Java 8 Interview Fragen (+ Antworten)

Dieser Artikel ist Teil einer Reihe: • Fragen zum Interview mit Java Collections

• Fragen zum Java Type System-Interview

• Fragen zum Java Concurrency-Interview (+ Antworten)

• Fragen zum Java-Klassenstruktur- und Initialisierungsinterview

• Java 8-Interviewfragen (+ Antworten) (aktueller Artikel) • Speicherverwaltung in Java-Interviewfragen (+ Antworten)

• Fragen zum Java Generics-Interview (+ Antworten)

• Fragen zum Java Flow Control-Interview (+ Antworten)

• Fragen zum Interview mit Java-Ausnahmen (+ Antworten)

• Fragen zum Interview mit Java Annotations (+ Antworten)

• Fragen zum Top Spring Framework-Interview

1. Einleitung

In diesem Artikel werden wir einige der JDK8-bezogenen Fragen untersuchen, die während eines Interviews auftauchen könnten.

Java 8 ist eine Plattformversion mit neuen Sprachfunktionen und Bibliotheksklassen. Die meisten dieser neuen Funktionen zielen darauf ab, saubereren und kompakteren Code zu erzielen, und einige fügen neue Funktionen hinzu, die in Java noch nie unterstützt wurden.

2. Java 8 Allgemeinwissen

Q1. Welche neuen Funktionen wurden in Java 8 hinzugefügt?

Java 8 wird mit mehreren neuen Funktionen ausgeliefert, die wichtigsten sind jedoch die folgenden:

  • Lambda-Ausdrücke - eine neue Sprachfunktion, mit der Aktionen als Objekte behandelt werden können
  • Methodenreferenzen - Aktivieren Sie die Definition von Lambda-Ausdrücken, indem Sie direkt auf Methoden verweisen, deren Namen verwendet werden
  • Optional - spezielle Wrapper-Klasse zum Ausdrücken der Optionalität
  • Funktionsschnittstelle - Eine Schnittstelle mit maximal einer abstrakten Methode. Die Implementierung kann mithilfe eines Lambda-Ausdrucks bereitgestellt werden
  • Standardmethoden - Geben Sie uns die Möglichkeit, neben abstrakten Methoden auch vollständige Implementierungen in Schnittstellen hinzuzufügen
  • Nashorn, JavaScript Engine - Java-basierte Engine zum Ausführen und Auswerten von JavaScript-Code
  • Stream API - eine spezielle Iteratorklasse, mit der Sammlungen von Objekten auf funktionale Weise verarbeitet werden können
  • Datums- API - eine verbesserte, unveränderliche, von JodaTime inspirierte Datums-API

Zusammen mit diesen neuen Funktionen werden viele Funktionsverbesserungen sowohl auf Compiler- als auch auf JVM-Ebene unter der Haube vorgenommen.

3. Methodenreferenzen

Q1. Was ist eine Methodenreferenz?

Eine Methodenreferenz ist ein Java 8-Konstrukt, mit dem eine Methode referenziert werden kann, ohne sie aufzurufen. Es wird zur Behandlung von Methoden als Lambda-Ausdrücke verwendet. Sie wirken nur als syntaktischer Zucker, um die Ausführlichkeit einiger Lambdas zu verringern. Auf diese Weise wird der folgende Code:

(o) -> o.toString();

kann werden:

Object::toString();

Eine Methodenreferenz kann durch einen Doppelpunkt identifiziert werden, der einen Klassen- oder Objektnamen und den Namen der Methode trennt. Es hat verschiedene Variationen wie Konstruktorreferenz:

String::new;

Statische Methodenreferenz:

String::valueOf;

Referenz der gebundenen Instanzmethode:

str::toString;

Referenz für ungebundene Instanzmethoden:

String::toString;

Sie können eine detaillierte Beschreibung der Methodenreferenzen mit vollständigen Beispielen lesen, indem Sie diesem und diesem Link folgen.

Q2. Was bedeutet String :: Valueof Expression?

Es ist eine statische Methodenreferenz auf die valueOf- Methode der String- Klasse.

4. Optional

Q1. Was ist optional ? Wie kann es verwendet werden?

Optional ist eine neue Klasse in Java 8, die einen optionalen Wert kapselt, dh einen Wert, der entweder vorhanden ist oder nicht. Es ist ein Wrapper um ein Objekt, und Sie können es sich als Container mit null oder einem Element vorstellen.

Optional hat einen speziellen Wert für Optional.empty () anstelle von null . Daher kann es in vielen Fällen anstelle eines nullbaren Werts verwendet werden, um die NullPointerException zu entfernen .

Hier können Sie einen speziellen Artikel über Optional lesen .

The main purpose of Optional, as designed by its creators, was to be a return type of methods that previously would return null. Such methods would require you to write boilerplate code to check the return value and sometimes could forget to do a defensive check. In Java 8, an Optional return type explicitly requires you to handle null or non-null wrapped values differently.

For instance, the Stream.min() method calculates the minimum value in a stream of values. But what if the stream is empty? If it was not for Optional, the method would return null or throw an exception.

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.

Weiter » Fragen zur Speicherverwaltung in Java-Interviews (+ Antworten) « Vorherige Fragen zum Java-Klassenstruktur- und Initialisierungsinterview