Java Programming Questions & Answers

Q.101 Easy Lambda Expressions

What is the return type of the lambda expression: (a, b) -> a > b?

A int

B boolean

C void

D Cannot be determined

Correct Answer: B. boolean

EXPLANATION

The expression uses comparison operator (>), which returns a boolean value.

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Q.102 Easy Lambda Expressions

A lambda expression can only be used with which type of interface?

A Regular interface

B Functional interface with single abstract method

C Interface with multiple abstract methods

D Interface with default methods only

Correct Answer: B. Functional interface with single abstract method

EXPLANATION

Lambda expressions can only be assigned to functional interfaces that have exactly one abstract method.

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Q.103 Easy Lambda Expressions

Which of the following is a valid lambda expression in Java?

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

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

C (String s) -> s.length()

D All of the above

Correct Answer: D. All of the above

EXPLANATION

All three are valid lambda expression syntaxes with different parameter types and return statements.

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Q.104 Medium Lambda Expressions

What will be the output of: List list = Arrays.asList(1, 2, 3); list.replaceAll(x -> x * 2); System.out.println(list);

A [1, 2, 3]

B [2, 4, 6]

C [] (empty list)

D [2, 2, 2]

Correct Answer: B. [2, 4, 6]

EXPLANATION

replaceAll() with a lambda expression modifies the list in-place, multiplying each element by 2. Result is [2, 4, 6].

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Q.105 Easy Lambda Expressions

Which stream operation would you use to transform elements from one type to another?

A filter()

B map()

C forEach()

D collect()

Correct Answer: B. map()

EXPLANATION

map() is the intermediate operation used to transform elements from one type/value to another. It applies a function to each element.

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Q.106 Easy Lambda Expressions

What is the purpose of the @FunctionalInterface annotation in Java?

A To indicate an interface can be used with lambda expressions

B To enforce that an interface has exactly one abstract method

C To improve performance of lambda expressions

D To allow multiple lambda expressions in one interface

Correct Answer: B. To enforce that an interface has exactly one abstract method

EXPLANATION

@FunctionalInterface is a compile-time check that ensures an interface has exactly one abstract method, making it safe for lambda implementation.

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Q.107 Hard Lambda Expressions

Consider: Stream.of(1, 2, 3).map(x -> { System.out.println(x); return x * 2; }).collect(Collectors.toList()); What will print?

A 1 2 3

B 2 4 6

C Nothing until the collect operation completes

D 1 2 3 2 4 6

Correct Answer: A. 1 2 3

EXPLANATION

map() is an intermediate operation. The println executes because collect() is a terminal operation that triggers evaluation. It prints 1, 2, 3 (the original values).

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Q.108 Medium Lambda Expressions

What happens when a lambda expression references a local variable from its enclosing scope?

A The variable must be explicitly final or effectively final

B The variable is automatically copied into the lambda's scope

C Any variable can be accessed without restrictions

D The variable must be static

Correct Answer: A. The variable must be explicitly final or effectively final

EXPLANATION

Lambda expressions can only access local variables that are final or effectively final (not reassigned). This ensures thread-safety and consistency.

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Q.109 Medium Lambda Expressions

Which of the following correctly uses method reference with constructor?

A String::new

B new String::

C String->new

D new::String

Correct Answer: A. String::new

EXPLANATION

Constructor method references use ClassName::new syntax. String::new refers to the String constructor and can be used wherever a Supplier<String> is expected.

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Q.110 Hard Lambda Expressions

Consider: Function curried = a -> b -> a + b; What is this pattern called?

A Higher-order function with currying

B Nested lambda expression

C Function composition

D Stream reduction

Correct Answer: A. Higher-order function with currying

EXPLANATION

This is a curried function - a higher-order function that takes one argument and returns another function taking the next argument. It transforms multi-argument functions into sequences of single-argument functions.

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