Introduction To Java 8 Interview Questions And Answer

Java 8 is a new platform that is newly released with new libraries and classes. Mostly these features focus on having a cleaner and compact code. The new features added in this version can be listed as Lambda Expressions, method references, optional, functional interface, and default methods, Nashorn, Stream, and Date API. The need for change in Java was to Utilize Current Multi-Core CPUs Efficiently and to Utilize FP Features.

Now, if you are looking for a job which is related to Java 8 then you need to prepare for the 2018 Java 8 Interview Questions. It is true that every interview is different as per the different job profiles. Here, we have prepared the important Java 8 Interview Questions and Answers which will help you get success in your interview.

In this 2018 Java 8 Interview Questions article, we shall present 10 most important and frequently asked Java 8 interview questions. These questions will help students build their concepts around Java 8 and help them ace the interview. These interview questions are divided into two parts are as follows:

Part 1 – Java 8 Interview Questions (Basic)

This first part covers basic Java 8 Interview Questions and Answers

Q1. How is Java 8 different from previous Java versions?

Answer:
Different Java versions are as follows:

• Date API: It is an improved API which is immutable JodaTime inspired date API.
• Stream API: This is a special iterator class which helps in processing collection of objects in a functional manner.
• Lambda Expression: This is a new language feature that allows treating different actions performed as objects.
• Method Reference: This helps in defining Lambda expressions by referring to different methods directly by using their names.
• Optional: This is a special wrapper class which helps in specifying optional features.
• Functional interface: This is an interface with maximum one abstract method and its implementation is provided by Lambda expression
• Nashorn Javascript Engine: It is a Java-based engine which is used for executing and evaluating JavaScript code.

Q2. Is it possible to implement two interfaces having default method with same name and signature? Explain with example.

Answer:
This is the basic Java 8 Interview Questions asked in an interview. Consider the following code with implements two interfaces
public interface DefaultMethodInterface {
default public void defaultMethod(){
System.out.println("I am in Default method Interface I");
}
}
public interface DefaultMethodInterface2 {
default public void defaultMethod(){
System.out.println("I am in Default method Interface II");
}
}
public class HelloJava8 implements DefaultMethodInterface,DefaultMethodInterface2 {
public static void main(String[] args){
DefaultMethodInterface defMethIn = new HelloJava();
defMethIn.defaultMethod();
}
}
Here the compiler gives an error saying that “Duplicate Default Methods”. Hence it is not possible to implement two interfaces with the same name and signature.

Q3. Is it possible to define our own Functional Interface? Explain the rules to define a functional interface.

Answer:
It is possible to define our own functional interfaces. A user can use Java SE 8’s @FunctionalInterface annotation to mark an interface as Functional Interface. The following rules need to be kept in mind when creating a functional interface.

1. Only one interface must be defined having only one abstract method
2. More than on abstract methods cannot be defined
3. A user should make use of @FunctionalInterface annotation in the interface definition.
4. Any number of different methods like the default method, static method, etc. can be defined.
5. We can override java.lang.Object class’s method as an abstract method and this will not be counted as an abstract method.

Let us move to the next Java 8 Interview Questions.

Q4. What is Optional in Java 8? Explain its advantages.

Answer:
Optional is a final class which is introduced as a part of Java SE 8. It is part of java.util.package. It can be used to represent optional values that either exist or do not exist. This can have value as one value or zero value. If this class contains a value then we get the value else we do not get anything.
The main advantages of this class are that is can be used to avoid null checks and it can be used to avoid ‘NullPointerException’.

Q5. Will the below code compiles without error?

Answer:
@FunctionalInterface
public interface Function2 {
public V apply(T t, U u);
default void count() {
// increment counter
}
}
Yes, this code will compile without any error as it follows the rule of functional interface specification of defining only a single abstract method. The second method that is used is a default method that does not increase the abstract method count.

Part 2 – Java 8 Server Interview Questions (Advanced)

Let us now have a look at the advanced Java 8 Interview Questions and Answers

Q6. What is Nashorn in Java?

Answer:
This is the new Java processing engine for Java platform which is shipped in Java 8. Until JDK 7 Java platform used Rhino as the processing engine. It was a Javascript processing engine. Nashorn provides better compliance with the ECMA normalized JavaScript specification. It also provides better runtime performance than its previous versions.

Q7. What is the difference between Predicate and Function?

Answer:
Both these are functional interfaces. A predicate is a single argument function which returns either true or false. This expression can be used as an assignment target for lambda expression or any method reference.
Function is also a single argument function but the difference here is that it returns an object. Here T represents input to the function and R represents the type of result. Both these can be used as an assignment target for lambda expressions or method references.

Let us move to the next Java 8 Interview Questions.

Q8. Explain the difference between intermediate and terminal operations.

Answer:
Stream operations are used to process streams and combine them. All these operations are either intermediate or terminal. Intermediate operations return the Stream itself and allow further operations if they are to be performed. These operations are not performed at the same time. These operations can be performed only to process data when there is a terminal operation. To name a few operations, they are filter, map and flat map.
Terminal operations terminate the pipeline and initiate stream processing. This stream is passed through all intermediate operations during terminal operation. These operations include for Each, reduce, collect, sum, etc.

Q9. What is stream pipelining in Java 8?

Answer:
This is advanced Java 8 Interview Questions asked in an interview. It is the concept of chaining operations together. This process is done by splitting operations that can work with streams with two categories of intermediate and terminal operations. Each intermediate operation returns an instance of a stream when it runs. There must also be a terminal operation which will return a final value and will terminate the pipeline.

Q10. What is StringJoiner?

Answer:
StringJoiner is a util method which is used to construct different strings with desired delimiters. It can also help in creating sequences of different characters separated by delimiters. This was introduced in Java 8. The different constructors are Public StringJoiner(CharSequence delimiter) and Public StringJoiner(CharSequence delimiter,CharSequence prefix,CharSequence suffix).Below is an example of StringJoiner:
StringJoiner strJoiner = new StringJoiner(".");
strJoiner.add("Buggy").add("Bread");
System.out.println(strJoiner); // prints Buggy.Bread

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