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ExecutorService executorService = Executors.newSingleThreadExecutor();

Callable<String> hello = () -> {
            Thread.sleep(2000L);
            return "Hello";
        };

Callable<String> java = () -> {
            Thread.sleep(3000L);
            return "Java";

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// when there is no return value
CompletableFuture<Void> future2 = CompletableFuture.runAsync(() -> {
    System.out.println("Hello "+ Thread.currentThread().getName());
});
future2.get(); // get() should be called to execute the thread


// when there is a return value
CompletableFuture<String> future3 = CompletableFuture.supplyAsync(() -> {
    System.out.println("HEllo " + Thread.currentThread().getName());

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// Using a callback method
CompletableFuture<String> future4 = CompletableFuture.supplyAsync(() -> {
    System.out.println("Hello "+ Thread.currentThread().getName());
    return "Hello";
}).thenApply((s) -> { // thenApply, thenAccept or thenRun can also be used here
    System.out.println(Thread.currentThread().getName());
    return s.toUpperCase(Locale.ROOT);
});

System.out.println(future4.get());

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// 1. Assign a function to a variable
const sayHello = function() {
   console.log("Hello");
  return "Hello, ";
}
// Invoke it using the variable
sayHello();

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// 2. Pass a function as a parameter
function sayHello() {
   return "Hello, ";
}
function greeting(helloMessage, name) {
  console.log(helloMessage() + name);
}

greeting(sayHello, "JavaScript!");

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// Creata a hash map class and put values into the map
Map<String, String> map = new HashMap<>();
map.put("1", "A");
map.put("2", "B");
map.put("3", "C");


// Create a class that only contains a collection.
public class GameRanking {

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public class LottoService {
  private static final int LOTTO_NUMBERS_SIZE = 6;
  
  public void createLottoNumber() {
    List<Long> lottoNumbers = createNonDuplicateNumbers();
    validateSize(lottoNumbers);
    validateDuplicate(lottoNumbers);
  }
  
  public void validateSize(List<Long> lottoNumbers) {

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public class LottoService {
  private static final int LOTTO_NUMBERS_SIZE = 6;
  
  public void createLottoNumber() {
    List<Long> lottoNumbers = createNonDuplicateNumbers();
    validateSize(lottoNumbers);
    validateDuplicate(lottoNumbers);
  }
  
  public void validateSize(List<Long> lottoNumbers) {

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public class LottoTicket {
  private static final int LOTTO_NUMBERS_SIZE = 6;
  
  public LottoTicket(List<Long> lottoNumbers) {
    List<Long> lottoNumbers = createNonDuplicateNumbers();
    validateSize(lottoNumbers);
    validateDuplicate(lottoNumbers);
  }
  
  public void validateSize(List<Long> lottoNumbers) {

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public class Orders {
  private final List<Order> orders;
  
  public Orders(List<Order> orders) {
    this.orders = orders;
  }
  
  public long getAmountSum() {
    return orders.stream()
                  .mapToLong(Order::getAmount)