Question 1

You can declare that an interface is a functional interface by preceding it with the @FunctionalInterface annotation. The compiler will then ensure that the interface contains ________; otherwise, it'll generate a compilation error.

Accepted Answer

A functional interface should contain only one abstract method. Preceding the interface with @FunctionalInterface annotation ensures that it contains only one abstract method. If the interface contains no abstract methods or more than one abstract method, then the compiler generates a compilation error.

Question 2

Collectors static method ________ returns a Collector that counts the number of objects in a given classification, rather than collecting them into a List.

Accepted Answer

The "counting" collector returns a Collector that counts the number of objects in a given classification. This is the most suitable collector for the given requirement. The other options are not relevant or inappropriate as "counter" is already used to count occurrences of elements, "count" is a terminal operation to count the elements in a stream, and "enumerate" is not a static method in the Collectors class.

Question 3

Which of the following statements is false?

Accepted Answer

Lambda expressions in Java do not have a "default type." The types of parameters in a lambda expression are inferred from the context in which the lambda is used, typically the target functional interface type.

Question 4

Which of the following statements is false?

Accepted Answer

An infinite stream is a stream that has no fixed size and can potentially produce an unlimited number of elements. However, the statement that you use an intermediate operation to complete processing on an infinite stream is misleading. It's the terminal operation that completes the processing of streams, including infinite streams, by producing a result or side-effect. Intermediate operations are operations that transform a stream into another stream, such as filter or map, and are lazy, meaning they don't start processing the elements until a terminal operation is invoked.

Question 5

Collectors static method groupingBy with two arguments receives a Function that classifies the objects in the stream and another Collector (known as the ________ Collector).

Accepted Answer

The second argument of the groupingBy method is referred to as the "downstream" Collector, since it operates on the values in each group produced by the first "upstream" Collector.

Question 6

Map method entrySet returns a Set of Map.Entry objects containing the Map's ________.

Accepted Answer

The entrySet method returns a Set of Map.Entry objects, which contain both the keys and values of the map as a pair. Therefore, option D key-value pairs is the correct answer.

Question 7

Which of the following statements is false?

Accepted Answer

This statement is false. Streams do not have their own storage and cannot be reused once processed. They are created from a data source (such as a collection) and perform operations on it, but do not maintain a copy of the original data source.

Question 8

Which of the following statements is false?

Accepted Answer

The answer of Which of the following statements is false?
...

Question 9

The intermediate Stream operation ________ results in a stream containing only the elements that satisfy a condition.

Accepted Answer

The filter operation allows for a condition to be applied to each element in the stream, with only the elements that meet that condition being included in the resulting stream.

Question 10

Terminal operations are ________; they perform the requested operation when they are called.

Accepted Answer

Terminal operations are eager because they perform the requested operation immediately when they are called, processing the entire stream to produce a result or side-effect.

Question 11

Intermediate operations are________; they aren't performed until a terminal operation is invoked. This allows library developers to optimize stream-processing performance.

Accepted Answer

Intermediate operations are called lazy because they do not perform any processing until a terminal operation is invoked. This allows library developers to optimize stream-processing performance by only executing the necessary operations.

Question 12

Which of the following statements is false?

Accepted Answer

Functional interfaces can have one abstract method but can also contain multiple default or static methods. The key characteristic of a functional interface is that it has exactly one abstract method, but it can have multiple default or static methods which do not count towards the abstract method count.

Question 13

Which of the following statements is false?

Accepted Answer

All the statements provided are true. Statement A is correct because some event-listener interfaces in Java, like ActionListener, are indeed functional interfaces, having only one abstract method. Statement B is true because for functional interfaces, you can use lambda expressions to implement their abstract method, which applies to event handlers as well. Statement C is also true because using a lambda expression for a simple event handler does significantly reduce the amount of code compared to using an anonymous class or implementing the interface in a separate class.

Question 14

Which of the following statements is false?

Accepted Answer

The false statement is A) because the Files method that creates a Stream<String> containing the lines of text from a file is named lines, not "lines creates a String". The method returns a Stream, not a single String containing all lines.

Question 15

Stream method ________ maps objects to double values and returns a DoubleStream. The method receives an object that implements the functional interface ToDoubleFunction (package java.util.function).

Accepted Answer

The correct method to map objects to double values and return a DoubleStream is mapToDouble, which takes a ToDoubleFunction as an argument. Option A (doubleMap) and option D (toDoubleStream) are not valid methods in the Java streams API. Option B (toDouble) does exist in the API, but it converts a primitive value to a Double object, rather than mapping an object to a primitive double.

Question 16

Which of the following statements is false?

Accepted Answer

The IntPredicate interface does not have a default method named invert. Instead, it provides methods like test, and, or, and negate for evaluating conditions and logical operations. The negate method is used to reverse the boolean value, not a method named invert.

Question 17

Which of the following statements is false?

Accepted Answer

The answer of Which of the following statements is false?
...

Question 18

A lambda expression represents a(n) ________ method-a shorthand notation for implementing a functional interface.

Accepted Answer

A lambda expression is an anonymous function that can be used as a shorthand notation for implementing a functional interface. It is anonymous because it does not have a name, but it can be assigned to a variable or passed as an argument to a method. Therefore, choice D, anonymous, is the best answer.

Question 19

The type of a lambda is the type of the ________ that the lambda implements.

Accepted Answer

A lambda expression can only be used in the context of a functional interface, which means that the lambda expression must implement the single abstract method defined by the functional interface. Therefore, the type of the lambda is the same as the functional interface that it implements.

Question 20

What is the meaning of ( ) in the following lambda?
 () -> System.out.println("Welcome to lambdas!")

Accepted Answer

The () in the lambda signifies an empty parameter list, indicating that the lambda takes no input arguments.

Quiz 17: Lambdas and Streams

You can declare that an interface is a functional interface by preceding it with the @FunctionalInterface annotation. The compiler will then ensure that the interface contains ________; otherwise, it'll generate a compilation error.

Collectors static method ________ returns a Collector that counts the number of objects in a given classification, rather than collecting them into a List.

Which of the following statements is false?

Which of the following statements is false?

Collectors static method groupingBy with two arguments receives a Function that classifies the objects in the stream and another Collector (known as the ________ Collector) .

Map method entrySet returns a Set of Map.Entry objects containing the Map's ________.

Which of the following statements is false?

Which of the following statements is false?

The intermediate Stream operation ________ results in a stream containing only the elements that satisfy a condition.

Terminal operations are ________; they perform the requested operation when they are called.

Intermediate operations are________; they aren't performed until a terminal operation is invoked. This allows library developers to optimize stream-processing performance.

Which of the following statements is false?

Which of the following statements is false?

Which of the following statements is false?

Stream method ________ maps objects to double values and returns a DoubleStream. The method receives an object that implements the functional interface ToDoubleFunction (package java.util.function) .

Which of the following statements is false?

Which of the following statements is false?

A lambda expression represents a(n) ________ method-a shorthand notation for implementing a functional interface.

The type of a lambda is the type of the ________ that the lambda implements.

What is the meaning of ( ) in the following lambda? () -> System.out.println("Welcome to lambdas!")