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In this code, a double variable amount is initialized to 42,000.00. The NumberFormat.getCompactNumberInstance(Locale.FRANCE, NumberFormat.Style.SHORT) method is used to obtain a compact number formatter for the French locale with the short style. The format method is then called to format the amount.

The compact number formatting is designed to represent numbers in a shorter form, based on the patterns provided for a given locale. In the French locale, the short style represents thousands with a lowercase 'k'. Therefore, 42,000 is formatted as 42 k.

Option Evaluations:

A. 42000E: This format is not standard in the French locale for compact number formatting.

B. 42 000,00 €: This represents the number as a currency with two decimal places, which is not the compact form.

C. 42000: This is the plain number without any formatting, which does not match the compact number format.

D. 42 k: This is the correct compact representation of 42,000 in the French locale with the short style.

Thus, option D (42 k) is the correct output.

The DoubleSummaryStatistics class in Java is part of the java.util package and is used to collect and summarize statistics for a stream of double values. Let's analyze how the methods work:

Initialization and Data Insertion

stats1.accept(4.5); → Adds 4.5 to stats1.

stats1.accept(6.0); → Adds 6.0 to stats1.

stats2.accept(3.0); → Adds 3.0 to stats2.

stats2.accept(8.5); → Adds 8.5 to stats2.

Combining stats1 and stats2

stats1.combine(stats2); merges stats2 into stats1, resulting in one statistics summary containing all values {4.5, 6.0, 3.0, 8.5}.

Calculating Output Values

Sum= 4.5 + 6.0 + 3.0 + 8.5 = 22.0

Max= 8.5

Average= (22.0) / 4 = 5.5

Thus, the output is:

yaml

Sum: 22.0, Max: 8.5, Avg: 5.5

References:

Java SE 21 & JDK 21 - DoubleSummaryStatistics

Java SE 21 - Streams and Statistical Operations

In Java, records are a special kind of class designed to act as transparent carriers for immutabledata. They automatically provide implementations for equals(), hashCode(), and toString(), and their fields are final and private by default.

Option A: ExtendingClass

Analysis: Records in Java implicitly extend java.lang.Record and cannot extend any other class because Java does not support multiple inheritance. Attempting to extend another class, such as Exception, will result in a compilation error.

Conclusion: Does not compile.

Option B: WithInstanceField

Analysis: Records do not allow the declaration of instance fields outside of their components. The declaration of double fuz; is not permitted and will cause a compilation error.

Conclusion: Does not compile.

Option C: ImplementingInterface

Analysis: Records can implement interfaces. In this case, ImplementingInterface implements Cloneable, which is valid.

Conclusion: Compiles successfully.

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

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

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 Java generics, wildcards are used to relax the type constraints of generic types. The extends wildcard () denotes an upper bounded wildcard, allowing any type that is a subclass of Type. Conversely, the super wildcard () denotes a lower bounded wildcard, allowing any type that is a superclass of Type.

Option A:

java

public List getListSuper() {

return new ArrayList();

}

Here, List represents a list that can hold IOException objects and objects of its supertypes. Since Exception is a superclass of IOException, ArrayList is compatible with List. Therefore, this method compiles successfully.

Option B:

java

public List getListExtends() {

return new ArrayList();

}

In this case, List represents a list that can hold objects of IOException and its subclasses. Since FileNotFoundException is a subclass of IOException, ArrayList is compatible with List. Thus, this method compiles successfully.

Option C:

java

public List getListExtends() {

return new ArrayList();

}

Here, List expects a list of IOException or its subclasses. However, Exception is a superclass of IOException, not a subclass. Therefore, ArrayList is not compatible with List, and this method will not compile.

Option D:

java

public List getListSuper() {

return new ArrayList();

}

In this scenario, List expects a list that can hold IOException objects and objects of its supertypes. Since FileNotFoundException is a subclass of IOException, ArrayList is not compatible with List, and this method will not compile.

Therefore, the methods in options A and B compile successfully, while those in options C and D do not.

In this code, the SmartPhone interface declares a method ring() with a boolean return type. The Iphone15 class implements the SmartPhone interface and provides an implementation for the ring() method.

However, in the Iphone15 class, the ring() method is declared without the public access modifier. In Java, when a class implements an interface, it must provide implementations for all the interface's methods with the same or a more accessible access level. Since interface methods are implicitly public, the implementing methods in the class must also be public. Failing to do so results in a compilation error.

Therefore, the Iphone15 class does not compile because the ring() method is not declared as public.

The CopyOnWriteArrayList is a thread-safe variant of ArrayList in which all mutative operations (such as add, set, and remove) are implemented by creating a fresh copy of the underlying array. This design allows for safe iteration over the list without requiring external synchronization, as iterators operate over a snapshot of the array at the time the iterator was created. Consequently, modifications made to the list after the creation of an iterator are not reflected in that iterator.

docs.oracle.com

Evaluation of Options:

Option A:Correct. This statement accurately describes the behavior of CopyOnWriteArrayList.

Option B:Incorrect. CopyOnWriteArrayList is thread-safe and is designed to prevent interference among concurrent threads.

Option C:Incorrect. Iterators of CopyOnWriteArrayList do not reflect additions, removals, or changes made to the list after the iterator was created; they operate on a snapshot of the list's state at the time of their creation.

Option D:Incorrect. CopyOnWriteArrayList allows null elements.

Option E:Incorrect. Element-changing operations on iterators, such as remove, set, and add, are not supported in CopyOnWriteArrayList and will throw UnsupportedOperationException.

In this code, an ArrayDeque named deque is created, and the integers 1 and 2 are added to it using the offer method. The offer method inserts the specified element at the end of the deque.

State of deque after offers:[1, 2]

The peek method retrieves, but does not remove, the head of the deque, returning 1. Therefore, i1 is assigned the value 1.

State of deque after peek:[1, 2]

Value of i1:1

The poll method retrieves and removes the head of the deque, returning 1. Therefore, i2 is assigned the value 1.

State of deque after poll:[2]

Value of i2:1

Another peek operation retrieves the current head of the deque, which is now 2, without removing it. Therefore, i3 is assigned the value 2.

State of deque after second peek:[2]

Value of i3:2

The System.out.println statement then outputs the values of i1, i2, and i3, resulting in 1 1 2.

In this code, a Stream of String elements is created containing "United" and "States". A BinaryOperator named operator is defined to concatenate the first string (s1) with the uppercase version of the second string (s2). The reduce method is then used with "-" as the identity value and operator as the accumulator.

The reduce method processes the elements of the stream as follows:

Initial Identity Value: "-"

First Iteration:

Accumulator Operation: "-".concat("United".toUpperCase())

Result: "-UNITED"

Second Iteration:

Accumulator Operation: "-UNITED".concat("States".toUpperCase())

Result: "-UNITEDSTATES"

Therefore, the final result stored in result is "-UNITEDSTATES", and the output of theSystem.out.println(result); statement is -UNITEDSTATES.

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

Comprehensive and Detailed In-Depth Explanation:

Understanding Java Module Compilation (javac)

Java modules are compiled using the javac command with specific options to specify:

Where the source files are located (--module-source-path)

Where required dependencies (external modules) are located (-p / --module-path)

Where the compiled output should be placed (-d)

Breaking Down the Correct Compilation Command

css

CopyEdit

javac --module-source-path src -p lib/com.eiffel.membership.jar -d out -m com.eiffeltower.shop

--module-source-path src → Specifies the directory where module sources are located.

-p lib/com.eiffel.membership.jar → Specifies the module path (JAR dependency in lib).

-d out → Specifies the output directory for compiled .class files.

-m com.eiffeltower.shop → Specifies the module to compile (com.eiffeltower.shop).

Error in for Loop Initialization

The initialization part of a for loopcannot declare multiple variables with different types in a single statement.

Error:

java

for (int i = 0, int j = 3; i < j; i++) {

Fix:Declare variables separately:

java

for (int i = 0, j = 3; i < j; i++) {

lyrics.lines() in Java 21

The lines() method of String returns aStream<String>, splitting the string by line breaks.

Calling .toList() on a streamconverts it to a list.

Valid Code After Fixing the Loop:

java

var lyrics = """

Quand il me prend dans ses bras

Qu'il me parle tout bas

Je vois la vie en rose

""";

for (int i = 0, j = 3; i < j; i++) {

System.out.println(lyrics.lines()

toList()

get(i));

}

Expected Output After Fixing:

vbnet

Quand il me prend dans ses bras

Qu'il me parle tout bas

Je vois la vie en rose

Thus, the correct answer is:Compilation fails.

References:

Java SE 21 - String.lines()

Java SE 21 - for Statement Rules

Understanding java.util.function.Predicate<T>

The Predicate interface represents a function thattakes an input and returns a boolean(true or false).

It is often used for filtering operations in functional programming and streams.

Analyzing the Methods:

and(Predicate<? super T> other)→Default method

Combines two predicates usinglogical AND(&&).

java

Predicate startsWithA = s -> s.startsWith("A");

Predicate hasLength3 = s -> s.length() == 3;

Predicate combined = startsWithA.and(hasLength3);

❌isEqual(Object targetRef)→Static method

Not a default method, because it doesnot operate on an instance.

java

Predicate isEqualToHello = Predicate.isEqual("Hello");

negate()→Default method

Negates a predicate (! operator).

java

Predicate notEmpty = s -> !s.isEmpty();

Predicate isEmpty = notEmpty.negate();

❌not(Predicate<? super T> target)→Static method (introduced in Java 11)

Not a default method, since it is static.

or(Predicate<? super T> other)→Default method

Combines two predicates usinglogical OR(||).

❌test(T t)→Abstract method

Not a default method, because every predicatemust implement this method.

Thus, the correct answers are:and(Predicate<? super T> other), negate(), or(Predicate<? super T> other)

References:

Java SE 21 - Predicate Interface

Java SE 21 - Functional Interfaces

Understanding Try-With-Resources (AutoCloseable)

BoomBoom implements AutoCloseable, meaning its close() method isautomatically calledat the end of the try block.

Step-by-Step Execution

Step 1: Enter Try Block

java

try (BoomBoom boomBoom = new BoomBoom()) {

System.out.print("bim ");

throw new Exception();

}

"bim " is printed.

Anexception (Exception) is thrown, butbefore it is handled, the close() method is executed.

Step 2: close() is Called

java

@Override

public void close() throws Exception {

System.out.print("bam ");

throw new RuntimeException();

}

"bam " is printed.

A new RuntimeException is thrown, but it doesnot override the existing Exception yet.

Step 3: Exception Handling

java

} catch (Exception e) {

System.out.print("boom ");

}

The catch (Exception e)catches the original Exception from the try block.

"boom " is printed.

Final Output

nginx

bim bam boom

Theoriginal Exception is caught, not the RuntimeException from close().

TheRuntimeException from close() is ignoredbecause thecatch block is already handling Exception.

Thus, the correct answer is:bim bam boom

References:

Java SE 21 - Try-With-Resources

Java SE 21 - AutoCloseable Interface

In Java's java.nio.file package, the Path class represents a file path in a file system. The Paths.get(String first, String... more) method is used to create a Path instance by converting a path string or URI.

In the provided code snippet, the Path object path is created with the string "C:\\home\\joe\\foo". This represents an absolute path on a Windows system.

The getName(int index) method of the Path class returns a name element of the path as a Path object. The index is zero-based, where index 0 corresponds to the first element in the path's name sequence. It's important to note that the root component (e.g., "C:\" on Windows) is not considered a name element and is not included in this sequence.

Therefore, for the path "C:\\home\\joe\\foo":

Root Component:"C:\"

Name Elements:

Index 0: "home"

Index 1: "joe"

Index 2: "foo"

When path.getName(0) is called, it returns the first name element, which is "home". Thus, the output of the System.out.println statement is home.

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.

In this code, a Stream of String elements is created containing the characters "J", "a", "v", and "a". The reduce method is then used with String::concat as the accumulator function.

The reduce method with a single BinaryOperator parameter performs a reduction on the elements of the stream, using an associative accumulation function, and returns an Optional describing the reduced value, if any. In this case, it concatenates the strings in the stream.

Since the stream contains elements, the reduction operation concatenates them to form the string "Java". The result is wrapped in an Optional, resulting in Optional[Java]. The print statement outputs this Optional object, displaying Optional[Java].

In this code, the Calculable interface defines a single abstract method calculate that takes an int parameter and returns a long. The main method contains three lambda expressions assigned to variables c1, c2, and c3 of type Calculable.

Line 1:Calculable c1 = i -> i + 1;

This lambda expression takes an integer i and returns the result of i + 1. Since the expression i + 1 results in an int, and Java allows implicit widening conversion from int to long, this line compiles successfully.

Line 2:Calculable c2 = i -> Long.valueOf(i);

Here, the lambda expression takes an integer i and returns the result of Long.valueOf(i). The Long.valueOf(int i) method returns a Long object. However, Java allows unboxing of the Long object to a long primitive type when necessary. Therefore, this line compiles successfully.

Line 3:Calculable c3 = i -> { throw new ArithmeticException(); };

This lambda expression takes an integer i and throws an ArithmeticException. Since the method calculate has a return type of long, and throwing an exception is a valid way to exit the method without returning a value, this line compiles successfully.

Since all three lines adhere to the method signature defined in the Calculable interface and there are no type mismatches or syntax errors, the program compiles successfully.

In this program, the following operations are performed:

Paths Initialization:

Path p1 is set to "f1.txt".

Path p2 is set to "f2.txt".

File Move Operation:

Files.move(p1, p2); attempts to move (or rename) f1.txt to f2.txt.

File Delete Operation:

Files.delete(p1); attempts to delete f1.txt.

Analysis:

If f1.txt Does Not Exist:

The Files.move(p1, p2); operation will throw a NoSuchFileException because the source file f1.txt is missing.

If f1.txt Exists and f2.txt Does Not Exist:

The Files.move(p1, p2); operation will successfully rename f1.txt to f2.txt.

Subsequently, the Files.delete(p1); operation will throw a NoSuchFileException because p1 (now f1.txt) no longer exists after the move.

If Both f1.txt and f2.txt Exist:

The Files.move(p1, p2); operation will throw a FileAlreadyExistsException because the target file f2.txt already exists.

If f2.txt Exists While f1.txt Does Not:

Similar to the first scenario, the Files.move(p1, p2); operation will throw a NoSuchFileException due to the absence of f1.txt.

In all possible scenarios, an exception is thrown during the execution of the program.

In this scenario, we have a Person class that extends ListResourceBundle and a properties file named Person.properties. Both define a resource with the key "name" but with different values:

Person class (ListResourceBundle):Defines the key "name" with the value "Jeanne".

Person.properties file:Defines the key "name" with the value "James".

When the ResourceBundle.getBundle("Person") method is called, the Java runtime searches for a resource bundle with the base name "Person". The search order is as follows:

Class-Based Resource Bundle:The runtime first looks for a class named Person (i.e., Person.class).

Properties File Resource Bundle:If the class is not found, it then looks for a properties file named Person.properties.

In this case, since the Person class is present and accessible, the runtime will load the Person class as the resource bundle. Therefore, the getBundle method returns an instance of the Person class.

Subsequently, when bundle.getString("name") is called, it retrieves the value associated with the key "name" from the Person class, which is "Jeanne".

Thus, the output of the program is:

nginx

Jeanne

In this code, two LocalDate instances are created representing May 4, 2023, and May 4, 2024. The Period.between() method is used to calculate the period between these two dates, and the Duration.between() method is used to calculate the duration between them.

Period Calculation:

The Period.between() method calculates the amount of time between two LocalDate objects in terms of years, months, and days. In this case, the period between May 4, 2023, and May 4, 2024, is exactly one year. Therefore, p is P1Y, which stands for a period of one year. Printing p will output P1Y.

Duration Calculation:

The Duration.between() method is intended to calculate the duration between two temporal objects that have time components, such as LocalDateTime or Instant. However, LocalDate represents a date without a time component. Attempting to use Duration.between() with LocalDate instances will result in an UnsupportedTemporalTypeException because Duration requires time-based units, which LocalDate does not support.

Exception Details:

The UnsupportedTemporalTypeException is thrown when an unsupported unit is used. In this case, Duration.between() internally attempts to access time-based fields (like seconds), which are not supported by LocalDate. This behavior is documented in the Java Bug System underJDK-8170275.

Correct Usage:

To calculate the duration between two dates, including time components, you should use LocalDateTime or Instant. For example:

java

LocalDateTime start = LocalDateTime.of(2023, Month.MAY, 4, 0, 0);

LocalDateTime end = LocalDateTime.of(2024, Month.MAY, 4, 0, 0);

Duration d = Duration.between(start, end);

System.out.println(d); // Outputs: PT8784H

This will correctly calculate the duration as PT8784H, representing 8,784 hours (which is 366 days, accounting for a leap year).

Conclusion:

The output of the given code will be:

pgsql

P1Y

Exception in thread "main" java.time.temporal.UnsupportedTemporalTypeException: Unsupported unit: Seconds

Therefore, the correct answer is D:

nginx

P1Y

UnsupportedTemporalTypeException

In this code, the Test class has a static integer field count and a constructor that is declared with the synchronized modifier. In Java, the synchronized modifier can be applied to methods to control access to critical sections, but it cannot be applied directly to constructors. Attempting to declare a constructor as synchronized will result in a compilation error.

Compilation Error Details:

The Java Language Specification does not permit the use of the synchronized modifier on constructors. Therefore, the compiler will produce an error indicating that the synchronized modifier is not allowed in this context.

Correct Usage:

If you need to synchronize the initialization of instances, you can use a synchronized block within the constructor:

java

public class Test {

static int count;

Test() {

synchronized (Test.class) {

count++;

}

}

public static void main(String[] args) throws InterruptedException {

Runnable task = Test::new;

Thread t1 = new Thread(task);

Thread t2 = new Thread(task);

t1.start();

t2.start();

t1.join();

t2.join();

System.out.println(count);

}

}

In this corrected version, the synchronized block within the constructor ensures that the increment operation on count is thread-safe.

Conclusion:

The original program will fail to compile due to the illegal use of the synchronized modifier on the constructor. Therefore, the correct answer is E: Compilation fails.

Why does the compilation fail?

TheJava switch statement does not support primitive type pattern matchingin switch expressions as of Java 21.

The case pattern case int i -> "integer"; isinvalidbecausepattern matching with primitive types (like int or long) is not yet supported in switch statements.

The error occurs at case int i -> "integer";, leading to acompilation failure.

Correcting the Code

Since myVar is of type Object,autoboxing converts 0 into an Integer.

To make the code compile, we should use Integer instead of int:

java

Object myVar = 0;

String print = switch (myVar) {

case Integer i -> "integer";

case Long l -> "long";

case String s -> "string";

default -> "";

};

System.out.println(print);

Output:

bash

integer

Thus, the correct answer is:Compilation fails.

References:

Java SE 21 - Pattern Matching for switch

Java SE 21 - switch Expressions