1. Why Does Compilation Fail?

The error is in this line:

java

final Stream strings = Files.readAllLines(Paths.get("orders.csv"));

Files.readAllLines(Paths.get("orders.csv")) returns a List<String>,not a Stream<String>.

A List cannot be assigned to a Stream.

2. Correcting the Code

The correct way to create a stream from the file:

java

Stream strings = Files.lines(Paths.get("orders.csv"));

This correctly creates a Stream from the file.

3. Expected Output After Fixing

java

Files.lines(Paths.get("orders.csv"))

skip(1) // Skips the header row

limit(2) // Limits to first two data rows

forEach(System.out::println);

Output:

arduino

1,Kylian Mbappé,Keyboard,2,25.50

2,Teddy Riner,Mouse,1,15.99

Thus, the correct answer is:Compilation fails.

References:

Java SE 21 - Files.readAllLines

Java SE 21 - Files.lines

Understanding the do-while Loop

The do-while loopexecutes at least oncebefore checking the condition.

The condition i++ > 0 increments iafterchecking.

Step-by-Step Execution

Iteration 1:

i = 0

Prints: "Chanel"

i++ updates i to 1

Condition 1 > 0is true, so the loop exits.

Why Doesn't the Loop Continue?

Since i starts at 0, the conditioni++ > 0 is false after the first iteration.

The loopexits immediately after printing "Chanel".

Final Output

nginx

Chanel

Thus, the correct answer is:Chanel

References:

Java SE 21 - do-while Loop

Java SE 21 - Post-Increment Behavior

In Java, the Stream API provides several methods to create streams. However, not all approaches are valid.

In this code, two Optional objects are created:

o1 is an empty Optional.

o2 is an Optional containing the integer 1.

A stream is created from o1 and o2. The filter method retains only the Optional instances that are present (i.e., non-empty). This results in a stream containing only o2.

The findAny method returns an Optional describing some element of the stream, or an empty Optional if the stream is empty. Since the stream contains o2, findAny returns Optional[Optional[1]].

The flatMap method is then used to flatten this nested Optional. It applies the provided mapping function (o -> o) to the value, resulting in Optional[1].

Finally, o3.orElse(2) returns the value contained in o3 if it is present; otherwise, it returns 2. Since o3 contains 1, the output is 1.

Understanding ExecutorService Methods

execute(Runnable command)

Runs the task but only supports Runnable (not Callable).

❌execService.execute(task2); fails because task2 is Callable.

submit(Runnable task)

Submits a Runnable task for execution.

execService.submit(task1); executes "Executing Task-1".

submit(Callable<T> task)

Submits a Callable task for execution.

execService.submit(task2); executes "Executing Task-2".

call() Does Not Exist in ExecutorService

❌execService.call(task1); and execService.call(task2); are invalid.

run() Does Not Exist in ExecutorService

❌execService.run(task1); and execService.run(task2); are invalid.

Correct Code to Print Both Messages:

java

execService.submit(task1);

execService.submit(task2);

Thus, the correct answer is:execService.submit(task1); execService.submit(task2);

References:

Java SE 21 - ExecutorService

Java SE 21 - Callable and Runnable

In this code, a Stream containing the elements "a" and "b" is processed using the collect method. The collect method is a terminal operation that performs a mutable reduction on the elements of the stream using a Collector. In this case, custom implementations for the supplier, accumulator, and combiner are provided.

Components of the collect Method:

Supplier:

() -> new StringBuilder("c")

This supplier creates a new StringBuilder initialized with the string "c".

Accumulator:

StringBuilder::append

This accumulator appends each element of the stream to the StringBuilder.

Combiner:

(a, b) -> b.append(a)

This combiner is used in parallel stream operations to merge two StringBuilder instances. It appends the contents of a to b.

Execution Flow:

Stream Elements:"a", "b"

Initial StringBuilder:"c"

Accumulation:

The first element "a" is appended to "c", resulting in "ca".

The second element "b" is appended to "ca", resulting in "cab".

Combiner:

In this sequential stream, the combiner is not utilized. The combiner is primarily used in parallel streams to merge partial results.

Final Result:

The StringBuilder contains "cab". Therefore, the output of the program is:

nginx

cab

In this code, three static methods from the Boolean class are used: logicalAnd, logicalOr, and logicalXor. Each method takes two boolean arguments and returns a boolean result based on the respective logical operation.

Evaluation of Each Statement:

Boolean.logicalAnd(1 == 1, 2 < 1)

Operands:

1 == 1 evaluates to true.

2 < 1 evaluates to false.

Operation:

Boolean.logicalAnd(true, false) performs a logical AND operation.

The result is false because both operands must be true for the AND operation to return true.

Output:

System.out.print(false); prints false.

Boolean.logicalOr(1 == 1, 2 < 1)

Operands:

1 == 1 evaluates to true.

2 < 1 evaluates to false.

Operation:

Boolean.logicalOr(true, false) performs a logical OR operation.

The result is true because at least one operand is true.

Output:

System.out.print(true); prints true.

Boolean.logicalXor(1 == 1, 2 < 1)

Operands:

1 == 1 evaluates to true.

2 < 1 evaluates to false.

Operation:

Boolean.logicalXor(true, false) performs a logical XOR (exclusive OR) operation.

The result is true because exactly one operand is true.

Output:

System.out.print(true); prints true.

Combined Output:

Combining the outputs from each statement, the final printed result is:

nginx

falsetruetrue

Correct module descriptor file name

A module declaration must be placed inside a file namedmodule-info.java.

The incorrect filename module-info.perfumery.shop.javais invalid(Option A).

The incorrect filename module.javais invalid(Option C).

Correct module declaration

The module declaration must match the name of the module (perfumery.shop).

The requires perfumery.provider; directive specifies that perfumery.shop depends on perfumery.provider.

The exports perfumery.shop.eaudeparfum; statement allows the perfumery.shop.eaudeparfum package to beaccessible by other modules.

The incorrect syntax exports perfumery.shop.eaudeparfum.*; in Option A isinvalid, as wildcards (*) arenot allowedin module exports.

Thus, the correct answer is:File name: module-info.java

References:

Java SE 21 - Modules

Java SE 21 - module-info.java File

In this code, now is assigned the current date using LocalDate.now(). The goal is to format this date to the ISO week date format, which represents dates in the YYYY-'W'WW-E pattern, where:

YYYY: Week-based year

'W': Literal 'W' character

WW: Week number

E: Day of the week

Given that the present day is December 31, 2024, this date falls in the first week of the week-based year 2025. Therefore, the ISO week date representation would be 2025-W01-2, where '2' denotes Tuesday.

Among the provided formatters:

format1: This line attempts to create a DateTimeFormatter using a constructor, which is incorrect because DateTimeFormatter does not have a public constructor that accepts a pattern directly. This would result in a compilation error.

format2: This is correctly assigned the predefined DateTimeFormatter.ISO_WEEK_DATE, which formats dates in the ISO week date format.

format3: This line attempts to create a DateFormat instance using a field, which is incorrect because DateFormat does not have such a constructor. This would result in a compilation error.

format4: This line attempts to get a DateFormat instance using an integer field, which is incorrect because DateFormat.getDateInstance() does not accept such parameters. This would result in a compilation error.

Therefore, the only correct and applicable formatter is format2. Using format2 in the now.format() method will produce the desired output: 2025-W01-2.

In this code, a list of integers integers is created containing the elements 0, 1, and 2. A stream is then created from this list, and the following operations are performed in sequence:

peek(System.out::print):

The peek method is an intermediate operation that allows performing an action on each element as it is encountered in the stream. In this case, System.out::print is used to print each element. However, since peek is intermediate, the printing occurs only when a terminal operation is executed.

limit(2):

The limit method is another intermediate operation that truncates the stream to contain no more than the specified number of elements. Here, it limits the stream to the first 2 elements.

forEach(i -> {}):

The forEach method is a terminal operation that performs the given action on each element of the stream. In this case, the action is an empty lambda expression (i -> {}), which does nothing for each element.

The sequence of operations can be visualized as follows:

Original Stream Elements: 0, 1, 2

After peek(System.out::print): Elements are printed as they are encountered.

After limit(2): Stream is truncated to 0, 1.

After forEach(i -> {}): No additional action; serves to trigger the processing.

Therefore, the output of the code is 01, corresponding to the first two elements of the list being printed due to the peek operation.

In this code, the try block throws an IOException. The catch block catches this exception and throws a new RuntimeException. Regardless of exceptions thrown in the try or catch blocks, the finally block is always executed. In this case, the finally block throws an ArithmeticException.

When an exception is thrown in a finally block, it overrides any previous exceptions that were thrown in the try or catch blocks. Therefore, the ArithmeticException thrown in the finally block is the exception that propagates out of the method. As a result, the program terminates with an ArithmeticException.

Understanding post++ (Post-increment)

post++uses the value first, then increments it.

postResult = post++ + 10;

post starts as 5.

post++ returns 5, then post is incremented to 6.

postResult = 5 + 10 = 15.

Final value of post after this line is 6.

Understanding ++pre (Pre-increment)

++preincrements the value first, then uses it.

preResult = ++pre + 10;

pre starts as 5.

++pre increments pre to 6, then returns 6.

preResult = 6 + 10 = 16.

Final value of pre after this line is 6.

Thus, the final output is:

yaml

postResult: 15, preResult: 16, Final value of post: 6, Final value of pre: 6

References:

Java SE 21 - Operators and Expressions

Java SE 21 - Arithmetic Operators

(BufferedWriter writer = new BufferedWriter("file.txt") is incorrect.)

Theincorrect statementisoption Bbecause BufferedWriterdoes nothave a constructor that accepts a String (file name) directly. The correct way to use BufferedWriter is to wrap it around a FileWriter, like this:

java

try (BufferedWriter writer = new BufferedWriter(new FileWriter("file.txt"))) {

writer.write("Hello");

}

Evaluation of Other Options:

Option A (Files.write)→ Correct

Uses Files.write() to write bytes to a file.

Efficient and concise method for writing small text files.

Option C (FileOutputStream)→ Correct

Uses a FileOutputStream to write raw bytes to a file.

Works for both text and binary data.

Option D (PrintWriter)→ Correct

Uses PrintWriter for formatted text output.

Option F (FileWriter)→ Correct

Uses FileWriter to write text data.

Option E (None of the suggestions)→ Incorrect becauseoption Bis incorrect.

In Java, FileOutputStream and ObjectOutputStream are used for writing data to files, but they have different purposes and methods. Let's analyze each statement:

fos.write("Today");

The FileOutputStream class is designed to write raw byte streams to files. The write method in FileOutputStream expects a parameter of type int or byte[]. Since "Today" is a String, passing it directly to fos.write("Today"); will cause a compilation error because there is no write method in FileOutputStream that accepts a String parameter.

fos.writeObject("Today");

The FileOutputStream class does not have a method named writeObject. The writeObject method is specific to ObjectOutputStream. Therefore, attempting to call fos.writeObject("Today"); will result in a compilation error.

oos.write("Today");

The ObjectOutputStream class is used to write objects to an output stream. However, it does not have a write method that accepts a String parameter. The available write methods in ObjectOutputStream are for writing primitive data types and objects. Therefore, oos.write("Today"); will cause a compilation error.

oos.writeObject("Today");

The ObjectOutputStream class provides the writeObject method, which is used to serialize objects and write them to the output stream. Since String implements the Serializable interface, "Today" can be serialized. Therefore, oos.writeObject("Today"); is valid and compiles successfully.

In summary, the only statement that compiles without errors is oos.writeObject("Today");.

References:

Java SE 21 & JDK 21 - ObjectOutputStream

Java SE 21 & JDK 21 - FileOutputStream

1. Understanding Supplier Functional Interfaces

The Supplier interface is part of java.util.function and provides valueswithout taking any arguments.

Java also provides primitive specializations of Supplier:

BooleanSupplier→ Returns a boolean. Exists

DoubleSupplier→ Returns a double. Exists

LongSupplier→ Returns a long. Exists

Supplier<T>→ Returns a generic T. Exists

2. What about BiSupplier<T, U, R>?

There is no BiSupplier<T, U, R> in Java.

In Java, suppliers donot take arguments, so abi-supplierdoes not exist.

If you need a function thattakes two arguments and returns a value, use BiFunction.

Thus, the correct answer is:BiSupplier<T, U, R> does not exist.

References:

Java SE 21 - Supplier

Java SE 21 - Functional Interfaces

In the provided code, the class ThisCalls has two constructors:

No-Argument Constructor (ThisCalls()):

This constructor calls the boolean constructor with this(true);.

Boolean Constructor (ThisCalls(boolean flag)):

This constructor attempts to call the no-argument constructor with this();.

This setup creates a circular call between the two constructors:

The no-argument constructor calls the boolean constructor.

The boolean constructor calls the no-argument constructor.

Such a circular constructor invocation leads to a compile-time error in Java, specifically "recursiveconstructor invocation." The Java Language Specification (JLS) states:

"It is a compile-time error for a constructor to directly or indirectly invoke itself through a series of one or more explicit constructor invocations involving this."

Therefore, the code will not compile due to this recursive constructor invocation.

Understanding CopyOnWriteArrayList

CopyOnWriteArrayList is a thread-safe variant of ArrayList whereall mutative operations (add, set, remove, etc.) create a new copy of the underlying array.

This meansiterations will not reflect modifications made after the iterator was created.

Instead of modifying the existing array, a new copy is created for modifications, ensuring that readers always see a consistent snapshot.

Thread Execution Behavior

Thread 1 (Writer Thread)adds "D" to the list.

Thread 2 (Reader Thread)iterates over the list.

The reader thread gets a snapshot of the listbefore"D" is added.

The output may look like:

mathematica

Read element: A

Read element: B

Read element: C

Element added: D

"D" may not appear in the output of the reader threadbecause the iteration occurs on a snapshot before the modification.

Why doesn't it print all elements including changes?

Since CopyOnWriteArrayList doesnot allow changes to be visible during iteration, the reader threadwill not see "D"if it started iterating before "D" was added.

Thus, the correct answer is:"It prints all elements, but changes made during iteration may not be visible."

References:

Java SE 21 - CopyOnWriteArrayList

Option A (Map<String, ArrayList<String>> authorsMap1 = new HashMap<>();)

❌Compilation Fails

frenchAuthors is declared as List,notArrayList.

The correct way to declare a Map that allows storing List is to use List as the generic type,notArrayList.

Fix:

java

Map> authorsMap1 = new HashMap<>();

authorsMap1.put("FR", frenchAuthors);

Reason:The type ArrayList is more specific than List, and this would cause atype mismatcherror.

Option B (Map<String, ? extends List<String>> authorsMap2 = new HashMap<String, ArrayList<String>>();)

❌Compilation Fails

? extends Listmakes the map read-onlyfor adding new elements.

The line authorsMap2.put("FR", frenchAuthors); causes acompilation errorbecause wildcard (? extends List) prevents modifying the map.

Fix:Remove the wildcard:

java

Map> authorsMap2 = new HashMap<>();

authorsMap2.put("FR", frenchAuthors);

Option C (var authorsMap3 = new HashMap<>();)

Compiles Successfully

The var keyword allows the compiler to infer the type.

However,the inferred type is HashMap<Object, Object>, which may cause issues when retrieving values.

Option D (Map<String, List<String>> authorsMap4 = new HashMap<String, ArrayList<String>>();)

Compiles Successfully

Valid declaration:HashMap can be assigned to Map.

Using new HashMap>() with Map> isalloweddue to polymorphism.

Correct syntax:

java

Map> authorsMap4 = new HashMap>();

authorsMap4.put("FR", frenchAuthors);

Option E (Map<String, List<String>> authorsMap5 = new HashMap<String, List<String>>();)

Compiles Successfully

HashMap> isa valid instantiation.

Correct usage:

java

Map> authorsMap5 = new HashMap<>();

authorsMap5.put("FR", frenchAuthors);

Thus, the correct answers are:C, D, E

References:

Java SE 21 - Generics and Type Inference

Java SE 21 - var Keyword

Pattern Matching in switch

The switch expression introduced inJava 21supportspattern matchingfor different types.

However,a switch expression must be exhaustive, meaningit must cover all possible cases or provide a default case.

Why does compilation fail?

input is an Object, and the switch expression attempts to pattern-match it to String, Double, and Integer.

If input had been of another type (e.g., Float or Long), there would beno matching case, leading to anon-exhaustive switch.

Javarequires a default caseto ensure all possible inputs are covered.

Corrected Code (Adding a default Case)

java

Object input = 42;

String result = switch (input) {

case String s -> "It's a string with value: " + s;

case Double d -> "It's a double with value: " + d;

case Integer i -> "It's an integer with value: " + i;

default -> "Unknown type";

};

System.out.println(result);

With this change, the codecompiles and runs successfully.

Output:

vbnet

It's an integer with value: 42

Thus, the correct answer is:Compilation failsdue to a missing default case.

References:

Java SE 21 - Pattern Matching for switch

Java SE 21 - switch Expressions

In this code, a text block is defined using the """ syntax introduced in Java 13. Text blocks allow for multiline string literals, preserving the format as written in the code.

Text Block Analysis:

The text block is defined as:

java

String textBlock = """

j \

a \t

contentReference[oaicite:0]{index=0}

In the provided code, a list abc is created containing the strings "a", "b", and "c". The first forEach operation attempts to convert each element to uppercase by assigning x = x.toUpperCase();. However, this assignment only changes the local variable x within the lambda expression and does not modify the elements in the original list abc. Strings in Java are immutable, meaning their values cannot be changed once created. Therefore, the original list remains unchanged.

The second forEach operation iterates over the original list and prints each element. Since the list was not modified, the output will be the concatenation of the original elements: abc.

To achieve the output ABC, you would need to collect the transformed elements into a new list, as shown below:

java

List abc = List.of("a", "b", "c");

List upperCaseAbc = abc.stream()

map(String::toUpperCase)

collect(Collectors.toList());

upperCaseAbc.forEach(System.out::print);

In this corrected version, the map operation creates a new stream with the uppercase versions of the original elements, which are then collected into a new list upperCaseAbc. The forEach operation then prints ABC.

In this code, we have two interfaces and a class with a main method:

WithDefaultMethod Interface:

Declares a default method print() that outputs "default".

WithStaticMethod Interface:

Extends WithDefaultMethod.

Declares a static method print() that outputs "static".

DefaultAndStaticMethods Class:

Contains the main method, which calls WithStaticMethod.print().

Key Points:

Static Methods in Interfaces:

Static methods in interfaces are not inherited by implementing or extending classes or interfaces. They belong solely to the interface in which they are declared.

Default Methods in Interfaces:

Default methods can be inherited by implementing classes, but they cannot be overridden by static methods in subinterfaces.

Execution Flow:

The main method calls WithStaticMethod.print().

This invokes the static method print() defined in the WithStaticMethod interface, which outputs "static".

Therefore, the program compiles successfully and prints static.