Java 1.8 interview Questions

Name a few recent features introduced in Java 8.

Below are the most recent features which is introduced in Java 8:

• Lambda Expressions: It is a Java function that you can share or refer to as an object.
• Method interference: It uses function as a criterion to implement a method.
• Functional Interference: Every functional interference is associated with a single abstract method which is known as the functional method.
• Default Method: It is useful in implementing methods in the interfaces that help enable the ‘interface evolution’ potential.
• Date Time API: It is an improved yet inspired version of java time APIs to deal with the drawbacks of the last version.
• Stream API is referred to as the abstract layer, which helps to pipeline the processing data.
• Optional: The wrapper class is useful in checking the null values and processing the further data.
• JavaScript and Nashorn Engine: It is the improved version of the JavaScript Engine, which is useful in enabling its functionality in Java, replacing Rhino.

In which programming paradigm Java 8 falls?

• Object-oriented programming language.
• Functional programming language.
• Procedural programming language.
• Logic programming language 

What are the significant advantages of Java 8?

• Compact, readable, and reusable code.
• Less boilerplate code.
• Parallel operations and execution.
• Can be ported across operating systems.
• High stability.
• Stable environment.
• Adequate support

What is MetaSpace? How does it differ from PermGen?

PremGen: MetaData information of classes was stored in PremGen (Permanent-Generation) memory type before Java 8. PremGen is fixed in size and cannot be dynamically resized. It was a contiguous Java Heap Memory.

MetaSpace: Java 8 stores the MetaData of classes in native memory called 'MetaSpace'. It is not a contiguous Heap Memory and hence can be grown dynamically which helps to overcome the size constraints. This improves the garbage collection, auto-tuning, and de-allocation of metadata.

What is the use of the @FunctionalInterface annotation?

This annotation is used in functional interfaces to add more readability to the code as an informative aspect. However, it does not affect the runtime or the semantics of the code.

What is the meaning of functional interfaces in Java 8?

Functional interfaces in Java 8 are interfaces having a single abstract method.

Following are the three types of methods that can be present:

• The static method
• The default method
• The overridden class method

What is the meaning of method reference in Java 8?

Method references are used in Java 8 to refer to methods of functional interfaces. It can be considered as a short-code version of using a lambda expression.

The following is the expression for a method reference:

Class::methodname

For e.g.: 

Integer::parseInt(str) \\ method reference

str -> Integer.ParseInt(str); \\ equivalent lambda

What is the lambda expression in Java and How does a lambda expression relate to a functional interface?

Lambda expression is a type of function without a name. It may or may not have results and parameters. It is known as an anonymous function as it does not have type information by itself. It is executed on-demand. It is beneficial in iterating, filtering, and extracting data from a collection.

As lambda expressions are similar to anonymous functions, they can only be applied to the single abstract method of Functional Interface. It will infer the return type, type, and several arguments from the signature of the abstract method of functional interface.

Describe the syntax of a lambda expression.

Lambda expressions can be divided into three parts as shown in the below syntax:

//Lambda expression: (int a, int b) -> { System.out.println(a+b); return a+b;}

1. Arguments: A lambda expression can have zero or more arguments at any point in time.
2. Array token: It is used to point to the body of the expression.
3. Body: The body consists of expressions and statements. Braces are not required if it has onl a single statement.

 What are the types and common ways to use lambda expressions?

A lambda expression does not have any specific type by itself. A lambda expression receives type once it is assigned to a functional interface. That same lambda expression can be assigned to different functional interface types and can have a different type.

For eg consider expression s -> s.isEmpty() :

Predicate<String> stringPredicate = s -> s.isEmpty(); 
Predicate<List> listPredicate = s -> s.isEmpty();
Function<String, Boolean> func = s -> s.isEmpty();
Consumer<String> stringConsumer = s -> s.isEmpty();

Common ways to use the expression

Assignment to a functional Interface —> Predicate<String> stringPredicate = s -> s.isEmpty();
Can be passed as a parameter that has a functional type —> stream.filter(s -> s.isEmpty())
Returning it from a function —> return s -> s.isEmpty()
Casting it to a functional type —> (Predicate<String>) s -> s.isEmpty()        

Can a functional interface extend/inherit another interface?

A functional interface cannot extend another interface with abstract methods as it will void the rule of one abstract method per functional interface. E.g:

interface Parent {

public int parentMethod();

}

@FunctionalInterface // This cannot be FunctionalInterface

interface Child extends Parent {

public int childMethod();

// It will also extend the abstract method of the Parent Interface

// Hence it will have more than one abstract method

// And will give a compiler error

}

It can extend other interfaces which do not have any abstract method and only have the default, static, another class is overridden, and normal methods. For eg:

interface Parent {

public void parentMethod(){

System.out.println("Hello");

}

}

@FunctionalInterface

interface Child extends Parent {

public int childMethod();

}

What is the default method, and why is it required?

A method in the interface that has a predefined body is known as the default method. It uses the keyword default. default methods were introduced in Java 8 to have 'Backward Compatibility in case JDK modifies any interfaces. In case a new abstract method is added to the interface, all classes implementing the interface will break and will have to implement the new method. With default methods, there will not be any impact on the interface implementing classes. default methods can be overridden if needed in the implementation. Also, it does not qualify as synchronized or final.

@FunctionalInterface // Annotation is optional

public interface Foo() {

// Default Method - Optional can be 0 or more

public default String HelloWorld() {

return "Hello World";

}

// Single Abstract Method

public void bar();

}

What are static methods in Interfaces?

Static methods, which contains method implementation is owned by the interface and is invoked using the name of the interface, it is suitable for defining the utility methods and cannot be overridden.

 

 

What are some standard Java pre-defined functional interfaces?

Some of the famous pre-defined functional interfaces from previous Java versions are Runnable, Callable, Comparator, and Comparable. While Java 8 introduces functional interfaces like Supplier, Consumer, Predicate, etc. Please refer to the java.util.function doc for other predefined functional interfaces and its description introduced in Java 8.

Runnable: use to execute the instances of a class over another thread with no arguments and no return value. 

Callable: use to execute the instances of a class over another thread with no arguments and it either returns a value or throws an exception.

Comparator: use to sort different objects in a user-defined order

Comparable: use to sort objects in the natural sort order.

What are the various categories of pre-defined function interfaces?

Function<T,R>: Represents a function that takes an argument of type T and produces a result of type R

Ex: Function<Integer, String> intToString = num -> "Number: " + num; System.out.println(intToString.apply(10)); // "Number: 10"

Predicate<R>: To perform a test and return a Boolean value.

Ex: Predicate<String> isNotEmpty = str -> !str.isEmpty(); System.out.println(isNotEmpty.test("Hello")); // true System.out.println(isNotEmpty.test("")); // false

Consumer: Represents an operation that accepts a single input argument and returns no result.

Ex: Consumer<String> printUpperCase = str -> System.out.println(str.toUpperCase()); printUpperCase.accept("hello"); // "HELLO"

Supplier: Represents a supplier of results. It provides a result of type T and does not take any arguments.

Ex: Supplier<String> getGreeting = () -> "Hello, World!"; System.out.println(getGreeting.get()); // "Hello, World!"

Operator: Perform a reduction type operation that accepts the same input types

Summary

• Predicate<T>: Tests a condition.
• Function<T, R>: Transforms an input to an output.
• Consumer<T>: Accepts an input and performs an action without returning a result.
• Supplier<T>: Provides a result without any input.
• BinaryOperator<T>: Operates on two operands of the same type.
• UnaryOperator<T>: Operates on a single operand of the same type.

What are the advantages of using the Optional class?

The optional keyword is used in Java 8 to avoid the occurrence of the NullPointerException.

It encapsulates optional values, i.e., null or not-null values, which helps in avoiding null checks, which results in better, readable, and robust code It acts as a wrapper around the object and returns an object instead of a value, which can be used to avoid run-time NullPointerExceptions.

What are Java 8 streams?

A stream is an abstraction to express data processing queries in a declarative way. 

Stream pipelining is a concept that is implemented in Java 8 so that users can chain more than one operation at a time. This works on the principle of splitting the operation into two categories:

• Intermediate operations: Return the instance of the stream when running
• Terminal operations: Used to terminate the operation and return the final value

When is an ideal situation to use the Stream API in Java 8?

The Stream API in Java 8 can be effectively used if the Java project calls for the following operations:

1. Perform database operations
2. Execute operations lazily
3. Write functional-style programming
4. Perform parallel processing
5. Use pipeline operations
6. Use internal iteration

 What are the main components of a Stream?

Components of the stream are:

• A data source
• Set of Intermediate Operations to process the data source
• Single Terminal Operation that produces the result 

 What are the sources of data objects a Stream can process?

A Stream can process the following data:

• A collection of an Array.
• An I/O channel or an input device.
• A reactive source (e.g., comments in social media or tweets/re-tweets) 
• A stream generator function or a static factory.

What are Intermediate and Terminal operations?

Intermediate Operations:

• Process the stream elements.
• Typically transforms a stream into another stream.
• Are lazy, i.e., not executed till a terminal operation is invoked.
• Does internal iteration of all source elements.
• Any number of operations can be chained in the processing pipeline.
• Operations are applied as per the defined order.
• Intermediate operations are mostly lambda functions.

Terminal Operations:

• Kick-starts the Stream pipeline.
• used to collect the processed Stream data.

int count = Stream.of(1, 2, 3, 4, 5)

.filter(i -> i <4) // Intermediate Operation filter

.count(); // Terminal Operation count 

What are the most commonly used Intermediate operations?

Filter(Predicate<T>) - Allows selective processing of Stream elements. It returns elements that are satisfying the supplied condition by the predicate.

map(Funtion<T, R>) - Returns a new Stream, transforming each of the elements by applying the supplied mapper function.= sorted() - Sorts the input elements and then passes them to the next stage.

distinct() - Only pass on elements to the next stage, not passed yet.

limit(long maxsize) - Limit the stream size to maxsize.

skip(long start) - Skip the initial elements till the start.

peek(Consumer) - Apply a consumer without modification to the stream.

flatMap(mapper) - Transform each element to a stream of its constituent elements and flatten all the streams into a single stream.

What is the stateful intermediate operation? Give some examples of stateful intermediate operations.

To complete some of the intermediate operations, some state is to be maintained, and such intermediate operations are called stateful intermediate operations. Parallel execution of these types of operations is complex.

For Eg: sorted() , distinct() , limit() , skip() etc. 

Sending data elements to further steps in the pipeline stops till all the data is sorted for sorted() and stream data elements are stored in temporary data structures.

What is the most common type of Terminal operations?

• collect() - Collects single result from all elements of the stream sequence.
• reduce() - Produces a single result from all elements of the stream sequence
• count() - Returns the number of elements on the stream.
• min() - Returns the min element from the stream.
• max() - Returns the max element from the stream.
• Search/Query operations
• anyMatch() , noneMatch() , allMatch() , ... - Short-circuiting operations.
• Takes a Predicate as input for the match condition.
• Stream processing will be stopped, as and when the result can be determined.
• Iterative operations
• forEach() - Useful to do something with each of the Stream elements. It accepts a consumer.
• forEachOrdered() - It is helpful to maintain order in parallel streams.

What is the difference between findFirst() and findAny()?

findFirst()	findAny()
Returns the first element in the Stream	Return any element from the Stream
Deterministic in nature	Non-deterministic in nature

How are Collections different from Stream?

Collections are the source for the Stream. Java 8 collection API is enhanced with the default methods returning Stream<T> from the collections.

Collections	Streams
Data structure holds all the data elements	No data is stored. Have the capacity to process an infinite number of elements on demand
External Iteration	Internal Iteration
Can be processed any number of times	Traversed only once
Elements are easy to access	No direct way of accessing specific elements
Is a data store	Is an API to process the data

What are Map and FlatMap stream operations?

Map and FlatMap are regarded as the stream and intermediate stream operations which accepts a function. It also helps in applying the given function to the rest of the elements.

What were the issues that were fixed with the new Date and Time API of Java 8?

With the older versions of Java, java.util.The date was mutable. This means it has absolutely no thread safety.

Also, java.text.SimpleDateFormat was not thread-safe in the older versions. The older Date and Time API was difficult to understand for programmers in terms of readability too.

What is the feature of the new Date and Time API in Java 8?

• Immutable classes and Thread-safe 
• Timezone support
• Fluent methods for object creation and arithmetic
• Addresses I18N issue for earlier APIs
• Influenced by popular joda-time package
• All packages are based on the ISO-8601 calendar system

What are the important packages for the new Data and Time API?

• java.time
• dates 
• times 
• Instants 
• durations 
• time-zones 
• periods
• Java.time.format
• Java.time.temporal
• java.time.zone

Explain with example, LocalDate, LocalTime, and LocalDateTime APIs.

LocalDate

• Date with no time component
• Default format - yyyy-MM-dd (2020-02-20)
• LocalDate today = LocalDate.now();  // gives today’s date
• LocalDate aDate = LocalDate.of(2011, 12, 30); //(year, month, date)

LocalTime

• Time with no date with nanosecond precision
• Default format - hh:mm:ss:zzz (12:06:03.015) nanosecond is optional
• LocalTime now = LocalTime.now();  // gives time now
• LocalTime aTime2 = LocalTime.of(18, 20, 30); // (hours, min, sec)

LocalDateTime

• Holds both Date and Time
• Default format - yyyy-MM-dd-HH-mm-ss.zzz (2020-02-20T12:06:03.015)
• LocalDateTime timestamp = LocalDateTime.now(); // gives timestamp now
• //(year, month, date, hours, min, sec)
• LocalDateTime dt1 = LocalDateTime.of(2011, 12, 30, 18, 20, 30);

Define Nashorn in Java 8

Nashorn is a JavaScript processing engine that is bundled with Java 8. It provides better compliance with ECMA (European Computer Manufacturers Association) normalized JavaScript specifications and better performance at run-time than older versions.

What is the use of JJS in Java 8?

As part of Java 8, JJS is a command-line tool that helps to execute the JavaScript code in the console. Below is the example of CLI commands:

JAVA>jjs
jjs> print("Hello, Java 8 - I am the new JJS!")
Hello, Java 8 - I am the new JJS!
jjs> quit()
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