Java Quick Reference

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

int count = 10;

double price = 9.99;

boolean active = true;

var name = "Ada";  // inferred

final int MAX = 100;

System.out.println(x);

System.out.printf("%s is %d%n", name, age);

Scanner in = new Scanner(System.in);
String line = in.nextLine();

Integer.parseInt("42")

String.valueOf(42)

(int) 3.9  // 3

s.length()

s.charAt(0)

s.equals(other)

s.equalsIgnoreCase(other)

s.contains("java")

s.indexOf("a")

s.substring(0, 3)

s.toUpperCase()

s.strip()

s.split(",")

String.join(", ", list)

String.format("%.2f", price)

"Hi %s".formatted(name)

var sb = new StringBuilder();
sb.append("a").append("b");
String result = sb.toString();

String json = """
    { "name": "Ada" }
    """;

int[] nums = {3, 1, 2};

int[] zeros = new int[5];

Arrays.sort(nums);

List<String> names = new ArrayList<>();

List.of("a", "b")  // immutable

names.add("Ada");

names.get(0);

names.remove("Ada");

names.size()

names.contains("Ada")

Collections.sort(names);

names.sort(Comparator.comparing(String::length));

Collections.reverse(names);

names.reversed()  // Java 21+ view

Map<String, Integer> ages = new HashMap<>();
ages.put("Ada", 36);
ages.getOrDefault("Bob", 0);

for (var e : ages.entrySet()) {
    System.out.println(e.getKey() + "=" + e.getValue());
}

Set<String> tags = new HashSet<>(List.of("a", "b"));

if (x > 0) {
    // ...
} else if (x == 0) {
    // ...
} else {
    // ...
}

String label = n % 2 == 0 ? "even" : "odd";

String day = switch (n) {
    case 1, 7 -> "weekend";
    default -> "weekday";
};

for (int i = 0; i < 5; i++) { }

for (String name : names) { }

while (count < 10) {
    count++;
}

if (obj instanceof String s) {
    System.out.println(s.length());
}

public class Dog {
    private final String name;

    public Dog(String name) {
        this.name = name;
    }

    public String getName() {
        return name;
    }
}

public class Puppy extends Dog {
    public Puppy(String name) {
        super(name);
    }
}

interface Shape {
    double area();
}

record Point(int x, int y) {}

enum Status { ACTIVE, DISABLED }

public static int add(int a, int b) {
    return a + b;
}

Comparator<String> byLength = (a, b) -> a.length() - b.length();

List<String> result = names.stream()
    .filter(n -> n.startsWith("A"))
    .map(String::toUpperCase)
    .toList();

int total = nums.stream().mapToInt(Integer::intValue).sum();

Map<String, List<User>> byRole = users.stream()
    .collect(Collectors.groupingBy(User::role));

Optional<User> u = users.stream().findFirst();
u.map(User::name).orElse("none");

try {
    int n = Integer.parseInt(text);
} catch (NumberFormatException e) {
    System.err.println(e.getMessage());
} finally {
    cleanup();
}

throw new IllegalArgumentException("bad input");

try (var reader = Files.newBufferedReader(path)) {
    // reader closes automatically
}

String text = Files.readString(Path.of("in.txt"));

Files.writeString(Path.of("out.txt"), text);