Domande di intervista su Java 8 (+ risposte)

Questo articolo fa parte di una serie: • Domande di intervista sulle raccolte Java

• Domande di intervista sul sistema di tipo Java

• Domande di intervista sulla concorrenza Java (+ risposte)

• Struttura delle classi Java e domande di colloquio sull'inizializzazione

• Domande di intervista Java 8 (+ risposte) (articolo corrente) • Gestione della memoria in domande di intervista Java (+ risposte)

• Domande di intervista su Java Generics (+ risposte)

• Domande di intervista sul controllo del flusso Java (+ risposte)

• Domande di intervista sulle eccezioni Java (+ risposte)

• Domande di intervista sulle annotazioni Java (+ risposte)

• Domande principali per l'intervista su Spring Framework

1. Introduzione

In questo articolo, esploreremo alcune delle domande relative a JDK8 che potrebbero apparire durante un'intervista.

Java 8 è una versione della piattaforma ricca di nuove funzionalità di linguaggio e classi di libreria. La maggior parte di queste nuove funzionalità è orientata al raggiungimento di un codice più pulito e compatto e alcune aggiungono nuove funzionalità che non sono mai state supportate in Java.

2. Conoscenza generale di Java 8

Q1. Quali nuove funzionalità sono state aggiunte in Java 8?

Java 8 viene fornito con diverse nuove funzionalità, ma le più significative sono le seguenti:

  • Espressioni lambda : una nuova funzionalità del linguaggio che consente di trattare le azioni come oggetti
  • Riferimenti ai metodi : abilita la definizione delle espressioni Lambda facendo riferimento ai metodi utilizzando direttamente i loro nomi
  • Facoltativo : classe wrapper speciale utilizzata per esprimere opzionalità
  • Interfaccia funzionale : un'interfaccia con al massimo un metodo astratto, l'implementazione può essere fornita utilizzando un'espressione Lambda
  • Metodi predefiniti : ci danno la possibilità di aggiungere implementazioni complete nelle interfacce oltre a metodi astratti
  • Nashorn, JavaScript Engine - Motore basato su Java per l'esecuzione e la valutazione del codice JavaScript
  • Stream API : una speciale classe iteratore che consente di elaborare raccolte di oggetti in modo funzionale
  • Date API : un'API Date migliorata e immutabile ispirata a JodaTime

Insieme a queste nuove funzionalità, vengono apportati molti miglioramenti alle funzionalità, sia a livello di compilatore che di JVM.

3. Riferimenti al metodo

Q1. Che cos'è un metodo di riferimento?

Un riferimento a un metodo è un costrutto Java 8 che può essere utilizzato per fare riferimento a un metodo senza richiamarlo. Viene utilizzato per trattare i metodi come espressioni Lambda. Funzionano solo come zucchero sintattico per ridurre la verbosità di alcuni lambda. In questo modo, il codice seguente:

(o) -> o.toString();

può diventare:

Object::toString();

Un riferimento al metodo può essere identificato da un doppio due punti che separa il nome di una classe o di un oggetto e il nome del metodo. Ha diverse varianti come il riferimento del costruttore:

String::new;

Riferimento al metodo statico:

String::valueOf;

Riferimento al metodo dell'istanza associata:

str::toString;

Riferimento al metodo dell'istanza non associata:

String::toString;

Puoi leggere una descrizione dettagliata dei riferimenti ai metodi con esempi completi seguendo questo collegamento e questo.

Q2. Qual è il significato di String :: Valueof Expression?

È un metodo statico di riferimento al metodo valueOf della classe String .

4. Facoltativo

Q1. Cosa è facoltativo ? Come si puó usare?

Opzionale è una nuova classe in Java 8 che incapsula un valore opzionale, cioè un valore che è presente o meno. È un involucro attorno a un oggetto e puoi pensarlo come un contenitore di zero o di un elemento.

Optional ha uno speciale valore Optional.empty () invece di un valore nullo avvolto . Pertanto, può essere utilizzato al posto di un valore nullable per eliminare NullPointerException in molti casi.

Puoi leggere un articolo dedicato su Opzionale qui.

Lo scopo principale di Optional , come progettato dai suoi creatori, era di essere un tipo restituito di metodi che in precedenza avrebbe restituito null . Tali metodi richiederebbero di scrivere codice boilerplate per controllare il valore restituito e talvolta potrebbero dimenticare di eseguire un controllo difensivo. In Java 8, un tipo restituito facoltativo richiede esplicitamente di gestire in modo diverso i valori racchiusi null o non null.

Ad esempio, il metodo Stream.min () calcola il valore minimo in un flusso di valori. Ma cosa succede se il flusso è vuoto? Se non fosse per Facoltativo , il metodo restituirebbe null o genererebbe un'eccezione.

But it returns an Optional value which may be Optional.empty() (the second case). This allows us to easily handle such case:

int min1 = Arrays.stream(new int[]{1, 2, 3, 4, 5}) .min() .orElse(0); assertEquals(1, min1); int min2 = Arrays.stream(new int[]{}) .min() .orElse(0); assertEquals(0, min2); 

It's worth noting that Optional is not a general purpose class like Option in Scala. It is not recommended to be used as a field value in entity classes, which is clearly indicated by it not implementing the Serializable interface.

5. Functional Interfaces

Q1. Describe Some of the Functional Interfaces in the Standard Library.

There are a lot of functional interfaces in the java.util.function package, the more common ones include but not limited to:

  • Function – it takes one argument and returns a result
  • Consumer – it takes one argument and returns no result (represents a side effect)
  • Supplier – it takes not argument and returns a result
  • Predicate – it takes one argument and returns a boolean
  • BiFunction – it takes two arguments and returns a result
  • BinaryOperator – it is similar to a BiFunction, taking two arguments and returning a result. The two arguments and the result are all of the same types
  • UnaryOperator – it is similar to a Function, taking a single argument and returning a result of the same type

For more on functional interfaces, see the article “Functional Interfaces in Java 8”.

Q2. What Is a Functional Interface? What Are the Rules of Defining a Functional Interface?

A functional interface is an interface with no more, no less but one single abstract method (default methods do not count).

Where an instance of such interface is required, a Lambda Expression can be used instead. More formally put: Functional interfaces provide target types for lambda expressions and method references.

The arguments and return type of such expression directly match those of the single abstract method.

For instance, the Runnable interface is a functional interface, so instead of:

Thread thread = new Thread(new Runnable() { public void run() { System.out.println("Hello World!"); } });

you could simply do:

Thread thread = new Thread(() -> System.out.println("Hello World!"));

Functional interfaces are usually annotated with the @FunctionalInterface annotation – which is informative and does not affect the semantics.

6. Default Method

Q1. What Is a Default Method and When Do We Use It?

A default method is a method with an implementation – which can be found in an interface.

We can use a default method to add a new functionality to an interface while maintaining backward compatibility with classes that are already implementing the interface:

public interface Vehicle { public void move(); default void hoot() { System.out.println("peep!"); } }

Usually, when a new abstract method is added to an interface, all implementing classes will break until they implement the new abstract method. In Java 8, this problem has been solved by the use of default method.

For example, Collection interface does not have forEach method declaration. Thus, adding such method would simply break the whole collections API.

Java 8 introduces default method so that Collection interface can have a default implementation of forEach method without requiring the classes implementing this interface to implement the same.

Q2. Will the Following Code Compile?

@FunctionalInterface public interface Function2 { public V apply(T t, U u); default void count() { // increment counter } }

Yes. The code will compile because it follows the functional interface specification of defining only a single abstract method. The second method, count, is a default method that does not increase the abstract method count.

7. Lambda Expressions

Q1. What Is a Lambda Expression and What Is It Used for

In very simple terms, a lambda expression is a function that can be referenced and passed around as an object.

Lambda expressions introduce functional style processing in Java and facilitate the writing of compact and easy-to-read code.

Because of this, lambda expressions are a natural replacement for anonymous classes as method arguments. One of their main uses is to define inline implementations of functional interfaces.

Q2. Explain the Syntax and Characteristics of a Lambda Expression

A lambda expression consists of two parts: the parameter part and the expressions part separated by a forward arrow as below:

params -> expressions

Any lambda expression has the following characteristics:

  • Optional type declaration – when declaring the parameters on the left-hand side of the lambda, we don't need to declare their types as the compiler can infer them from their values. So int param -> … and param ->… are all valid
  • Optional parentheses – when only a single parameter is declared, we don't need to place it in parentheses. This means param -> … and (param) -> … are all valid. But when more than one parameter is declared, parentheses are required
  • Optional curly braces – when the expressions part only has a single statement, there is no need for curly braces. This means that param – > statement and param – > {statement;} are all valid. But curly braces are required when there is more than one statement
  • Optional return statement – when the expression returns a value and it is wrapped inside curly braces, then we don't need a return statement. That means (a, b) – > {return a+b;} and (a, b) – > {a+b;} are both valid

To read more about Lambda expressions, follow this link and this one.

8. Nashorn Javascript

Q1. What Is Nashorn in Java8?

Nashorn is the new Javascript processing engine for the Java platform that shipped with Java 8. Until JDK 7, the Java platform used Mozilla Rhino for the same purpose. as a Javascript processing engine.

Nashorn provides better compliance with the ECMA normalized JavaScript specification and better runtime performance than its predecessor.

Q2. What Is JJS?

In Java 8, jjs is the new executable or command line tool used to execute Javascript code at the console.

9. Streams

Q1. What Is a Stream? How Does It Differ from a Collection?

In simple terms, a stream is an iterator whose role is to accept a set of actions to apply on each of the elements it contains.

The stream represents a sequence of objects from a source such as a collection, which supports aggregate operations. They were designed to make collection processing simple and concise. Contrary to the collections, the logic of iteration is implemented inside the stream, so we can use methods like map and flatMap for performing a declarative processing.

Another difference is that the Stream API is fluent and allows pipelining:

int sum = Arrays.stream(new int[]{1, 2, 3}) .filter(i -> i >= 2) .map(i -> i * 3) .sum();

And yet another important distinction from collections is that streams are inherently lazily loaded and processed.

Q2. What Is the Difference Between Intermediate and Terminal Operations?

Stream operations are combined into pipelines to process streams. All operations are either intermediate or terminal.

Intermediate operations are those operations that return Stream itself allowing for further operations on a stream.

These operations are always lazy, i.e. they do not process the stream at the call site, an intermediate operation can only process data when there is a terminal operation. Some of the intermediate operations are filter, map and flatMap.

Terminal operations terminate the pipeline and initiate stream processing. The stream is passed through all intermediate operations during terminal operation call. Terminal operations include forEach, reduce, Collect and sum.

To drive this point home, let us look at an example with side effects:

public static void main(String[] args) { System.out.println("Stream without terminal operation"); Arrays.stream(new int[] { 1, 2, 3 }).map(i -> { System.out.println("doubling " + i); return i * 2; }); System.out.println("Stream with terminal operation"); Arrays.stream(new int[] { 1, 2, 3 }).map(i -> { System.out.println("doubling " + i); return i * 2; }).sum(); }

The output will be as follows:

Stream without terminal operation Stream with terminal operation doubling 1 doubling 2 doubling 3

As you can see, the intermediate operations are only triggered when a terminal operation exists.

Q3. What Is the Difference Between Map and flatMap Stream Operation?

There is a difference in signature between map and flatMap. Generally speaking, a map operation wraps its return value inside its ordinal type while flatMap does not.

For example, in Optional, a map operation would return Optional type while flatMap would return String type.

So after mapping, one needs to unwrap (read “flatten”) the object to retrieve the value whereas, after flat mapping, there is no such need as the object is already flattened. The same concept is applied to mapping and flat mapping in Stream.

Both map and flatMap are intermediate stream operations that receive a function and apply this function to all elements of a stream.

The difference is that for the map, this function returns a value, but for flatMap, this function returns a stream. The flatMap operation “flattens” the streams into one.

Here's an example where we take a map of users' names and lists of phones and “flatten” it down to a list of phones of all the users using flatMap:

Map
    
      people = new HashMap(); people.put("John", Arrays.asList("555-1123", "555-3389")); people.put("Mary", Arrays.asList("555-2243", "555-5264")); people.put("Steve", Arrays.asList("555-6654", "555-3242")); List phones = people.values().stream() .flatMap(Collection::stream) .collect(Collectors.toList());
    

Q4. What Is Stream Pipelining in Java 8?

Stream pipelining is the concept of chaining operations together. This is done by splitting the operations that can happen on a stream into two categories: intermediate operations and terminal operations.

Each intermediate operation returns an instance of Stream itself when it runs, an arbitrary number of intermediate operations can, therefore, be set up to process data forming a processing pipeline.

There must then be a terminal operation which returns a final value and terminates the pipeline.

10. Java 8 Date and Time API

Q1. Tell Us About the New Date and Time API in Java 8

A long-standing problem for Java developers has been the inadequate support for the date and time manipulations required by ordinary developers.

The existing classes such as java.util.Date and SimpleDateFormatter aren’t thread-safe, leading to potential concurrency issues for users.

Poor API design is also a reality in the old Java Data API. Here's just a quick example – years in java.util.Date start at 1900, months start at 1, and days start at 0 which is not very intuitive.

These issues and several others have led to the popularity of third-party date and time libraries, such as Joda-Time.

In order to address these problems and provide better support in JDK, a new date and time API, which is free of these problems, has been designed for Java SE 8 under the package java.time.

11. Conclusion

In this article, we've explored a few very important questions for technical interview questions with a bias on Java 8. This is by no means an exhaustive list but only contains questions that we think are most likely to appear in each new feature of Java 8.

Even if you are just starting up, ignorance of Java 8 isn't a good way to go in an interview, especially when Java appears strongly on your resume. It is, therefore, important that you take some time to understand the answers to these questions and possibly do more research.

Good luck in your interview.

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