Chapter 10 Streams

Question 1

Can I create a record inside a Java class? If false, why?

Answer 1

True.

Question 2

When Optional is empty the method get returns:

Answer 2

Throws exception.

Question 3

When Optional is empty the method ifPresent(Consumer c) returns:

Answer 3

Does nothing.

Question 4

When Optional is empty the method isPresent() returns:

Answer 4

Returns false.

Question 5

When Optional is empty the method orElse(T other) returns:

Answer 5

Returns other parameter.

Question 6

When Optional is empty the method orElseGet(Supplier s) returns:

Answer 6

Returns result of calling Supplier.

Question 7

When Optional is empty the method orElseThrow() returns:

Answer 7

Throws NoSuchElementException.

Question 8

When Optional is empty the method orElseThrow(Suppliers) returns:

Answer 8

Throws exception created by calling Supplier.

Question 9

Intermediate operations are executed upon method call. If false, why?

Answer 9

False – Intermediate operations are lazy and only executed when a terminal operation is invoked.

Question 10

Terminal operations can exist multiple times in a pipeline. If false, why?

Answer 10

False – Only one terminal operation is allowed per pipeline; it triggers stream evaluation.

Question 11

The return type of intermediate operations is the stream type.

Answer 11

True

Question 12

A stream remains valid after a terminal operation is called.

Answer 12

False – Once a terminal operation is executed, the stream is consumed and no longer valid.

Question 13

A terminal operation is required for a pipeline to be useful.

Answer 13

True

Question 14

True or False: opt.orElseThrow(() -> throw new Exception()); will compile correctly.

Answer 14

False: It will not compile because orElseThrow expects a lambda returning an exception, not one that throws an exception.

Question 15

True or False: orElseThrow() in Java 17 can only accept method references as arguments.

Answer 15

False: It can accept both method references and lambda expressions that return exceptions.

Question 16

True or False: Removing the throw from () -> throw new Exception() will make the code compile.

Answer 16

True: The lambda should return an exception (() -> new Exception()) instead of throwing it.

Question 17

True or False: The correct lambda form for orElseThrow is () -> new Exception().

Answer 17

True: This lambda returns an exception, meeting orElseThrow’s requirement.

Question 18

True or False: orElseThrow is new to Java 17.

Answer 18

False: orElseThrow has been available since Java 10 as part of Optional.

Question 19

True or False: Stream.empty() creates a finite stream with zero elements.

Answer 19

True

Question 20

True or False: Stream.of(varargs) creates an infinite stream.

Answer 20

False: It creates a finite stream with the specified elements.

Question 21

True or False: coll.parallelStream() is infinite.

Answer 21

False: coll.parallelStream() creates a finite stream from the collection but processes it in parallel.

Question 22

True or False: Stream.generate(supplier) creates an infinite stream.

Answer 22

True: It generates an infinite stream by calling the supplier for each element.

Question 23

True or False: Stream.iterate(seed, predicate, unaryOperator) always creates an infinite stream.

Answer 23

False: It stops when the predicate returns false, so it can be finite.

Question 24

The Optional.ofNullable(value) method is a factory method that either wraps a value or returns an empty Optional.

Answer 24

True

Question 25

Optional.empty() contains a default value.

Answer 25

(It contains no value at all.)

Question 26

Calling get() on an empty Optional returns null.

Answer 26

(It throws a NoSuchElementException.)

Question 27

ifPresent(Consumer c) runs the Consumer even if the Optional is empty.

Answer 27

(It does nothing when empty.)

Question 28

isPresent() returns true for an empty Optional.

Answer 28

(It returns false.)

Question 29

orElse(T other) throws an exception when the Optional is empty.

Answer 29

(It returns the provided default value.)

Question 30

orElseThrow() returns null when the Optional is empty.

Answer 30

(It throws a NoSuchElementException.)

Question 31

orElseGet(Supplier s) behaves identically to orElse(T other).

Answer 31

(It calls the Supplier function only if empty.)

Question 32

Optional.of(value) allows null values.

Answer 32

(It throws NullPointerException if value is null.)

Question 33

The ternary operator is required to handle null values with Optional.

Answer 33

(Optional.ofNullable(value) does this automatically.)

Question 34

The orElseGet(() -> Math.random()) method returns the same value every time it is called.

Answer 34

False (It calls the Supplier each time, generating a new random value.)

Question 35

The orElseThrow() method, when used without a Supplier, throws a NullPointerException.

Answer 35

False (It throws a NoSuchElementException by default.)

Question 36

The orElse(Double.NaN) method requires a Supplier to provide a default value.

Answer 36

False (It takes a direct value, not a Supplier.)

Question 37

Returning null instead of an Optional provides a clearer API.

Answer 37

False (Returning Optional is clearer because it explicitly represents the possibility of absence.)

Question 38

Optional cannot be used in functional programming styles.

Answer 38

False (Optional supports functional programming with methods like ifPresent().)

Question 39

Intermediate operations in a stream execute immediately when called.

Answer 39

False (They use lazy evaluation and execute only when a terminal operation runs.)

Question 40

A stream can be reused after executing a terminal operation.

Answer 40

False (A stream is consumed after a terminal operation and cannot be reused.)

Question 41

A terminal operation is optional in a stream pipeline.

Answer 41

False (A terminal operation is required to produce a result.)

Question 42

An intermediate operation in a stream pipeline can exist only once.

Answer 42

False (Multiple intermediate operations can be used in a pipeline.)

Question 43

A terminal operation in a stream pipeline can be executed multiple times on the same stream.

Answer 43

False (Once a terminal operation is executed, the stream is no longer valid.)

Question 44

A terminal operation always returns another stream.

Answer 44

False (Terminal operations return a non-stream result, such as a collection or a primitive value.)

Question 45

A stream is a sequence of data that follows a pipeline model.

Answer 45

True

Question 46

The orElseThrow() method can be customized to throw a specific exception.

Answer 46

True

Question 47

The return type of an intermediate operation is always a non-stream type.

Answer 47

False (Intermediate operations return another stream.)

Question 48

What is the purpose of the peek method in Java Streams?

Answer 48

The peek method allows performing an action on each element of a stream as it is consumed, primarily for debugging purposes.

Question 49

Is peek an intermediate or terminal operation in Java Streams?

Answer 49

peek is an intermediate operation, meaning it returns a new stream and does not trigger execution.

Question 50

When using peek in a parallel stream, what must be considered regarding shared state?

Answer 50

In parallel streams, the action in peek may be executed in different threads, so proper synchronization is required if modifying shared state.

Question 51

Can peek be used to modify stream elements?

Answer 51

No, peek should not be used for modifying elements. It is meant for observation, and modifying elements may lead to unpredictable behavior.

Question 51

What type of argument does the peek() method expect?

Answer 51

The peek() method expects a Consumer'<'T'>' object as an argument.

Question 52

What happens if you call peek() without an argument?

Answer 52

Calling peek() without an argument will fail to compile because it requires a Consumer'<'T'>' as a parameter.

Question 53

How can you use peek() correctly in a stream?

Answer 53

You can use peek() with a lambda expression or method reference, e.g.,

Question 54

What does the allMatch(Predicate predicate) method do?

Answer 54

It checks whether all elements in a stream match the given predicate. If all elements satisfy the condition, it returns true; otherwise, it returns false.

Question 55

What happens if the stream is empty when calling allMatch(predicate)?

Answer 55

It returns true, and the predicate is not evaluated.

Question 56

Why is allMatch(predicate) considered a short-circuiting terminal operation?

Answer 56

Because it may not evaluate the predicate on all elements if it determines the result early, stopping as soon as it finds an element that does not match.

Question 57

What does the dropWhile() method do in a Java stream?

Answer 57

It skips elements from the start of the stream as long as the predicate is true, and includes the rest.

Question 58

Does dropWhile() evaluate the entire stream?

Answer 58

No — it stops checking as soon as it finds the first element that does not match the predicate, and includes that and everything after.

Question 59

What is the output of the following code? ``` List.of(1, 2, 3, 0, 5).stream() .dropWhile(n -> n < 3) .toList(); ```

Answer 59

[3, 0, 5]