Java cheat sheet

A scannable Java reference: 27 short snippets across 12 topics, each linking back to the lesson it came from.

At a glance

Quick snippets

Java: getting started

Your first class

// Hello.java
public class Hello {
    public static void main(String[] args) {
        System.out.println("Hello, world");
    }
}

Your first class

javac Hello.java     # produces Hello.class
java Hello           # runs it; note: no .class suffix

Modern base syntax

// since Java 10: local variable type inference
var name = "Ada";
var scores = new int[]{90, 85, 77};

// text blocks (Java 15+) for multi-line strings
var message = """
    Line one
    Line two""";

// var still needs a type; it is not dynamic typing
// name = 42;  // compile error

Full lesson: Java: getting started →

Java types and variables

Eight primitives

int count = 10;
long big = 10_000_000_000L;   // underscores aid readability
double rate = 0.075;
char grade = 'A';             // single quotes, char only
boolean active = true;

Objects and autoboxing

Integer boxed = 1000;      // autoboxed
int unboxed = boxed;       // auto-unboxed

Integer a = 1000, b = 1000;
System.out.println(a == b);      // false: different objects
System.out.println(a.equals(b)); // true: same value

Integer c = 100, d = 100;
System.out.println(c == d);      // true: small values are cached (-128..127)

Strings and equality

String s1 = "hello";
String s2 = new String("hello");
System.out.println(s1 == s2);      // false: reference comparison
System.out.println(s1.equals(s2)); // true: value comparison

// string pool: literals are interned and share storage
String joined = "a" + "b" + 1;     // "ab1"
String better = "%s has %d items".formatted("Cart", 3);

Full lesson: Java types and variables →

Java collections

Using them

var names = new ArrayList<String>();
names.add("Ada");
names.add("Grace");
names.add(0, "Alan");

var unique = new LinkedHashSet<>(names);
var byId = new HashMap<Integer, String>();
byId.put(1, "Ada");
byId.getOrDefault(99, "unknown");
byId.computeIfAbsent(2, k -> "created");

names.sort(Comparator.naturalOrder());

Using them

for (String n : names) System.out.println(n);
names.forEach(System.out::println);

var adults = people.stream()
    .filter(p -> p.age() >= 18)
    .map(Person::name)
    .toList();

equals and hashCode

public record User(int id, String email) {}   // equals/hashCode generated

// mutating a key after insertion makes an entry unreachable
var key = new MutableKey("a");
map.put(key, 1);
key.setName("b");
map.get(key);   // null - its hash changed

Full lesson: Java collections →

Exceptions and try-with-resources

Handling

try {
    int value = Integer.parseInt(text);
} catch (NumberFormatException e) {
    System.err.println("Not a number: " + text);
} finally {
    cleanup();          // always runs
}

// multi-catch for unrelated types
try { risky(); }
catch (IOException | SQLException e) { log(e); }

try-with-resources

try (var reader = Files.newBufferedReader(path);
     var conn = dataSource.getConnection()) {
    String line;
    while ((line = reader.readLine()) != null) {
        process(line);
    }
} catch (IOException | SQLException e) {
    throw new UncheckedIOException(new IOException(e));
}

Full lesson: Exceptions and try-with-resources →

Classes, records and interfaces

Records and enums

public record Point(int x, int y) {
    public Point {
        // compact constructor: validate
        if (x < 0 || y < 0) throw new IllegalArgumentException("negative coords");
    }
    public double distanceFromOrigin() { return Math.hypot(x, y); }
}

public enum Status { ACTIVE, SUSPENDED, CLOSED }

Full lesson: Classes, records and interfaces →

Setting up a modern Java toolchain

One JDK, a handful of commands

java -version          # the launcher and the runtime
javac -version         # the compiler that ships with a JDK, not a JRE

java Hello.java        # single-file source: compile and run in one step
jshell                 # interactive REPL
javadoc -d docs src/main/java/com/example/*.java
jar --create --file app.jar -C out .

jshell and the IDE

jshell> var names = List.of("Ada", "Grace")
names ==> [Ada, Grace]
jshell> names.stream().map(String::toUpperCase).toList()
$2 ==> [ADA, GRACE]
jshell> /vars          # every variable defined so far
jshell> /methods       # and every method
jshell> /reset         # clear state
jshell> /exit

Full lesson: Setting up a modern Java toolchain →

Control flow, methods and the modern main

Compact source files and instance main

// Hello.java — a compact source file: no class declaration, no static, no args
void main() {
    String name = IO.readln("Name? ");
    IO.println("Hello, " + name);
}

Compact source files and instance main

java Hello.java        # compiles and runs in one step, no build file needed

Full lesson: Control flow, methods and the modern main →

Dates, text and numbers

java.time

Instant now = Instant.now();                                   // UTC timeline point
LocalDate today = LocalDate.now(ZoneId.of("Europe/London"));
LocalDateTime meeting = LocalDateTime.of(2026, 9, 18, 9, 30);

Duration elapsed = Duration.between(meeting, meeting.plusHours(2));   // PT2H
Period span = Period.between(LocalDate.of(2026, 1, 1), today);        // P8M17D

ZonedDateTime tokyo = meeting.atZone(ZoneId.of("Asia/Tokyo"));
Instant asInstant = tokyo.toInstant();

Formatting and parsing

DateTimeFormatter friendly =
    DateTimeFormatter.ofPattern("d MMM uuuu HH:mm", Locale.UK);

String text = meeting.format(friendly);              // "18 Sep 2026 09:30"
LocalDateTime parsed = LocalDateTime.parse(text, friendly);

String machine = DateTimeFormatter.ISO_INSTANT.format(Instant.now());

Full lesson: Dates, text and numbers →

Files, I/O and JSON

JSON with records

public record Customer(String id, String email, List<String> tags) {}

ObjectMapper mapper = new ObjectMapper()
    .registerModule(new JavaTimeModule())
    .disable(SerializationFeature.WRITE_DATES_AS_TIMESTAMPS)
    .enable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES);

String json = mapper.writeValueAsString(customer);        // records serialise directly
Customer back = mapper.readValue(json, Customer.class);

Full lesson: Files, I/O and JSON →

Modules, packages and build configuration

Packages and visibility

package com.example.orders;          // the directory must match exactly

public class OrderService {

    public Receipt place(Order order) { ... }      // the published API
    Receipt recalculate(Order order) { ... }       // package-private helper

    private BigDecimal taxFor(Order order) { ... } // visible only inside this class
}

JPMS modules

// src/main/java/module-info.java
module com.example.orders {
    requires java.sql;
    requires transitive com.example.core;    // callers also see core

    exports com.example.orders.api;          // only this package is public
    opens com.example.orders.model to com.fasterxml.jackson.databind;

    uses com.example.orders.spi.Pricer;
    provides com.example.orders.spi.Pricer
        with com.example.orders.internal.SimplePricer;
}

Multi-module builds and runnable jars

<!-- parent pom.xml: the reactor builds these in dependency order -->
<modules>
  <module>core</module>
  <module>api</module>
</modules>

Full lesson: Modules, packages and build configuration →

Concurrency, virtual threads and structured concurrency

Threads and executors

Thread platform = Thread.ofPlatform().name("worker-1").start(() -> doWork());

try (var pool = Executors.newFixedThreadPool(8)) {
    Future<Integer> future = pool.submit(() -> compute());
    int result = future.get(2, TimeUnit.SECONDS);       // blocks, wraps failures
    pool.shutdown();
}

try (var virtual = Executors.newVirtualThreadPerTaskExecutor()) {
    for (Order order : orders) virtual.submit(() -> enrich(order));
}

CompletableFuture

var io = Executors.newVirtualThreadPerTaskExecutor();

CompletableFuture<Quote> quoted = CompletableFuture
    .supplyAsync(() -> fetchQuote(sku), io)
    .thenApply(Quote::withTax)
    .exceptionally(err -> Quote.failed(sku));

CompletableFuture<Void> all = CompletableFuture.allOf(a, b, c);
CompletableFuture<Object> fastest = CompletableFuture.anyOf(a, b);

Virtual threads and structured concurrency

try (var scope = new StructuredTaskScope.ShutdownOnFailure()) {
    Supplier<User> user = scope.fork(() -> loadUser(id));
    Supplier<Orders> orders = scope.fork(() -> loadOrders(id));

    scope.join();                 // wait for every forked task
    scope.throwIfFailed();        // propagate the first failure
    return new Profile(user.get(), orders.get());
}

Full lesson: Concurrency, virtual threads and structured concurrency →

Testing, logging and JVM tooling

Logging and JVM tooling

private static final Logger log = LoggerFactory.getLogger(Checkout.class);

log.info("order placed id={} total={}", order.id(), total);
log.warn("upstream slow id={}", order.id());

try {
    gateway.charge(order, total);
} catch (PaymentException failure) {
    log.error("charge failed id={}", order.id(), failure);   // the throwable goes last
}

Logging and JVM tooling

<!-- logback.xml: one line per event, on stdout, for the platform to collect -->
<configuration>
  <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
    <encoder>
      <pattern>%d{ISO8601} %-5level [%thread] %logger{36} - %msg%n</pattern>
    </encoder>
  </appender>
  <root level="INFO"><appender-ref ref="STDOUT"/></root>
</configuration>

Full lesson: Testing, logging and JVM tooling →

FAQ

Related cheat sheets

Node.js PHP HTTP Go Rust Spring Boot

Last refreshed 2026-09-27.