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
Method reference (::) is syntactic sugar for lambda expressions. System.out::println is equivalent to x -> System.out.println(x).
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
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.
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
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.
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
Optional is used to handle absence of values gracefully. Methods like ifPresent(lambda) and orElse() help prevent null pointer exceptions.
Consider: Function f = x -> x * x; What will f.apply(5) return?
Function<Integer, Integer> takes an Integer and returns an Integer. f.apply(5) computes 5 * 5 = 25.
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]
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].
Which functional interface is used for operations that take no arguments and return a value?
A
Supplier
B
Function
C
Predicate
D
BiConsumer
Correct Answer:
A. Supplier
Supplier<T> is a functional interface with method get() that takes no parameters and returns a value of type T.
Consider the code: List names = Arrays.asList("Raj", "Priya", "Amit"); names.forEach(n -> System.out.println(n)); What does this code do?
A
Prints each name with a newline
B
Creates a new list with uppercase names
C
Filters names starting with 'A'
D
Sorts the names alphabetically
Correct Answer:
A. Prints each name with a newline
forEach with the lambda expression n -> System.out.println(n) iterates through each element and prints it with a newline.
What is the return type of the map() method when used with lambda expressions in Java Streams?
A
Stream
B
List
C
Set
D
Map
Correct Answer:
A. Stream
The map() method in Stream API returns a new Stream with transformed elements of type R, maintaining the stream pipeline.
In Java 8, a lambda expression can access local variables from its enclosing scope. What is the constraint on these variables?
A
They must be declared as final or effectively final
B
They must be static variables
C
They must be instance variables of the class
D
They can be modified freely within the lambda
Correct Answer:
A. They must be declared as final or effectively final
Lambda expressions can only access local variables that are final or effectively final to ensure thread safety and immutability.
