Java Programming — Lambda Expressions Questions & Answers

Q.21 Easy Lambda Expressions

Which of the following lambda expressions is syntactically incorrect?

A () -> System.out.println("Hello")

B (int x) -> x * x

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

D x -> x > 5

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

EXPLANATION

The syntax (x, y,) is incorrect because there's a trailing comma after y. The correct syntax would be (x, y) -> x + y.

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Q.22 Medium Lambda Expressions

What will happen if you try to access a local variable from an enclosing scope that is not final or effectively final in a lambda expression?

A It will compile and run successfully

B Compilation error: variable must be final or effectively final

C Runtime exception will be thrown

D The variable value will be copied

Correct Answer: B. Compilation error: variable must be final or effectively final

EXPLANATION

Lambda expressions can only access local variables that are final or effectively final. This is because lambda expressions are translated to methods that need access to stable variable values.

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Q.23 Medium Lambda Expressions

Consider a lambda expression: (String s) -> s.length(). What is the correct functional interface for this?

A Function

B Supplier

C Consumer

D Predicate

Correct Answer: A. Function

EXPLANATION

Function<String, Integer> takes a String input and returns an Integer (the length). This matches the lambda that takes a String parameter and returns s.length().

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Q.24 Easy Lambda Expressions

Which annotation is used to mark an interface as a functional interface in Java?

A @Function

B @FunctionalInterface

C @Lambda

D @Interface

Correct Answer: B. @FunctionalInterface

EXPLANATION

@FunctionalInterface is the standard annotation introduced in Java 8 to explicitly mark an interface as a functional interface. It helps in compile-time checking.

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Q.25 Medium Lambda Expressions

What is the return type of a lambda expression (x, y) -> x + y where x and y are integers?

A void

B Integer

C Depends on the functional interface it's assigned to

D int

Correct Answer: C. Depends on the functional interface it's assigned to

EXPLANATION

The return type of a lambda expression is inferred from the functional interface it's assigned to. The same lambda can return Integer or int depending on the context.

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Q.26 Easy Lambda Expressions

Which of the following is a valid functional interface that can be used with lambda expressions?

A An interface with exactly one abstract method

B An interface with multiple abstract methods

C An interface with no abstract methods

D An interface with static methods only

Correct Answer: A. An interface with exactly one abstract method

EXPLANATION

A functional interface must have exactly one abstract method. This is the defining characteristic that allows it to be used with lambda expressions and method references.

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Q.27 Medium Lambda Expressions

Analyze: What is the output?
BiConsumer printer = (a, b) -> System.out.println(a + b);
printer.accept("Java", "8");

A Java8

B JavaCompilation error

C Java 8

D Compilation error

Correct Answer: A. Java8

EXPLANATION

BiConsumer accepts two parameters and performs the action. String concatenation of "Java" + "8" produces "Java8".

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Q.28 Hard Lambda Expressions

What is the correct way to chain lambda expressions using Function interface?
Function f1 = x -> x * 2;
Function f2 = x -> x + 5;
How to apply f1 first, then f2?

A f1.andThen(f2).apply(3)

B f1.compose(f2).apply(3)

C f2.apply(f1.apply(3))

D A and C

Correct Answer: D. A and C

EXPLANATION

Both andThen and explicit composition order result in applying f1 first (3*2=6, then 6+5=11), equivalent to option C.

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Q.29 Medium Lambda Expressions

Which lambda expression is equivalent to the method reference Integer::parseInt?

A (s) -> Integer.parseInt(s)

B () -> Integer.parseInt()

C (Integer i) -> i.parseInt()

D (s) -> s.parseInt()

Correct Answer: A. (s) -> Integer.parseInt(s)

EXPLANATION

Method reference Integer::parseInt is equivalent to lambda (s) -> Integer.parseInt(s).

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Q.30 Medium Lambda Expressions

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

A Sorts strings in ascending order

B Sorts strings in descending order

C Compares string lengths

D Checks string equality

Correct Answer: B. Sorts strings in descending order

EXPLANATION

By reversing the compareTo order (s2 compared to s1), it sorts in descending order.

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