avoid: prefer orElseThrow()

Exceptions & try-with-resources

try { ... } catch (IOException e) { ... }

Catch a checked exception

catch (IOException | SQLException e)

Multi-catch in one block

finally { cleanup(); }

Always runs (even on throw)

try (var in = Files.newBufferedReader(p)) { ... }

Auto-close resources

throw new IllegalArgumentException("bad id");

Throw with a message

class AppException extends RuntimeException {}

Custom unchecked exception

RuntimeException vs Exception

Unchecked vs checked (declared)

e.getMessage(); e.getCause();

Inspect an exception

Objects.requireNonNull(x, "x is required")

Fail fast on null arguments

Concurrency

Thread.startVirtualThread(() -> work());

Cheap virtual thread (21+)

Executors.newVirtualThreadPerTaskExecutor()

One virtual thread per task

var pool = Executors.newFixedThreadPool(4);

Bounded platform thread pool

Future<T> f = pool.submit(task); f.get();

Submit and await a result

CompletableFuture.supplyAsync(() -> fetch())

Async computation

.thenApply(r -> parse(r))

Transform the async result

.thenCombine(other, (a, b) -> a + b)

Join two futures

.exceptionally(e -> fallback)

Recover from async failure

CompletableFuture.allOf(f1, f2).join();

Wait for all to finish

new AtomicInteger().incrementAndGet()

Lock-free counter

new ConcurrentHashMap<K, V>()

Thread-safe map

synchronized (lock) { ... }

Mutual exclusion block

I/O & files (java.nio)

Path p = Path.of("data", "file.txt");

Build a platform-safe path

String s = Files.readString(p);

Read a whole file as text

Files.writeString(p, text);

Write text (creates/truncates)

List<String> lines = Files.readAllLines(p);

Read all lines

try (var st = Files.lines(p)) { ... }

Stream lines lazily

Files.exists(p); Files.size(p);

Existence and size checks

Files.createDirectories(dir);

mkdir -p equivalent

Files.copy(src, dst); Files.move(src, dst);

Copy / move files

Files.deleteIfExists(p);

Delete without throwing

try (var files = Files.list(dir)) { ... }

List directory entries

Files.walk(dir)

Recursive directory stream

Files.createTempFile("up-", ".tmp")

Temporary file

Build & CLI

javac Main.java && java Main

Compile then run

java Main.java

Run a source file directly

java -jar app.jar

Run an executable jar

java -Xmx512m -jar app.jar

Cap the max heap size

jshell

Interactive Java REPL

mvn clean package

Maven: build the artifact

mvn test

Maven: run the test suite

mvn dependency:tree

Maven: inspect dependencies

./gradlew build

Gradle: full build via wrapper

./gradlew test --tests "UserTest"

Gradle: run selected tests

jar tf app.jar

List a jar's contents

java --version

Show the JDK version

No entry matches “:q”.

About Java Cheat Sheet

This Java cheat sheet compresses modern Java into one searchable page: basics and types, strings and text blocks, collections, streams and lambdas, records and sealed classes, pattern matching and switch, OOP essentials, Optional, exceptions and try-with-resources, concurrency, file I/O with java.nio, and the build and CLI commands.

It covers both the fundamentals you write every day — a List, a HashMap, a for-each loop, a try-with-resources block — and the newer language features that changed how Java reads, including records, sealed types, switch expressions with pattern matching and text blocks.

The sheet is free and client-side: filter rows live with the search box, hop between sections with the sticky table of contents, copy any snippet with one click and print the page for a reference beside your keyboard.

How to use Java Cheat Sheet

Skim the sections, from Basics & types and Collections through Concurrency to Build & CLI.
Search for a keyword such as stream, record or Optional to filter every row live.
Jump to Pattern matching & switch or Streams & lambdas when you need the modern syntax.
Click a snippet or its copy icon to copy the Java code to your clipboard.
Print the sheet for an offline Java reference.

Frequently asked questions

Twelve sections: basics and types, strings and text blocks, collections, streams and lambdas, records and sealed classes, pattern matching and switch, OOP essentials, Optional, exceptions and try-with-resources, concurrency, java.nio file I/O, and build and CLI commands.

Yes. Records, sealed classes, text blocks, var, enhanced switch expressions and pattern matching each get their own rows alongside the classic syntax.

It is built around what you actually write: List, Set, Map and Deque creation and iteration, then map, filter, collect, groupingBy, reduce and the common collectors.

Yes. Click any snippet or its hover copy icon and it lands on your clipboard with a brief confirmation.

Yes, completely free — searchable, printable and rendered in your browser with no sign-up.

Java Cheat Sheet

Basics & types

Strings & text blocks

Collections

Streams & lambdas

Records & sealed classes

Pattern matching & switch

OOP essentials

Optional

Exceptions & try-with-resources

Concurrency

I/O & files (java.nio)

Build & CLI

About Java Cheat Sheet

How to use Java Cheat Sheet

Frequently asked questions

What does the Java cheat sheet include?

Does it cover modern Java features?

Is the collections and streams section practical?

Can I copy code with one click?

Is the Java reference free?

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