Java 25 Cheat Sheet

void main() {                          // compact, Java 25
    IO.println("Hello");
    var name = IO.readln("Name? ");
}

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

java Program.java     javac Program.java     java Program     jshell

int i = 42;        long l = 42L;      double d = 3.14;    float f = 3.14f;
boolean b = true;  char c = 'A';      String s = "text";  var x = 42;
final double VAT = 0.19;

s.length()   s.isEmpty()   s.isBlank()   s.strip()   s.repeat(3)
s.toUpperCase()   s.substring(2, 5)   s.charAt(0)   s.indexOf("a")
s.contains("a")   s.startsWith("a")    s.replace("a", "b")   s.split(",")
s.equals(t)  s.equalsIgnoreCase(t)     s.compareTo(t)
"%s is %d".formatted(name, age)        String.join(", ", list)

var block = """
    multiline
    """;

if (a > b) { … } else if (a == b) { … } else { … }

var t = cond ? "yes" : "no";

var text = switch (value) {
    case 1, 2 -> "small";
    case Integer i when i > 100 -> "big";
    case String str -> "text: " + str;
    case null -> "nothing";
    default -> { yield "other"; }
};

if (o instanceof String str && str.length() > 3) { … }

for (var e : list) { … }
for (var i = 0; i < n; i++) { … }
while (cond) { … }
do { … } while (cond);
break;   continue;

var list = new ArrayList<String>();
list.add("a");  list.get(0);  list.remove("a");  list.size();  list.contains("a");
var fixed = List.of("a", "b");

var set = new HashSet<String>();
var map = new HashMap<String, Integer>();
map.put("a", 1);   map.get("a");   map.getOrDefault("b", 0);
map.merge("a", 1, Integer::sum);
map.computeIfAbsent("k", k -> new ArrayList<>());
map.forEach((k, v) -> IO.println(k + "=" + v));

int[] arr = {1, 2, 3};   arr.length;   Arrays.sort(arr);

int add(int a, int b) { return a + b; }
void doIt() { … }
int sum(int... numbers) { … }
static int help(int x) { … }

record Person(String name, int age) {
    Person { if (age < 0) throw new IllegalArgumentException(); }
    String initial() { return name.substring(0, 1); }
}

public class Account {
    private long balance;
    public Account(long start) { this.balance = start; }
    public long balance() { return balance; }
}

interface Payable {
    void pay(long cents);
    default String info() { return "payment"; }
}

sealed interface Shape permits Circle, Square {}
record Circle(double r) implements Shape {}
record Square(double a) implements Shape {}

enum Status { OPEN, DONE }

try {
    risky();
} catch (IOException | NumberFormatException e) {
    IO.println(e.getMessage());
} finally {
    cleanUp();
}

try (var reader = Files.newBufferedReader(path)) { … }

throw new IllegalArgumentException("message");
void m() throws IOException { … }

Predicate<String> p = s -> s.length() > 3;
Function<String, Integer> f = String::length;

list.stream()
    .filter(x -> x > 0)
    .map(String::valueOf)
    .sorted()
    .distinct()
    .limit(10)
    .toList();

list.stream().count();          list.stream().anyMatch(p);
list.stream().findFirst();      // Optional
list.stream().mapToInt(Integer::intValue).sum();
list.stream().collect(groupingBy(Person::city));
list.stream().map(Person::name).collect(joining(", "));

opt.orElse("default");   opt.orElseThrow();   opt.ifPresent(IO::println);

var p = Path.of("file.txt");
Files.readString(p);        Files.writeString(p, "content");
Files.readAllLines(p);      Files.lines(p);          // stream, close it
Files.exists(p);   Files.size(p);   Files.createDirectories(p.getParent());

LocalDate.now()            LocalDate.of(2026, 8, 12)
LocalDateTime.now()        LocalTime.of(14, 30)
date.plusDays(7)           date.minusMonths(1)
date.format(DateTimeFormatter.ofPattern("MM/dd/yyyy"))
Period.between(a, b)       Duration.ofMinutes(90)

Thread.startVirtualThread(() -> …);

try (var ex = Executors.newVirtualThreadPerTaskExecutor()) {
    var f = ex.submit(() -> compute());
    var result = f.get();
}

var counter = new AtomicInteger();
var map = new ConcurrentHashMap<String, Integer>();

java --version          jshell
javadoc File.java       jar --create --file x.jar -C out .
mvn test                mvn package
./gradlew build         ./gradlew run
java --enable-preview --source 25 File.java