Which of the following is a valid generic class declaration in Java?

The correct answer is A: public class Box { }. Valid generic class declarations allow multiple type parameters with upper bounds. Option A is syntactically correct. Option C uses invalid 'super' keyword in class declaration. Option D has malformed syntax.

Which of the following statements about @FunctionalInterface is true?

The correct answer is B: It's optional but helps compiler verify exactly one abstract method. @FunctionalInterface is optional annotation that helps compiler verify interface has exactly one abstract method, catching errors early.

Consider: Comparator comp = (s1, s2) -> s2.compareTo(s1); What does this lambda expression do?

The correct answer is B: Sorts strings in descending order. By reversing the compareTo order (s2 compared to s1), it sorts in descending order.

Which of the following lambda expressions is invalid?

The correct answer is D: -> x + y. Lambda must have parameter list (even if empty with ()). Missing parameter list '() -> x + y' is invalid syntax.

What will happen when you try to create an array of generics? List[] array = new List[10];

The correct answer is A: Compilation error. You cannot create arrays of generic types because of type erasure. The compiler prevents this with a compilation error.

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Java Programming
Lambda Expressions

Java OOP, collections, multithreading

51 Q 10 Topics Take Mock Test

Difficulty: All Easy Medium Hard 31–40 of 51

Topics in Java Programming

All Basics & Syntax 100 OOP in Java 100 Collections Framework 100 Multithreading 100 Exception Handling 100 I/O Streams 100 JDBC 100 Generics 100 Lambda Expressions 100 Spring Framework Basics 58

Q.31 Medium Lambda Expressions

Consider: Stream.of(1, 2, 3, 4, 5).filter(x -> x > 2).map(x -> x * 2).forEach(System.out::println); What will be the output?

A 6 8 10

B 2 4 6 8 10

C 3 4 5

D 1 2 3 4 5

Correct Answer: A. 6 8 10

EXPLANATION

filter(x -> x > 2) keeps 3, 4, 5. map(x -> x * 2) transforms them to 6, 8, 10. forEach prints each value.

Test

Q.32 Medium Lambda Expressions

What will be the output of: IntStream.range(1, 4).map(x -> x * 2).sum();

A 12

B 9

C 6

D 24

Correct Answer: A. 12

EXPLANATION

range(1, 4) generates [1, 2, 3]. After map(x -> x * 2): [2, 4, 6]. Sum = 2 + 4 + 6 = 12.

Test

Q.33 Medium Lambda Expressions

In the lambda expression (a, b) -> a.compareTo(b), what is the implicit functional interface?

A Comparator

B BiFunction

C BiPredicate

D BinaryOperator

Correct Answer: A. Comparator

EXPLANATION

The Comparator<T> functional interface has the method compare(T a, T b) which returns an int, matching this lambda's behavior.

Test

Q.34 Medium Lambda Expressions

What is the difference between peek() and forEach() when used with lambda expressions in Streams?

A peek() is intermediate and doesn't consume the stream, forEach() is terminal and consumes it

B forEach() is faster than peek()

C peek() can only be used with Collectors

D forEach() returns a value while peek() returns void

Correct Answer: A. peek() is intermediate and doesn't consume the stream, forEach() is terminal and consumes it

EXPLANATION

peek() is an intermediate operation used for debugging; it doesn't end the stream. forEach() is a terminal operation that consumes the stream.

Test

Q.35 Medium Lambda Expressions

What is method reference (::) in Java and how does it relate to lambda expressions?

A Method reference is a shorthand for lambda expressions that call a single method

B Method reference is used for declaring new methods

C Method reference is faster than lambda but less readable

D Method reference can only be used with static methods

Correct Answer: A. Method reference is a shorthand for lambda expressions that call a single method

EXPLANATION

Method reference (::) is syntactic sugar for lambda expressions. System.out::println is equivalent to x -> System.out.println(x).

Test

Q.36 Medium Lambda Expressions

Consider: Stream.of(1, 2, 3, 4, 5).filter(x -> x > 2).map(x -> x * x). What will the terminal operation need to be?

A collect(Collectors.toList())

B forEach(System.out::println)

C Either a or b

D reduce(0, Integer::sum)

Correct Answer: C. Either a or b

EXPLANATION

Both collect() and forEach() are valid terminal operations. collect() returns a list, forEach() performs an action. reduce() is also valid but would sum the squares.

Test

Q.37 Medium Lambda Expressions

Which of the following lambda expressions is INCORRECT?

A () -> 42

B x -> x + 1

C (int x, int y) -> x + y

D (x, y) -> (x > y) ? x : y

Correct Answer: C. (int x, int y) -> x + y

EXPLANATION

In option C, you cannot specify types in lambda parameters when using implicit typing. It should be either (x, y) or explicitly define all parameters.

Test

Q.38 Medium Lambda Expressions

What is the purpose of the Optional class when used with lambda expressions in Stream operations?

A To handle null values and avoid NullPointerException

B To increase stream processing speed

C To convert streams to lists automatically

D To sort elements in reverse order

Correct Answer: A. To handle null values and avoid NullPointerException

EXPLANATION

Optional is used to handle absence of values gracefully. Methods like ifPresent(lambda) and orElse() help prevent null pointer exceptions.

Test

Q.39 Medium Lambda Expressions

Consider: Function f = x -> x * x; What will f.apply(5) return?

A 25

B 10

C 5

D 30

Correct Answer: A. 25

EXPLANATION

Function<Integer, Integer> takes an Integer and returns an Integer. f.apply(5) computes 5 * 5 = 25.

Test

Q.40 Medium Lambda Expressions

What will be the result of executing: List nums = Arrays.asList(1, 2, 3, 4); nums.stream().map(x -> x * 2).collect(Collectors.toList());

A [2, 4, 6, 8]

B [1, 2, 3, 4]

C [4, 8, 12, 16]

D [3, 4, 5, 6]

Correct Answer: A. [2, 4, 6, 8]

EXPLANATION

The map() with lambda x -> x * 2 doubles each element: 1*2=2, 2*2=4, 3*2=6, 4*2=8, resulting in [2, 4, 6, 8].

Test

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Lambda Expressions