Java

class Animal {
    void sound() {
        System.out.println("Animal makes a sound");
    }
}

class Dog extends Animal {
    @Override
    void sound() {
        System.out.println("Dog barks");
    }
}

public class Main {
    public static void main(String[] args) {
        Animal myDog = new Dog();
        myDog.sound(); // Output: Dog barks
    }
}

String a = new String("hi");
String b = new String("hi");

System.out.println(a == b);       // false
System.out.println(a.equals(b));  // true

StringBuilder sb = new StringBuilder();
for (int i = 0; i < 3; i++) {
  sb.append(i).append(",");
}
System.out.println(sb.toString());

Integer x = null;
// int y = x; // NullPointerException due to unboxing

Integer a = 100;
Integer b = 100;
System.out.println(a == b); // true (Integer cache)

Integer c = 1000;
Integer d = 1000;
System.out.println(c == d); // false

final class User {
  private final String email;

  User(String email) {
    this.email = email;
  }

  @Override public boolean equals(Object o) {
    if (this == o) return true;
    if (!(o instanceof User)) return false;
    return email.equals(((User) o).email);
  }

  @Override public int hashCode() {
    return email.hashCode();
  }
}

class StopFlag {
  private volatile boolean stop = false;

  void requestStop() {
    stop = true;
  }

  void runLoop() {
    while (!stop) {
      // do work
    }
  }
}

CompletableFuture<Integer> result = CompletableFuture
  .supplyAsync(() -> 40)
  .thenApply(x -> x + 2)
  .exceptionally(e -> 0);

System.out.println(result.join());

Optional<User> user = repo.findById(id);
String name = user.map(User::getName).orElse("unknown");

try (var in = Files.newInputStream(path)) {
  return new String(in.readAllBytes(), StandardCharsets.UTF_8);
}

List<String> activeNames = users.stream()
  .filter(User::isActive)
  .map(User::getName)
  .toList();

List<String> xs = List.of("a", "b");
// xs.add("c"); // throws UnsupportedOperationException

try (var in = Files.newInputStream(path)) {
  return new String(in.readAllBytes(), StandardCharsets.UTF_8);
}