Java Programming — Lambda Expressions Questions & Answers

Q.11 Medium Lambda Expressions

Which stream operation uses a lambda expression to transform elements from one type to another?

A filter()

B map()

C forEach()

D reduce()

Correct Answer: B. map()

EXPLANATION

map() is used to transform elements. It takes a Function lambda that converts each element from one type/value to another.

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Q.12 Easy Lambda Expressions

Given: Function f = x -> x * 2; Integer result = f.apply(5); What is the value of result?

A 5

B 10

C 15

D 25

Correct Answer: B. 10

EXPLANATION

The lambda expression x -> x * 2 multiplies the input by 2. When applied to 5, it returns 10.

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Q.13 Easy Lambda Expressions

What is the correct syntax for a lambda expression with no parameters that returns a fixed value?

A -> "Hello"

B () -> "Hello"

C ( ) -> "Hello"

D { } -> "Hello"

Correct Answer: B. () -> "Hello"

EXPLANATION

When a lambda has no parameters, empty parentheses () are required before the arrow. Options A and D are syntactically incorrect.

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Q.14 Medium Lambda Expressions

Which of the following correctly uses a method reference as an alternative to a lambda expression?

A System.out::println instead of (x) -> System.out.println(x)

B String::length instead of (s) -> s.length()

C Integer::parseInt instead of (s) -> Integer.parseInt(s)

D All of the above

Correct Answer: D. All of the above

EXPLANATION

All three are valid method references that can replace their equivalent lambda expressions. Method references are a shorthand notation introduced in Java 8.

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Q.15 Medium Lambda Expressions

Consider: Comparator comp = (a, b) -> b - a; List list = Arrays.asList(3,1,2); Collections.sort(list, comp); What is the result?

A [1, 2, 3]

B [3, 2, 1]

C [2, 1, 3]

D [1, 3, 2]

Correct Answer: B. [3, 2, 1]

EXPLANATION

The comparator (a, b) -> b - a returns negative values when b < a, causing elements to be sorted in descending order. Result is [3, 2, 1].

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Q.16 Easy Lambda Expressions

In a lambda expression, what does the arrow (->) operator represent?

A Assignment operator

B Separation between parameters and body

C Comparison operator

D Logical AND operator

Correct Answer: B. Separation between parameters and body

EXPLANATION

The arrow (->) in lambda expressions separates the parameter list on the left from the method body on the right. It's a syntax element specific to lambda expressions.

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Q.17 Medium Lambda Expressions

Which lambda expression correctly implements a custom functional interface: public interface Math { int calculate(int a, int b); }

A (a, b) -> a + b

B () -> 0

C (int a, int b) -> a - b

D Both A and C

Correct Answer: D. Both A and C

EXPLANATION

Both expressions are valid. Option A uses type inference while Option C explicitly specifies types. Both can implement the calculate method.

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Q.18 Medium Lambda Expressions

What is the output of: List nums = Arrays.asList(1,2,3); nums.stream().filter(x -> x > 1).forEach(System.out::println);

A 1 2 3

B 2 3

C 1

D Nothing is printed

Correct Answer: B. 2 3

EXPLANATION

filter(x -> x > 1) keeps only elements greater than 1, which are 2 and 3. These are then printed on separate lines using forEach.

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Q.19 Medium Lambda Expressions

Consider the code: List names = Arrays.asList("Alice", "Bob"); names.forEach(name -> System.out.println(name)); What type of functional interface is used in forEach?

A Function

B Consumer

C Supplier

D Predicate

Correct Answer: B. Consumer

EXPLANATION

forEach accepts a Consumer functional interface. Consumer<T> takes an input and performs an action without returning anything, which matches the System.out.println action.

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Q.20 Easy Lambda Expressions

What is the purpose of a Predicate functional interface in Java?

A To transform one type of object to another

B To perform an action without returning a value

C To test a condition and return a boolean value

D To supply a value without taking any input

Correct Answer: C. To test a condition and return a boolean value

EXPLANATION

Predicate<T> is a functional interface that takes a single input of type T and returns a boolean. It's commonly used for filtering operations in streams.

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