Java Programming — Lambda Expressions Questions & Answers

Q.1 Medium Lambda Expressions

Consider the following code:
List fruits = Arrays.asList("apple", "banana", "cherry");
fruits.removeIf(s -> s.length() > 5);

What will be the contents of 'fruits' list after execution?

A ["apple", "banana", "cherry"]

B ["apple"]

C ["apple", "cherry"]

D []

Correct Answer: B. ["apple"]

EXPLANATION

removeIf() removes all elements that satisfy the lambda predicate. Strings with length > 5 are "banana" (6 chars) and "cherry" (6 chars). Only "apple" (5 chars) remains as its length is not greater than 5.

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Q.2 Medium Lambda Expressions

A stream processes a list of strings and applies a lambda to convert them to uppercase. Which intermediate operation should be used?

A filter(s -> s.toUpperCase())

B map(s -> s.toUpperCase())

C peek(s -> s.toUpperCase())

D collect(s -> s.toUpperCase())

Correct Answer: B. map(s -> s.toUpperCase())

EXPLANATION

The map() intermediate operation transforms each element using the provided lambda function. filter() returns boolean, peek() doesn't transform, and collect() is a terminal operation. map() is the correct choice for transformation.

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Q.3 Medium Lambda Expressions

Which of the following lambda expressions is INVALID in Java?

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

B (int x) -> x * x

C (x, y) -> { return x + y; }

D (x, y) -> return x + y;

Correct Answer: D. (x, y) -> return x + y;

EXPLANATION

In a lambda expression, if the body uses curly braces, 'return' keyword is required. But without curly braces, 'return' keyword cannot be used. Option D is invalid because it has 'return' without curly braces.

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Q.4 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.5 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.6 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.7 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.8 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.9 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.10 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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