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Core Java - Interview Questions and Answers for 'Java 8' - 37 question(s) found - Order By Rating

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Q1. What is concurrent interface ?

Ans. ConcurrentMap is an interface and it is a member of the Java Collections Framework. It represents a Map that is capable of handling concurrent access to it without affecting the consistency of entries in a map. ConcurrentMap interface present in java.util.concurrent package.

HashMap operations are not synchronized, while Hashtable provides synchronization. Though Hashtable is thread-safe, it is not very efficient. To solve this issue, the Java Collections Framework introduced ConcurrentMap in Java 1.5.

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Q2. What are the advantages and disadvantages of Lambda Expression ?

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Q3. Write a unit test case using java 8.

Ans. public List convertAllToUpperCase(List words) {
return words.stream().map(String::toUpperCase).collect(Collectors.toList());
}

@Test
public void testAllToUpperCase() {
List expected = Arrays.asList("JAVA8", "STREAMS");
List result = convertAllToUpperCase(Arrays.asList("java8", "streams"));
assertEquals(expected, result);
}

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Q4. How to get collections from a Stream

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Q5. What is the difference between map and filter methods in streams ?

Ans. By using map , you can transform the object values. The map operation allows us to apply a function, that takes input parameter of one type, and returns something else.

Filter is used for filtering the data, it always returns the boolean value. If it returns true, then item is added to list else its filtered out (ignored)

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Q6. Which Java version supports Interface Default methods ?

Ans. Java 1.8 or Java 8

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Q7. Can we have interface as a replacement for utility class ?

Ans. Yes, With Java 8 we can use Interfaces as collection of utility methods through the use of default methods.

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Q8. Have you used Java 8 Lambda expressions ?

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Q9. Can you explain filter and collectors in java 8 ?

Ans. Filters are used to filter object in stream on certain criteria.
Collector is used to collect outcome of the stream. Collector is a terminal operation.
List numbers = List.of(1,2,3,4,5,6,7);
List evenNumbers = numbers.stream().filter(n -> n%2==0).collect(Collectors.toList());
The above code will filter out odd numbers and return the list of even number.

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Q10. Write code to get count of every 5 letter word in a string using lambda expression

Ans. String str = "I had been saying that he had been there";

Map<String,Long> countWords = Arrays.asList(str.split(" ")).stream().filter(p->p.length() = 5).collect(Collectors.groupingBy(p->p,Collectors.counting()));

System.out.println(countWords);

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Q11. What will the following code return

list.stream().collect(Collectors.groupingBy(ClassA::getElement1,
Collectors.averagingDouble(ClassA::getElement2)));

Ans. Map<DataType1,DataType2> where DataType1 is the data type of element1 ( returned by getelement1 method ) and DataType2 is the data type of element2 ( returned by getElement2 method )

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Q12. What are we doing here

list.stream().collect(Collectors.groupingBy(ClassA::getElement1,
Collectors.averagingDouble(ClassA::getelement2)));

Ans. We are averaging elements of a collection by element1 grouped by element1

* list is the reference of collection
* element1 is the member element returned by getElement1 method of ClassA
* element2 is the member element returned by getElement2 method of ClassA

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Q13. How do you define a functional interface?

Ans. Create interface with the only one non-overriding abstract method and annotate it with @FunctionalInterface

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Q14. How can i split a linked list in two parts in java 8?

Ans. This can be done using a Spliterator.

LinkedList list = Arrays.asList("names","numbs","birds","animals");
Spliterator split1 = list.Spliterator();
Spliterator split2 = split1.Spliterator();

Now, the LinkedList is split into split1 and split2.
use split2 first then split1 to check the output.

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Q15. Given a collection of Employee objects, how can we get the customers with name starting with 'A' using lambda expression.

Ans. List emp = Arrays.asList("American", "Indian", "Finn");
emp.stream().filter(em -> em.startsWith("A")).forEach(System.out.println);

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Q16. Lets say we have a set "set1" with objects of Class A, Class has a non static string called element1, Write a code ( using lambda expression ) to see if there is any object in the set with element1 = "Hello".

Ans. set1.stream().filter(p->p.element1.equals("Hello")).findAny().isPreent();

or

set1.stream().map(A::getElement1).anyMatch("Hello"::equals)

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Q17. Why can't we have diamond problem with interfaces ?

Ans. Interfaces don't have member elements and method definitions that could cause diamond problem. With Java 8, Interfaces have default method definitions. This could have created diamond problem but Java introduced a compile time check for "duplicate default methods" in case same method is derived from multiple interfaces and no definition is overridden by the class.

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Q18. Explain Java 8 concurrency mechanism.

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Q19. Is it ok to use optional everywhere just to get over nullpointerexception ?

Ans. Optional is to be used for arguments / atrributes which are indeed optional i.e the request should continue even if they aren't provided. It should not be used for mandatory attributes or arguments as we would like application to shout out ( with error message / exception trace ) to signify a problem.

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Q20. What is a method reference in Java ?

Ans. Introduced with java 8 , Method References help us to point to methods by their name.

A method references is described using :: symbol

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Q21. How Spliterator in Java 8 different than iterator ?

Ans. Though there are many differences the way internally they both iterates the collections and streams respectively, but the main difference in performance that is achieved by spliterator as it can iterate Streams in Parallel whereas iterator only iterates collections sequentially.

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Rarely asked as it was introduced with Java 8.

Q22. What is the use of :: operator wef from Java 8 ?

Ans. We can refer to a function using this operator like System.out.println(intList.stream().reduce(Math::max).get());

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Q23. Does default methods introduce multiple inheritance and the Diamond problem in Java 8 ?

Ans. Default methods results in multiple inheritance of behavior and not of state. In case we try to implement multiple interfaces with default method having same name and signature, and don't override it in implementation class, it will throw an error.

For example -

interface MyInterface {
public void default myMethod(){
}
}

interface MyInterface2 {
public void default myMethod(){
}
}

class MyClass implements MyInterface,MyInterface2 {
}

This code will compilation error "Duplicate Default Method"

if we specify the definition of myMethod() in myClass, compiler won't complain and there is no conflict and MyClass can use overridden definition. But if we don't override myMethod() in MyClass, Java would be in conflict as to what definition should be carried to MyClass and hence throws compilation error.

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Q24. What was the driving force to introduce default methods in Interfaces wef from Java 8 ?

Ans. Earlier any class implementing an interface was supposed to implement all methods declared in an interface. There was no place for optionally implementing all or subset of methods.Though we have abstract classes wherein we could have provided such a mechanism by declaring some methods as abstract while providing definition for some. But as Abstract classes have a body and are comparatively heavier than interfaces and interfaces associate closely to the concept of providing interfacing than abstract classes, Java might have though of providing optional implementation for default methods. This way same interface can be reused in variety of ways rather than making copies of an interface to suit different needs.

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Not frequently asked as it was introduced with Java 8.

Q25. What is the @FunctionalInterface annotation ?

Ans. This is an informative annotation that specify that the interface is a functional interface. A Function Interface has only one abstract method and many default methods. Compiler generates an error if the interface specified with the annotation doesn't abide by the specifications for functional interface.

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Not frequently asked as it was introduced with Java 8.

Q26. Difference between final and effectively final ? Why is effectively final even required ?

Ans. Final variable means a variable that has been declared final and hence cannot be de referenced after initialization. Effective final means a variable that has not been declared final but haven't been reassigned the value after initialization.

First is the regulation that restricts the reassignment and will raise a compilation error if we try to do so. Second is the outcome without the restriction.

Effective Final is the eventual treatment of the variable that is required for many features. For eq - Java 8 requires that local variables referenced from a lambda expression must be final or effectively final.It means all local referenced from lambda expressions must be such that their value shouldn't be changed after initialization whether declared final or not.

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Rarely asked as it was introduced with Java 8.

Q27. Difference between DoubleSummaryStatistics , IntSummaryStatistics and LongSummaryStatistics ?

Ans. They all does the same task i.e to compute statistical information on the stream of data. They differ by the way they store the statistical information as they expect a different data type of the values being used.

IntSummaryStatistics and LongSummaryStatistics expect non floating point values and hence stores the statistical information like min,max and sum as non floating values ( int or long ) whereas DoubleSummaryStatistics stores these information as floating value.

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Not frequently asked as it was introduced with Java 8.

Q28. What is StringJoiner ?

Ans. StringJoiner is a util method to construct a string with desired delimiter. This has been introduced with wef from Java 8.

Sample Code

StringJoiner strJoiner = new StringJoiner(".");

strJoiner.add("Buggy").add("Bread");

System.out.println(strJoiner); // prints Buggy.Bread

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Not frequently asked as it was introduced with Java 8.

Q29. What is the use of Optional ?

Ans. Optional is a good way to protect application from runtime nullPointerException in case the the absent value has been represented as null. So basically Optional class provides the type checking during compile time and hence will never result in NPE.

For ex -

List> intList = new ArrayList>();
intList.add(Optional.empty());
intList.add(Optional.of(new Employee("abc")));
intList.add(Optional.of(new Employee("xyz")));
intList.add(Optional.of(new Employee("123")));
System.out.println(intList.get(0).getName());

So Now , even when the first list element is empty, this code will never throw an NullPointerException.

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Rarely asked as it was introduced with Java 8.

Q30. Name few "Optional" classes introduced with Java 8 ?

Ans. http://www.buggybread.com/2015/01/java-optional-classes-and-interfaces.html

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