Java | Фишки и трюки

for (initialization; condition; update) {
    // тело цикла
}

int[] numbers = {1, 2, 3, 4, 5};
for (int i = 0; i < numbers.length; i++) {
    System.out.println(i + ": " + numbers[i]);
}

for (type element : collection) {
    // тело цикла
}

int[] numbers = {1, 2, 3, 4, 5};
for (int number : numbers) {
    System.out.println(number);
}

while (condition) {
    // тело цикла
}

String code = UUID.randomUUID().toString();
boolean isCodeExists = repo.isCodeExists(code);
while (isCodeExists) {
    code = UUID.randomUUID().toString();
    isCodeExists = repo.isCodeExists(code);
}
repo.saveCode(code);

do {
    // тело цикла
} while (condition);

String command;
do {
    command = readCommand();
    doCommand(command);
} while (!"EXIT".equals(command));

class Base {
    public String className = "Base";

    public String getClassName() {
        return className;
    }
}

class Derived extends Base {
    public String className = "Derived";
}

public class Quest {
    public static void main(String[] args) {
        System.out.println(new Derived().getClassName());
    }
}

var list1 = List.of("apple", "banana", "cherry");
var list2 = List.of("orange", "pineapple", "mango");

var result = Stream.concat(list1.stream(), list2.stream())
        .filter(s -> s.length() > 5)
        .toList();

System.out.println(result);
// [banana, cherry, orange, pineapple]

super.method();

class Dog {
    public static void bark() {
        System.out.print("Woof ");
    }
}

class Basenji extends Dog {
    public static void bark() { }
}

public class Bark {
    public static void main(String[] args) {
        Dog woofer = new Dog();
        Dog nipper = new Basenji();
        woofer.bark();
        nipper.bark();
    }
}

import java.util.List;
import java.util.Map;
import java.util.stream.Collectors;

record Person(String name, String city) {}

public class StreamExample {
    public static void main(String[] args) {
        List<Person> people = List.of(
    new Person("Alice", "New York"),
    new Person("Bob", "Los Angeles"),
    new Person("Charlie", "New York"),
    new Person("David", "Los Angeles"),
    new Person("Edward", "San Francisco")
        );

        Map<String, List<Person>> peopleByCity = people.stream()
                .collect(Collectors.groupingBy(Person::city));

        peopleByCity.forEach((city, peopleInCity) -> {
            System.out.println(city + ": " + peopleInCity.stream()
                    .map(Person::name)
                    .collect(Collectors.joining(", ")));
        });
        // Вывод:
        // San Francisco: Edward
        // New York: Alice, Charlie
        // Los Angeles: Bob, David
    }
}

public final class StringBuilder extends AbstractStringBuilder implements java.io.Serializable, CharSequence {
    char[] value; // Массив символов, хранящий содержимое строки
    int count;    // Количество символов в текущем объекте StringBuilder
}

public StringBuilder append(String str) {
    if (str == null) str = "null";
    int len = str.length();
    ensureCapacityInternal(count + len); // расширить массив если нужно
    str.getChars(0, len, value, count);
    count += len;
    return this;
}